U.S. patent application number 15/156018 was filed with the patent office on 2016-09-08 for method and apparatus for providing elevation plane spatial beamforming.
The applicant listed for this patent is Quintel Technology Limited. Invention is credited to David Edwin BARKER, David Sam PIAZZA.
Application Number | 20160261326 15/156018 |
Document ID | / |
Family ID | 47712591 |
Filed Date | 2016-09-08 |
United States Patent
Application |
20160261326 |
Kind Code |
A1 |
BARKER; David Edwin ; et
al. |
September 8, 2016 |
METHOD AND APPARATUS FOR PROVIDING ELEVATION PLANE SPATIAL
BEAMFORMING
Abstract
In one embodiment, the present disclosure provides a method and
apparatus for spatially filtering inter-cell co-channel
interference in the elevation plane, which in turn will improve
network spectral efficiency.
Inventors: |
BARKER; David Edwin;
(Stockport, GB) ; PIAZZA; David Sam; (San Jose,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Quintel Technology Limited |
Bristol |
|
GB |
|
|
Family ID: |
47712591 |
Appl. No.: |
15/156018 |
Filed: |
May 16, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13589121 |
Aug 18, 2012 |
9344176 |
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15156018 |
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61525625 |
Aug 19, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01Q 3/2605 20130101;
H04B 7/0469 20130101; H04B 7/0671 20130101; H04B 7/0408 20130101;
H04B 7/0617 20130101; H04B 7/10 20130101; H04W 16/28 20130101 |
International
Class: |
H04B 7/06 20060101
H04B007/06; H04B 7/04 20060101 H04B007/04 |
Claims
1. A base station system, comprising: a precoding stage for mapping
of baseband data symbols associated with a mobile terminal into two
baseband signal branches and pre-coding the two baseband signal
branches via a multiplication of the two baseband signal branches
with complex pre-coding weights to impart a phase difference
between the two baseband signal branches for generating pre-coded
signals; an allocation stage for allocating resources to the
pre-coded signals; a modulation stage for modulating the pre-coded
signals into modulated signals; a baseband to radio frequency (RF)
up-conversion stage for up-converting the modulated signals into RF
signals; and a passive RF antenna assembly comprising a beamforming
network having two ports for receiving the RF signals, and for
creating a spatially coherent main beam with a variable elevation
plane beam tilt in response to differentially phased signals across
the two ports.
2. The base station system of claim 1, wherein the passive RF
antenna assembly comprises a plurality of antenna elements, wherein
the plurality of antenna elements comprises at least five antenna
elements.
3. The base station system of claim 1, wherein the baseband to
radio (RF) up-conversion stage further functions as a RF to
baseband down conversion stage, for processing receive RF signals
into receive baseband signals, the receive RF signals comprising
differentially phased signals or differentially amplitude weighted
signals from the passive RF antenna assembly.
4. The base station system of claim 3, further comprising: a
receiver combining stage for performing a vector summation on the
receive baseband signals.
5. The base station system of claim 4, wherein the vector summation
comprises a maximal ratio combining process.
6. The base station system of claim 5, wherein the maximal ratio
combining process comprises a post-detection maximal ratio
combining process.
7. A method, comprising: mapping baseband data symbols associated
with a mobile terminal into two baseband signal branches and
pre-coding the two baseband signal branches via a multiplication of
the two baseband signal branches with complex pre-coding weights to
impart a phase difference between the two baseband signal branches
for generating pre-coded signals; allocating resources to the
pre-coded signals; modulating the pre-coded signals into modulated
signals; up-converting the modulated signals into RF signals;
receiving the RF signals via two ports of a beamforming network of
a passive RF antenna assembly; and creating a spatially coherent
main beam with a variable elevation plane beam tilt in response to
differentially phased signals across the two ports.
8. The method of claim 7, wherein the passive RF antenna assembly
comprises a plurality of antenna elements, wherein the plurality of
antenna elements comprises at least five antenna elements.
9. The method of claim 7, wherein the baseband to radio (RF)
up-conversion stage further functions as a RF to baseband down
conversion stage, for processing receive RF signals into receive
baseband signals, the receive RF signals comprising differentially
phased signals or differentially amplitude weighted signals from
the passive RF antenna assembly.
10. The method of claim 9, further comprising: performing a vector
summation on the receive baseband signals.
11. The method of claim 10, wherein the vector summation comprises
a maximal ratio combining process.
12. The method of claim 11, wherein the maximal ratio combining
process comprises a post-detection maximal ratio combining process.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/589,121, filed Aug. 18, 2012, now U.S. Pat.
No. 9,344,176, which claims priority under 35 U.S.C. .sctn.119(e)
to U.S. provisional patent application serial no. 61/525,625 filed
on Aug. 19, 2011, both of which are hereby incorporated by
reference in their entireties.
[0002] The present disclosure relates generally to elevation plane
spatial beamforming and, more particularly, to a method and
apparatus for spatially filtering inter-cell co-channel
interference in the elevation plane, which in turn will improve
network spectral efficiency.
SUMMARY
[0003] In one embodiment, the present disclosure provides a method
and apparatus for spatially filtering inter-cell co-channel
interference in the elevation plane, which in turn will improve
network spectral efficiency. For example, in one embodiment, a base
station system comprises a first passive radio frequency (RF)
antenna assembly comprising a beamforming network having at least
two ports, for creating a spatially coherent main beam with a
variable elevation plane beam tilt in response to differentially
phased signals or differentially amplitude signals across the two
ports, a RF to baseband down conversion stage coupled to the first
passive RF antenna assembly, for processing the differentially
phased signals or differentially amplitude weighted signals into
baseband signals and a receiver combining stage for performing a
vector summation on the baseband signals.
[0004] In another embodiment, a base station system comprises a
precoding stage for mapping of baseband data symbols associated
with a mobile terminal into two baseband signal branches and
pre-coding the branches via a multiplication of the signal branches
with complex pre-coding weights to impart a phase difference
between the two signal branches for generating pre-coded signals,
an allocation stage for allocating resources to the pre-coded
signals, a modulation stage for modulating the pre-coded signals
into modulated signals, a baseband to RF up-conversion stage for
up-converting the modulated signals into RF signals, and a first
passive RF antenna assembly comprising a beamforming network having
at least two ports for receiving the RF signals, and for creating a
spatially coherent main beam with a variable elevation plane beam
tilt in response to differentially phased signals across the two
ports.
[0005] In another embodiment, a method comprises creating a
spatially coherent main beam with a variable elevation plane beam
tilt in response to differentially phased signals or differentially
amplitude signals across two ports of a beamforming network of a
first passive RF antenna assembly, processing the differentially
phased signals or differentially amplitude weighted signals into
baseband signals via a RF to baseband down conversion stage coupled
to the first passive RF antenna assembly, and performing a vector
summation on the baseband signals via a receiver combining
stage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The teaching of the present disclosure can be readily
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0007] FIG. 1 depicts an Antenna Beamforming network and Array of
Antenna elements which can create beam with variable tilt; where
tilt angle is a function of the phase difference applied across the
Beamformer ports;
[0008] FIG. 2 depicts an Antenna Beamforming network and Array of
Antenna elements which can create beam with variable tilt; where
tilt angle is a function of the amplitude difference applied across
the Beamformer ports;
[0009] FIG. 3 depicts a Radio Channel Multi-path Geometry assumed
for various embodiments;
[0010] FIG. 4 depicts Uplink Elevation Beamforming using 2T4R base
station with MRC receive combining process with a passive antenna
array based on variable tilt in response to differential phase
across VA & VB Ports;
[0011] FIG. 5 depicts Uplink Elevation Beamforming using base
station MRC receive combining process with a passive antenna array
based on variable tilt in response to differential amplitude across
VA & VB Ports;
[0012] FIG. 6 depicts Uplink Elevation Beamforming using 2T4R base
station with MRC receive combining process with a passive antenna
array based on variable tilt in response to differential phase
across VA & VB Ports;
[0013] FIG. 7 depicts Uplink Elevation Beamforming using 2T4R base
station with MRC receive combining process with a passive antenna
array based on variable tilt in response to differential amplitude
across VA & VB Ports; and
[0014] FIG. 8 depicts Downlink Elevation Beamforming using 4T4R
base station (only 2 of 4 ports shown) connected to a passive
antenna array based on variable tilt in response to differential
phase across VA & VB Ports.
[0015] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures.
DETAILED DESCRIPTION
[0016] The present disclosure relates generally to elevation plane
spatial beamforming and, more particularly, to a method and
apparatus for spatially filtering inter-cell co-channel
interference in the elevation plane, which in turn will improve
network spectral efficiency.
[0017] Multiple Antenna techniques for use with radio communication
systems encompass a broad range of applications, and antenna
configurations. It is beyond the scope of this introduction to
discuss fully what these are, but suffice to say they can be
broadly divided into three types of application; Diversity, Spatial
Multiplexing Beamforming, and Spatial Beamforming.
[0018] Antenna Diversity is a technique that can be employed in the
cellular industry, and is employed in various legacy cellular
networks in the form of Receiver Diversity at the base station to
provide an effective diversity gain for Uplink channels. With
technologies such as UMTS/HSPA+, Transmit Diversity can also be
applied at the base station for Downlink channels. Additionally,
mobile terminals (e.g., cell phones, smart phones, tablets, and the
like) equipped with two or more antennas can also deliver Transmit
Diversity for the Uplink, and benefit from Receive Diversity on the
Downlink. Combinations of Receive Diversity and Transmit Diversity
are also possible in a communications link. Diversity aims to
improve link robustness against multi-path fading radio channels,
which in turn can be translated to a link gain which is beneficial
for improving cellular network coverage, or indeed be translated to
a gain in throughput/capacity. Diversity is achieved by exploiting
de-correlated or partially correlated multi-path radio channels
between transmitter (Tx) antenna and each receiver antenna (Rx) in
the case of Receiver Diversity; the same applies for Transmit
Diversity. Multi-path de-correlation can be achieved by using
sufficient spatial separation and/or exploiting orthogonal
polarizations of two or more antennas. Sufficient spatial
separation for de-correlation is a function of the angular
dispersion of the radio channel as observed by the antenna
arrangement, which is a function of the relative positions of the
radio channel scatterers with respect to the antennas. At a macro
cellular base station end of a link, which may be well elevated
above the scattering environment, a sufficient distance may be of
the order of up to 40.quadrature..lamda., whereas sufficient
distance between antennas at a terminal end may be less than
.lamda./2; this is because multi-path tends to arrive from all
angles to/from a mobile terminal, which is typically surrounded by
scatterers, whereas at a base station antenna, Radio Frequency (RF)
energy to/from one mobile terminal tends to arrive within a finite
range of angles.
[0019] For receiver diversity, each signal received is combined to
maximize some metric, such as Signal to Noise Ratio (SNR). A
popular combining technique is that of Maximal Ratio Combining
(MRC), where each signal is co-phased and then weighted according
to its instantaneous SNR and then summed together, in effect a
vector summing process. The combining function usually occurring at
complex baseband after down-conversion, and where multiple MRC
processes may be running concurrently for all active communications
link in a multiple access communications system. MRC can be shown
to work as an optimal combining scheme in Noise Limited radio
channels.
[0020] In the case of interference limited radio channels, other
combining schemes such as Interference Rejection Combining (IRC)
can be used in Multiple Access cellular system in Uplink at the
base station, especially for intra-cell interference rejection. IRC
in this case aims to find complex weights which maximally suppress
or even cancel any co-channel intra-cell interference from the
wanted signal, and naturally assumes that the radio channels by
which the interference is transmitted can be known, which would be
the case for a base station attempting to reject intra-cell
interference. IRC schemes can be shown to be sensitive to noise and
hybrid schemes and those based upon Minimum Mean Square Error
(MMSE) processing are often used for stability reasons in noise,
but often all such receive combining schemes are generally referred
to MRC due to historical reasons.
[0021] Transmit Diversity exploits independent or partially
independent radio channels in a similar manner to Receive
Diversity. For radio access systems based upon Time Division Duplex
(TDD), it is possible to use the same co-phasing and weight
coefficients derived from Receive Diversity for Transmit Diversity,
assuming that the radio channel can be measured (via pilot or
reference symbols) at a rate equal to or greater than the coherence
bandwidth of the radio channel. In this case Transmit Diversity is
called pre-coding, where information symbols are pre-coded
(multiplied by complex weights to adjust amplitude and phase) prior
to transmission via the Transmit Diversity antennas. This is also
often termed pre-coder based Beamforming. However, with Frequency
Division Duplex (FDD) radio access systems which use different RF
spectrum for Uplink and Downlink channels, pre-coder based
Beamforming to deliver Transmit diversity is not realistically
possible unless a fast and large overhead in reverse link signaling
could be tolerated; for example, for the Uplink to inform the base
station of the instantaneous channel received on Downlink, so the
base station can pre-code data for Transmission. Transmit Diversity
for FDD systems often used Space Time Block Coding (STBC) or Space
Frequency Block Coding (SFBC) techniques as an alternative more
practical means. Here, pairs of consecutive information symbols are
transmitted from a first antenna and at the same time the same
symbols, but sign-reversed and complex conjugated are transmitted
from a second antenna, at a base station. At the terminal receiver
(using a single antenna), the terminal receives two consecutive
symbols, where each received symbol is a vector composite of the
transmitted symbols. Given that the radio channel is known (via the
Terminal measuring periodic Downlink Pilot symbols and the radio
channel doesn't de-correlate between Downlink radio channel
estimation intervals), and the Transmit Diversity scheme is known,
the original transmit symbols can be fully recovered using
simultaneous complex equations, which is in fact multiplication of
the received symbols by the channel inverse matrix.
[0022] N.times.M Spatial Multiplexing Beamforming is often termed
N.times.M MIMO (Multiple In--Multiple Out) meaning the technique
relies upon N multiple antennas at the Transmission end and M
multiple antennas at the Receiving end of a communications link.
For completeness, Receiver Diversity is often termed SIMO for
Single In--Multiple Out, and Transmit Diversity termed MISO for
Multiple In--Single Out. Strictly speaking MISO and SIMO operating
together is just combinations of Transmit and Receive Diversity,
and can also be called MIMO too, but this arrangement is not
performing Spatial Multiplexing. Spatial Multiplexing Beamforming
(normally just called Spatial Multiplexing) aims to increase
spectral efficiency, rather than signal quality or robustness, by
transmission and reception of multiple information streams or
layers, each using the same frequency and time resources, but where
orthogonality is attempted in the spatial dimension. Spatial
Multiplexing, as with Diversity, relies upon and exploits
de-correlated or partially correlated multi-path radio channels
between each transmit (Tx) antenna and each Receive (Rx) antenna;
this correlation typically denoted mathematically by a radio
channel complex Matrix to capture all Tx antenna to Rx antenna
radio channel combinations. De-correlation can be achieved by using
sufficient spatial separation and/or exploiting orthogonal
polarizations of the multiple antennas within each end of the
communications link, in the same manner as for Diversity. Spatial
Multiplexing theory can demonstrate that it is possible to increase
spectral efficiency by re-using the same time and frequency
resources for transmission of separate information between Tx and
Rx ends of a communications link up to K times, with K antennas at
each end of the link, and K.sup.2 de-correlated radio channels. One
way of considering a 2.times.2 Spatial Multiplexing channel is to
realize that two signals transmitted are two signals creating
interference to one another; the two signals received by the
receive antennas can then carry out IRC to suppress the other
signal thus resulting in two orthogonal information streams.
Another way of viewing this process is applying an inverse of the
complex radio channel matrix to the received signals to arrive at
the wanted signals. As aforementioned with IRC, even if there is
good radio channel de-correlation, if signals experience high
levels of noise, i.e., poor SNR then Spatial Multiplexing may not
be reliable. Spatial Multiplexing however in practice, such as in
LTE systems is achieved not by simply applying a channel inverse at
the receiver, but by pre-coding signals for transmission and
de-coding signals at reception, where the pre-coding and de-coding
together are performing the channel inverse function. This
pre-coding arrangement allows for better performance through
improved signal isolation at the receiver side. Pre-coded Spatial
Multiplexing also allows for combinations of pre-coded Beamforming
and Spatial Multiplexing in the event full radio channel matrix
de-correlation cannot be achieved. As discussed above for Transmit
Diversity, with FDD systems and considering the Downlink channel
case, ideal pre-coding would require full, rapid and persistent
feedback of the Downlink radio channels. However, this would
require a large overhead in terms of Uplink signaling, and instead
for systems such as LTE, the Terminal reports back to the base
station key parameters such as signal quality, how de-correlated
the Downlink radio channels are and then the base station can
decide upon a pre-coding matrix to use from a finite set of
pre-coding matrices with which to pre-code the information symbols;
and informing the Terminal which pre-coding matrix is being used
for the current time interval. This is called codebook based
Spatial Multiplexing or codebook based Beamforming (i.e.,
Beamforming for Spatial Multiplexing). Additionally LTE allows for
the Terminal to suggest a pre-coding matrix from the known codebook
for the base station to use. This partial channel information
feedback and codebook based pre-coding constraint allows for
practical spatial multiplexing.
[0023] Spatial Multiplexing and Diversity rely upon de-correlated
radio channels, whereas classical Spatial Beamforming is a
technique which exploits highly correlated radio channels, where
multiple antennas normally arranged in a linear array are co-phased
in such a manner as to direct energy to/from a particular spatial
direction, and/or reject (spatially filter) co-channel interference
from (an)other spatial direction(s). Note that Spatial Beamforming,
sometimes termed classic Beamforming, Coherent Beamforming, or just
Beamforming is different to pre-coder based Beamforming as used
with Transmit Diversity and Spatial Multiplexing. Pre-coder based
Beamforming can be viewed as the most general interpretation of
transmit side Beamforming, where Spatial Beamforming is a
particular sub-set. Spatial Beamforming exploits strong radio
channel correlation between Tx antenna-Rx antenna radio channels,
and as such antennas designed for (Spatial) Beamforming at a macro
base station are typically separated by small distances (typically
.lamda./2), and typically arranged as a classic array of antennas.
Beamforming can be applied at the transmitter end or the receiver
end of a communications link, or indeed both. Note: when referring
to Beamforming, an antenna in the array is commonly called an
Antenna element of the Antenna array, but the (antenna) array can
also be considered as a single antenna itself. Beamforming is
designed to enhance the radio link quality rather than directly
increasing channel throughput or robustness, as with Spatial
Multiplexing or Diversity, respectively. Beamforming enables
different coherent spatial radiation patterns to be formed via
control of phase and/or amplitude to each antenna element in the
array. Beamforming when used in cellular communication systems can
offer improvement in C/I levels which can be translated or
indirectly exploited to improving coverage or capacity. As stated
above, although Beamforming uses multiple antennas (antenna
elements) it is considered as a single antenna in the SIMO, MISO,
and MIMO taxonomy. It is therefore also possible to use hybrid
combinations of Spatial Multiplexing or Diversity with (Spatial)
Beamforming.
[0024] Spatial Multiplexing, Diversity and Beamforming are simply
classes or descriptors to help understand the application of using
multiple antennas. As stated earlier they all, in fact can be
considered as Beamforming in the most general sense, since phase
and amplitude manipulation and subsequent processing is carried out
for each information symbol associated with each antenna.
Sometimes, the term Beamforming is used to describe all these
multiple antenna techniques. However, when considering for example
spatial multiplexing or Diversity where orthogonal polarized
antennas are used, then a "beam" in the familiar polar or Cartesian
co-ordinates cannot really be visualized or described by a
conventional coherent or spatially convergent radiation pattern. In
fact when the term Beamforming is used in the context of Spatial
Multiplexing or Diversity, it strictly refers to the general class
of pre-coder based Beamforming in dispersive radio channels, and
could be tangibly viewed as creating very localized coherency of
wanted information signals and/or incoherency of unwanted
co-channel signals in "vector" space, meaning that wanted RF energy
is coherent at a particular location in space, time and frequency,
rather than coherency at an angle or direction from the antennas as
with more classical Spatial Beamforming. However, for the purposes
of this disclosure, the term Beamforming is used, as described in
the previous paragraph, to mean "classic" spatially coherent
Beamforming using an array of closely coupled antennas (i.e.,
antenna elements) in an array configuration where a Beam can be
fully described in the angular domain, i.e., a radiation pattern
using Polar or Cartesian co-ordinates can be used to fully describe
Beamforming.
[0025] There are many texts on the subject of Spatial Beamforming,
but can be classified into two broad classes; switched Beamforming
and adaptive Beamforming. With switched beam antennas, such as an
antenna array of N antennas (often called antenna elements) fed
from an N.times.N Butler matrix distribution network permits one of
N different and spatially orthogonal radiation patterns (beams) to
be generated via connection of an RF signal to one of N different
input ports of the Butler matrix, considering a Tx mode of
operation. As the Butler matrix is a passive distribution network,
the reciprocal case for Rx mode can also be made.
[0026] Adaptive arrays, also commonly referred to as Smart Antenna
arrays rely upon independent RF phase/amplitude variation at each
antenna (antenna element), thus allowing a very wide range of
possible radiation pattern outcomes. Classic coherent spatial
Beamforming theory can demonstrate that given N antenna elements,
then N freedoms of Phase/Amplitude control are possible which can
be used to create N spatial beams and N-1 spatial nulls in angular
terms. Control of Phase/Amplitude at the antenna elements can be
achieved using RF Beamforming or Digital Beamforming techniques. RF
Beamforming would be typically achieved, considering a Transmission
mode, using an RF Power Amplifier (PA) which can vary its amplitude
or gain, and an RF phase shifting arrangement, associated with each
antenna element. For Reception to the antenna, then the use of a
variable gain Low Noise Amplifier (LNA) and phase shifting
arrangement associated with each antenna element is required. A
trivial example of an RF Beamforming technique is a conventional
cellular base station antenna utilizing Variable Electrical Tilt
methods; in this case, a variable phase shifter is used at each
antenna element in an array (or often a phase shifter driving pairs
of elements) with the goal of varying main beam elevation pattern
tilt angle. However, a modulated cellular RF signal from and to a
base station carries multiple embedded information signals at the
same time, separated in frequency, code domains, or spatial domains
(if spatial multiplexing is used) normally associated with
different terminals/users which are generally in different spatial
locations for example as would be the case in a cellular network.
The disadvantage of RF Beamforming, for example in a cellular
network is that the all information (to be transmitted or received
by different terminals) shares the same, albeit adaptive radiation
pattern.
[0027] Digital Beamforming however performs RF to baseband
conversion or RF translation, then analogue to digital conversion
for the RF signal from each antenna element (considering a receive
channel). This process results in N.times.complex baseband signals
associated with N.times.antenna elements. By applying complex
weights to each of these complex baseband signals, it can be shown
that this varies phase/amplitude in the RF domain at the actual
antenna elements. The advantage of digital Beamforming removes the
need for RF phase/amplitude control at the antenna elements,
instead allowing use of fixed gain and fixed phase delays
associated with the antenna elements. Moreover, multiple copies of
the baseband signal can be created and processed independently and
concurrently. This permits independent Beamforming for individual
users/terminals which may be in different spatial locations or more
precisely at different bearings or directions to/from the antenna
array in a cellular network. In effect multiple, concurrent beams
can be generated for different users, with each beam carrying
specific information/data for each user/terminal.
[0028] For the Downlink channel in LTE cellular systems, Transmit
Diversity, Spatial Multiplexing MIMO and Spatial Beamforming are
all possible, and are achieved by applying complex weights to
information symbols at complex baseband, in what is called the
pre-coder. The pre-coder is, as aforementioned the most general
term to describe the baseband processing of coding associated with
all incarnations of Beamforming. For Diversity and Spatial
Multiplexing the pre-coding is designed to be used with antennas
with low multi-path radio channel correlation. With more classic
Beamforming, the pre-coder is designed to be used with antennas
with high multi-path radio channel correlation. With Spatial
Multiplexing the pre-coding weights change at a rate commensurate
with the multi-path channel fading rate, whereas classic
Beamforming, the pre-coder weights would change at a much slower
rate commensurate with a terminal having moved significantly across
a cell. Finally, for FDD LTE systems, Spatial Multiplexing is based
upon codebook based pre-coding whereas Spatial Beamforming can be
based upon non codebook pre-coding, i.e., arbitrary pre-coding
vectors can be used.
[0029] In cellular communications systems and in particular access
technologies such as 3GPP LTE permit a variety of multiple antenna
configurations to exploit Spatial Multiplexing, Antenna Diversity
and Beamforming. For example, a LTE cellular application can use
4.times. passive antenna arrays connected to what is commonly
referred to as a 4T4R base station, i.e., having 4.times. duplexed
Tx/Rx ports, assuming Frequency Division Duplex (FDD) operation.
This base station could connect to 4.times. co-polarized antennas
which have sufficiently large separation between antennas to ensure
radio channel (branch) de-correlation between Tx antennas and Rx
antennas; this would permit 4-branch antenna diversity or up 4
parallel layers of Spatial Multiplexing (assuming 4 antennas at the
terminal). The disadvantage of this configuration is that it
requires four antenna positions, per sector at a base station
site.
[0030] A more practical arrangement to achieve 4-branch antenna
diversity would be to use two Cross-polarized antennas which have
sufficient separation between them. This arrangement reduces the
number of physical antenna positions, per sector at a base station
site to two. With this arrangement or the 4.times. co-polarized
antenna arrangement, it is still possible that many radio channel
matrices may not exhibit full de-correlation and hence non-optimal
Spatial Multiplexing MIMO.
[0031] If the use of two antenna positions is prohibitive and/or
the Tx antenna-Rx antenna radio channel matrix is not de-correlated
sufficiently to sustain 4-branch diversity or 4 parallel spatial
multiplexing then the 4T4R base station can be connected to
4.times. linearly co-polarized antennas with inter antenna
distances of .about..lamda./2 or 2.times. cross polarized arrays
where inter co-polar antenna distances are .about..lamda./2., and
as such result in coherent Spatial Beamforming. These
configurations permit a single, albeit slightly larger, antenna
position to be used at a base station site. The latter
configuration may be considered more advantageous since this
permits polarization diversity (for 2-branch Antenna diversity or
the possibility for 2.times.2 Spatial Multiplexing MIMO) plus
coherent Beamforming function, i.e., across two co-polarized
antennas. In LTE, this is often referred to as "Dual Layer
Beamforming"; that is two spatially multiplexed data layers with
Spatial Beamforming within each layer, and is specified in 3GPP LTE
Release 9 specifications for Downlink (Base Station
Transmission).
[0032] In cellular systems such as LTE, Spatial Beamforming is
exemplified as azimuth plane Beamforming. This uses multiple
passive antennas or antenna arrays arranged in a horizontal
side-by-side formation. Each passive antenna array consists of a
column of multiple antenna elements with .about..lamda./2 spacing
aligned in the vertical plane connected to an RF distribution
network with a fixed radiation pattern; each passive array here is
a simple static Beamformer, with perhaps only a variable tilt
function (semi-static) imposing phase delays at the individual
antenna elements of the array. The Adaptive Beamforming (function)
is applied across the passive antenna arrays, thus having adaptive
Beamforming in the Azimuth plane and non-adaptive static or
semi-static beam in the Elevation plane. The passive antenna arrays
need to have a side-by-side separation distance of <.lamda. to
ensure coherence in spatial radiation pattern Beamforming in
Azimuth.
[0033] The above example describes a 4T4R base station connected to
4.times. antennas, and similar logic can be applied to a 2T4R base
station. In a 2T4R base station, 2.times. duplexed Tx/Rx channels
plus 2.times. Rx only channels are available. This would allow the
Uplink or receive channel to exploit up to 4 parallel Spatial
Multiplexing layers (assuming 4.times. antennas at the terminal),
or 4-branch Diversity, or Beamforming, or a combination as
discussed. The Downlink or transmit channel however could only
exploit two branches. The advantage of using a 2T4R base station,
over a more conventional 2T2R base station is that only two PAs are
required, with only the additional costs of two extra receivers,
which are considerably less expensive than PAs. Moreover, it is
often the Uplink channel which is the limiting link in a cellular
network, more often being power limited, and therefore by having
additional performance gains for Uplink channels through additional
receivers and processing is beneficial. In low dispersion radio
channels, and using a single antenna position with closely spaced
cross-polar arrays provides 2.5-3 dB of Uplink gain over the more
conventional 1T2R or 2T2R base station case and only a 1 dB less
diversity combining gain than had the two cross-polar antennas been
separated by 1 meter. A disadvantage of using two closely spaced
cross-polar antennas is that many antenna systems demanded by
cellular operators need to be multi-band capable. The closely
spaced side-by-side array arrangement as described above achieves a
single antenna position but the array topology does not lend itself
to also incorporating additional antenna arrays designed for
different spectrum bands. For example, a typical operator may wish
for a low-band array (say 790-960 MHz) and a high-band array (say
1710-2170 MHz) in a dual-band antenna. Given the approximate 2:1
ratio between the bands this can be achieved by having dual band
antenna elements placed in a vertical array plus single band
(high-band only) antenna elements placed between the dual-band
antenna elements. This is a common dual-banding technique known in
the industry, which maximizes space, antenna beam tracking and
squint metrics for both bands due to mechanical and electrical
symmetry along the array face. If two side-by-side arrays, say at
high band are used to support a 2T4R or 4T4R base station then it
is difficult to fabricate three side-by-side arrays as
High/Low/High and maintain performance and minimum form factor.
[0034] A base station may use a 4-branch MRC process to perform
optimal combining of the four receive branches at complex baseband.
MRC is one of a number of diversity combining techniques, where
individual receive branches are multiplied by complex weights to
vary amplitude and phase. MRC by definition combines and co-phases
receive branches (performing a vector sum at baseband), associated
with a terminal/specific uplink channel, where each receive branch
is weighted by a factor proportional to the square root of the SNR
of that branch. Noise is assumed to be de-correlated between
branches. When the two cross-polar antennas are side-by-side, then
the radio channel experienced by each pair of co-polarized antennas
is virtually the same at the base station (i.e., highly
correlated), and the MRC process can be considered as performing a
simple coherent and adaptive Beamforming function in Azimuth for
each of the two orthogonal polarizations in the Uplink channel.
[0035] Additionally, it should be noted that Diversity combining,
including MRC implementations can take many forms, as discussed
earlier. MRC in its strictest definition performs optimal combining
in a noise limited radio channel. When co-channel interference is
also correlated then MRC can also be combined with interference
rejection algorithms such as IRC, and MMSE algorithms. It is beyond
the scope of this patent application to detail all receive
combining algorithms. However it should be carefully noted that MRC
can be implemented in the receiver pre-detection or in the receiver
post-detection stages. In post-detection implementations of MRC,
phase information from each receive branch is lost, since the MRC
system is designed to track the envelope of a multipath fading
signal on each branch, using this as the basis for combining, and
hence the MRC process is simply applying weights then combining. In
pre-detection MRC implementations, phase information is preserved
and hence performing co-phasing, weighting and combining, i.e.
performing a vector summation.
[0036] Uplink Beamforming in the Azimuth plane with Polarization
diversity using a 4-branch receiver diversity combining process
with conventional base station antenna arrays can be done by
configuring two passive cross-polar antenna arrays in a
side-by-side arrangement, as discussed. However, it is not readily
possible to create Uplink Beamforming in the same manner for the
Elevation plane using a 4-branch diversity combining process. One
might consider the case of arranging two cross-polar passive
antenna arrays in the vertical plane (where one passive antenna
array is placed on top of the other passive antenna array).
However, for practical directivity gains for a cellular base
station antenna, a passive base station antenna array may consist
of many vertically stacked antenna elements, typically 5-14
elements, depending upon the spectrum band of operation and desired
directivity and vertical elevation pattern beamwidth. Therefore,
using two passive arrays vertically separated will result in the
phase centers of each passive array being multiple wavelengths
apart, and thus would result in a non-coherent Beamforming
operation, and as such only vertical space diversity could be
achieved. Also, it is understood that vertical space diversity
techniques invariably lead to branch imbalance between the two
cross-polar antenna arrays due to the fact the higher antenna array
has a more favorable radio propagation channel to/from mobile
terminals; branch imbalance results in sub-optimal diversity
combining.
[0037] Given the cellular tessellation geometry of cells in a
cellular network, and for certain radio access technologies,
Elevation Beamforming may be more advantageous than Azimuthal
Beamforming in terms of resulting network performance benefits.
Many cellular access technologies, such as CDMA, WCDMA/UMTS/HSPA+,
LTE/LTE-Advanced, WiMax, and even GSM/GPRS/EDGE with aggressive
fractional frequency hopping can use aggressive spectral re-use
between sectors of the same cell site, and between cell sites. It
should be noted that full spectral re-use is employed resulting in
a 1:1 re-use factor, and that the network can be considered as
interference limited rather than noise limited. Given the cell
tessellation geometry of a cellular network in Azimuth and
Elevation planes co-channel inter-cell site interference in a Macro
cellular network would tend to consistently come from a narrow
range of elevation angles around the horizon, assuming a reasonably
flat plane with which the cellular network operates. For systems
such as WCDMA, interference can be considered from both intra-cell
interference and inter-cell interference, the former being a
function of the channel dispersion. However, in LTE and GSM based
systems, it can be shown that interference is predominately
inter-cell interference. Also, for LTE and GSM systems Co-channel
Uplink interference in the Azimuth plane may be much more variable
as a function of azimuth angle, dependent upon where adjacent cell
sites and respective served mobile terminals are positioned
relative to a base station. When using two cross-polar passive
antenna arrays in a side-by-side arrangement as described
previously with a 4-branch receive combining process then this
essentially has two adaptive Beamforming freedoms per orthogonal
polarization. With two freedoms of Beamforming it is only possible
to Beamform in Azimuth, creating an Azimuthal radiation pattern
with up to two main-lobes but only one null. If co-channel
interference in a cellular network comes from multiple azimuthal
directions then Azimuthal Beamforming to suppress interference in
this manner may not be optimal. An optimal Beamformer in Azimuth
may need to create multiple nulls, which requires additional
Beamforming freedoms and hence additional antenna arrays and a
higher-order receiver branch combining process. This becomes
prohibitive (due to larger total antenna array aperture size), and
the fact most base stations are not readily equipped with beyond
4-branch receive combining processes.
[0038] One means of achieving a per-terminal orientated adaptive
Beamforming or adaptive tilting function in the elevation plane
from a vertically aligned antenna array is the use of a fully
active antenna. In an active antenna it is possible to have control
of phase and amplitude at complex baseband for each active user
(Uplink and/or Downlink) at each antenna element in the array. This
of course requires a PA, LNA, Duplexer arrangement, Up/Down
conversion at each antenna element. For practical directivity gains
for a cellular base station antenna, this may consist of many
elements, typically 5-14 elements, depending upon the spectrum band
of operation and desired directivity and resulting vertical
elevation pattern beamwidth. The disadvantage of an active array is
that multiple active electronics must be employed together with
high-order branch receive combining (Uplink Beamforming) algorithms
to impart the required Beamforming. However, active antennas using
high-order Beamforming may be very well suited for highly
dispersive radio channels where the angle of arrival of multi-path
components may extend over a wide range of angles, such as may be
typical of micro-cellular environments where multi-path scatterers
are relatively close to and surround (in Azimuth and Elevation
terms) the antenna.
[0039] A base station antenna array arrangement may comprise the
use of an antenna distribution network suitable for use in a
cellular communications network. The distribution network can drive
(in transmit mode) or be driven from (in receive mode) a plurality
of antenna elements which forms a coherent main beam in the
elevation plane suitable for cellular applications; the
distribution network or vector network combiner is fed
(transmit)/feeds (receive) from/to a differentially phased signal
input/output, where the angle of main beam elevation radiation
pattern tilt is a function of the phase difference of the
differential signal.
[0040] A base station antenna array arrangement may also use a
Butler Matrix for use in a cellular communications network. The
distribution network (Butler Network) can drive (in transmit mode)
a plurality of antenna elements which forms a main beam radiation
pattern in the elevation plane suitable for cellular applications;
the distribution network is fed (transmit) from a signal pair which
is a differential or complimentary power signal, connected to two
ports of a 4.times.4 Butler network which would form orthogonal
spatial beams if driven in isolation. Furthermore, phase to power
conversion can be achieved through the use of a simple hybrid
combiner connected to the complimentary power input pair, and thus
achieve a mainbeam variable tilt operation in response to the phase
difference of the input signal to the Hybrid combiner.
[0041] Similarly, a phase to power conversion operation could also
be achieved by the use of a Hybrid combiner connected to the
differential phase input ports of the antenna distribution
networks, in order to vary a mainbeam in elevation (tilt) as a
function of the power ratio signal input to the Hybrid
combiner.
[0042] In one embodiment, the objective of the current invention is
to deliver an adaptive spatially coherent Beamforming function in
the elevation plane, at a "per terminal or per user resolution"
using a completely passive antenna array connected to a simple base
station MRC or similar receive combining process for the Uplink
channels. The present invention also discloses the same arrangement
for use with Downlink channels to deliver adaptive elevation
spatial Beamforming using a simple 2-branch (2-freedom) Beamforming
algorithm, which adapts phase difference or power difference
between the 2 branches.
[0043] In particular, the present invention discloses the use of
passive antenna arrays which employ passive distribution networks
which are designed to vary mainbeam radiation elevation pattern
(tilt) as a function of differential power (power ratio) and/or
differential phase at the input ports to the antenna distribution
network (transmit mode), or at the output from the antenna
distribution network (when considering a receive mode).
[0044] By connecting such antenna distribution network or antenna
Beamforming network schemes to a base station using 4-branch
receive diversity combining (such as 4-branch MRC) or generically a
4-branch receive Beamformer algorithm, it is possible to achieve
per user or per terminal uplink Beamforming or tilting operation
for two orthogonally polarized antenna arrays. One embodiment of
the current invention depicts a 4-branch base station receiver
connected to a Cross-Polar antenna array, where each antenna array
uses the differential phase (or power) processing method. The
choice of whether to use an antenna system capable of variable tilt
in response to differential phase or power may be dependent upon
what particular Uplink diversity combining, Uplink Beamforming
algorithm and Downlink Beamforming algorithm (if any) is used in
the base station. If a 4-branch Receive diversity scheme is
available as would be the case for a 2T4R or 4T4R base station,
which employs a pre-detection MRC implementation then to enable
Uplink Beamforming, either antenna Beamforming network method
(phase or power processing) may be used since the MRC will be able
to process phase and amplitude for each active uplink channel at
complex baseband. On the other hand, if post-detection MRC is
employed and hence the MRC scheme is designed for envelope
tracking, and as such phase information is lost prior to MRC
combining, an antenna Beamforming network based on processing
differential power may be more appropriate.
[0045] In one embodiment, the combining process at the receiver can
take many forms such as the popular process of MRC. This combining
process effectively performs a vector sum of the receive signals
(branches) at complex baseband (i.e., after down-conversion and
de-modulation from RF); the vector sum is carried out in such a
manner as to weight each receive signal in accordance to the
instantaneous signal quality or signal strength, and to also apply
phase delay (co-phasing action) to each complex weighted receive
signal branch such that coherent combining can be achieved. Known
reference signals from the transmitter are transmitted at known and
periodic intervals; this allows the receiver to perform a radio
"channel estimation" to which signal strength, signal quality
measurements of the receive signals can be taken including
measurements of phase differences between the receive signals,
which in turn are used for determining the required complex weights
in the combining process. This receive combining process can be
applied to multiple and simultaneous radio links at the base
station, by taking multiple copies of the complex baseband receive
signals and processing as described above independently for each
active independent mobile terminal to base station radio
Uplink.
[0046] For Downlink Elevation Spatial Beamforming, it is expected
that generating differential phase between information signals
which are then power amplified may be a more suitable arrangement
than generating power differences between information signals.
Therefore, an antenna beamformer distribution network method which
can vary tilt in response to differential phase is preferred. The
reason why Downlink Beamforming may be more advantageous using
differential phasing is simply the fact that a differential power
scheme may require large swings in available power, and in the
limit (assuming all information signals are Beamformed to one tilt
angle) demand full power from one PA and no power from the other;
this clearly requiring larger power PAs, or headroom in the PAs. It
is also anticipated in the current invention that phase and power
processing methods be also used for the basis of Elevation
Beamforming for different duplex links, e.g., the differential
power method for the Uplink Elevation Beamforming and differential
Phase for the Downlink Beamforming. For example, another embodiment
of the current invention takes a Tx/Rx line and an Rx only line
(e.g., two of the four lines from a 2T4R base station) and connects
these to the two inputs of a 180.degree. Hybrid combiner; the two
outputs of the 180.degree. Hybrid combiner being then connected to
an antenna system driving an array of antenna elements where the
elevation pattern tilt angle is a function of the phase difference.
In this particular embodiment, the single Tx line is split with
equal power and phase via the Hybrid combiner and thus drives the
antenna beamformer network creating a directive radiation beam in
elevation with a certain angle of Downlink tilt. A variable RF
phase shifter can also be inserted on one of the outputs of the
Hybrid combiner and before connection to one of the two ports of
the antenna beamforming network to enable a variable downlink tilt
function. Now considering the Receive side of this embodiment, the
differential phase antenna beamformer will receive information on
the Uplink from a terminal and this Uplink receive signal will
emerge as a pair of signals with a differential phasing at the
ports of the beamforming network, depending upon the angle of
arrival incident upon the array. As these receive signals pass
through the 180.degree. Hybrid Combiner in receive mode, any phase
difference in the Uplink receive signals is translated into a
complementary power difference, which can then be processed by a
base station using a pre or post detection MRC diversity combining,
or similar scheme.
[0047] Various embodiments are depicted depending upon whether the
base station is capable of processing phase difference or power
difference for the information signals carried on the Uplink and
Downlink channels. The motivation for the present invention
exploits the fact that users in a cell tend to occupy different
distances from the cell site (cell centre) up to the cell boundary;
by being able to direct a spatial elevation pattern at/from each
active terminal then this reduces the required Uplink power a
terminal needs to communicate back to a base station. A reduction
in Uplink powers means reduced inter-cell co-channel interference,
which in turn permits improved C/I geometry across the cellular
network. Moreover, inter-cell co-channel interference in an LTE
network for example tends to arrive (considering the Uplink
channel) from a narrow range of elevation angles at around, or just
below the horizon. Since a tilt per terminal action is being
created with the current invention, the elevation pattern being
directed to/from a particular user is also receiving less
inter-cell interference by virtue of the elevation pattern being,
on average tilted down with respect to a more conventional static
elevation pattern. In an extreme case, perhaps with a base station
using a receiver combining algorithm based on minimum C/I such as
IRC or MMSE with 4-branches could result in the elevation radiation
pattern first upper null being effectively directed toward the
adjacent cell(s) to maximize C/I statistics. Given the elevation
geometry in a macro cell site where co-channel interference
dominates from typically one bearing (near to the Horizon) and a
wanted Uplink signal is at another bearing (consider the per
terminal case), then an antenna array beamformer with only
2-freedoms may be sufficient, i.e., the beamformer has control of
phase and/or amplitude for only two inputs thus only able to
control the bearing of N-1 nulls (i.e., 1 null). This achieves
spatial filtering (in angular space) of inter-cell interference in
the elevation plane.
[0048] Another viewpoint by which the current invention can be
considered is angle diversity or pattern diversity in the Uplink.
Consider a 4.times.4 Butler matrix network where the 4.times.
outputs connect to 8.times. antenna elements in element pairs,
arranged in the vertical plane as a column. Two of the four inputs
of the Butler matrix are terminated, and the remaining two input
ports of the Butler matrix are associated with adjacent orthogonal
beams. If power is applied to one of the non-terminated ports and
no power to the other non-terminated port, a beam is generated in a
particular direction in elevation; if the power is swapped over
then an orthogonal beam in generated in a different direction; if
power is varied between the two ports then a composite beam of
varying elevation tilt (direction) is produced. Likewise, in
receive mode for the Uplink channel, the reciprocal will occur; if
a plane wave is incident upon the array then power emerges from the
two ports of the Butler matrix, and emerges as a power ratio
depending upon the angle of incidence. The advantage of a Butler
matrix is that two orthogonal beams can co-exist with full array
length gain from the same physical array and also be isolated from
one another. A similar argument can be constructed for the antenna
beamforming methods based on the vector combining of differentially
phased signals, when a Hybrid combiner is connected directly to the
beamformer ports to convert phase differential to power
differential. By considering these two orthogonal beam in the
elevation plane as receive diversity path with an MRC process, then
because the MRC process applies different complex weights prior to
summing the receive then this in effect can be viewed as an
adaptive Uplink tilt action (tilting between the two orthogonal
beams) with the property of the elevation pattern having one strong
null and one directive beam. It is fully expected that cells where
there may be a high concentration of traffic close to the cell
centre may find most benefit from the current invention. In such a
scenario, one might visualize that a significant proportion of
traffic is communicating predominately via the orthogonal beam with
greatest tilt (and greatest nulling or rejection to adjacent cells
in the elevation plane).
[0049] The current invention may also be most appropriate in
macro-cellular environments, where the base station antenna is at a
height generally greater than the surrounding rooftops or
multi-path scatterers. As stated earlier, macro cell sites tend to
experience low angular Multi-Path dispersion or angular spread in
elevation terms. The angular dispersion in elevation is also
expected to decrease with increasing distance from the cell centre,
assuming a similar scattering volume or environment is maintained
around the terminal. In the limit, the angular dispersion of
multipath arriving at or near to the horizon angle is expected to
be relatively narrow; this is likely to be the dominated by any
co-channel interference from adjacent and distant cells. The
presence of an elevation pattern null around these angles is
beneficial to suppress such inter-cell interference.
[0050] Whilst the above description focuses upon adaptive antenna
Beamforming for the Uplink channel in a cellular communications
system, similar benefits and arguments can be made for adaptive
Spatial Beamforming on the Downlink channel too, when 4.times.
transmit ports are available from a base station, commonly known as
4.times.Tx/Rx or 4T4R. Cellular access technologies such as HSPA+
and LTE can support various antenna configurations from a 4T4R base
station. For example, two closely spaced cross-polar antennas
connected to a 4T4R base station may deliver Uplink and Downlink
adaptive Beamforming in Azimuth plus 2-way (de-correlated) receiver
diversity for Uplink, and 2-way transmission diversity or 2.times.2
Spatial Multiplexing for Downlink.
[0051] The embodiments of the present invention consider a mobile
terminal T1 which would be located near to the ground and typically
surrounded by local scatterers in the radio channel communicates
via the Uplink communications channel to the base station via
antenna assembly (100). It is assumed that the base station antenna
is sufficiently elevated above the local buildings and hence
elevated above the scattering environment and as such we can assume
that the RF signal from T1 arrives at the base station antenna
(100) via scattering which subtends a relatively narrow range of
elevation angles, and we make the assumption that this angular
dispersion is typically similar to, or less than the vertical
beamwidth of the antenna array. The radio channel geometry of the
radio link is illustrated in FIG. 3.
[0052] FIG. 3 illustrates the multi-path radio channel in elevation
terms. Multi-path dispersion exists in both Azimuth and Elevation
planes, and therefore FIG. 3 depicts the resolved multi-path
components in elevation only for the purposes of this discussion.
Five dominant multi-path components are shown 1-5. Multi-path
component 1 represents a direct path, although not a line of sight
component, it travels through one building (110(a) of a plurality
of buildings 110(a)-100(f) between base station antenna and
terminal. Multi-path component 2 is a similar direct path but
includes a ground reflection near to the terminal 120. Multi-path
component 3 is the strongest multi-path component and arrives via a
reflection from the side of a nearby building 110(c) which is
between base station antenna and terminal. Multi-path component 4
is a diffractive path via the rooftop of the tallest building
110(a), and Multi-path component 5 has the largest temporal
dispersion via a reflection from another building 110(d), which is
not between the base station antenna 100 and terminal T.sub.1. All
these Multi-path components subtend the beamwidth of the base
station antenna 100, and as such it is not possible to combine
independent multi-paths (radio channel equalization in the
elevation plane) as this would warrant a much larger (longer) and
hence impractical base station antenna array to create very narrow
beams to resolve each multi-path component.
[0053] The temporal dispersion in the radio channel can be
equalized using known techniques at the base station, including
adaptive channel equalization. For communication links where the
information bandwidth is less than the coherence bandwidth of the
radio channel, i.e., multi-path delay is much less than the
information symbol period then the radio channel is considered
narrowband and all multi-path echoes fall comfortably within the
symbol period and minimal Inter Symbol Interference (ISI) results;
such access technologies might include GSM or LTE when considering
most radio environments. When the multi-path delay profile is
greater than the symbol duration, the radio channel is considered
wideband and then RAKE receiver architectures for example can be
employed to equalize the radio channel, as used in access
technologies such as CDMA and WCDMA/U MTS.
[0054] It should be noted that various embodiments of a base
station system are disclosed below. In describing various stages or
modules of the base station system, various methods of signal
processing will also be described. As such, the steps of these
various methods will be described in view of FIGS. 4-8. Thus, the
steps of these methods are supported by FIGS. 4-8 as flowcharts for
these methods. It should be noted that although only 2T4R base
stations are shown, 4T4R base stations are based on similar
architecture as discussed above and can be implemented, e.g., where
a second parallel base station port connection arrangement is
present for connection to a second antenna assembly which may be of
an orthogonal polarization to the first antenna assembly and where
the receiver diversity combining process has four branches (or even
greater than 4 branches) rather than two branches.
4.1 Embodiment 1
[0055] FIG. 4 depicts Uplink Elevation Beamforming using a 2T4R
base station with MRC receive combining process with a passive
antenna array based on variable tilt in response to differential
phase across VA & VB Ports. A first embodiment of the current
invention is illustrated in FIG. 4, using an antenna assembly based
upon an adaptive beamforming network (40) designed for processing
differential phased signals at the ports of the beamforming network
(VA and VB). Such a beamforming network (40) suitable for operation
includes those based upon U.S. Pat. No. 7,450,066, U.S. Pat. No.
7,400,296, or U.S. Pat. No. 7,420,507. The RF signal from the
mobile terminal T1 arrives at the Base Station Antenna Assembly
(100), and according to FIG. 3. The antenna assembly comprises of
an antenna array of N antenna elements (62.sub.1 to 62.sub.N) which
are typically spaced apart, e.g., by 0.7.lamda. and .lamda.. Each
antenna element may be connected to phase shifters as shown
(64.sub.1-64.sub.1N), or indeed (although not shown) groups of
antenna elements may be connected to phase shifters. The purpose of
the phase shifters (64.sub.1-64.sub.1N) is to provide the facility
to move the mid-value beam tilt angle of the adaptive beamforming
range around. The beamforming range is the range of coherent beams
which can be produced over a range of phase differences across the
VA and VB ports where the beam maintains some minimum
specifications such as directivity, gain, vertical beamwidth,
sidelobe levels, etc. This is shown as beam 10A (minimum tilt
angle), 10C (maximum tilt angle) and 10B (mid-tilt angle). Assuming
an Uplink channel, the RF energy arriving at the antenna array is
processed via the Beamforming Network (40) which results in signals
at the ports VA and VB of the Beamforming Network. The signals,
associated with terminal T1 at VA and VB will be substantially of
equal amplitude, but will have a phase differential between; the
phase difference being a function of the angle of arrival in
elevation, assuming that the angle of arrival falls within the
valid Beamforming range of the Beamforming network. The signals at
VA and VB are connected to duplexing filters, 47A and 47B,
respectively, which are designed to accommodate a Downlink signal
for the antenna array, which will be discussed later in this
embodiment. Broadly, the method has created a spatially coherent
main beam with a variable elevation plane beam tilt in response to
differentially phased signals or differentially amplitude signals
across two ports of a beamforming network of a first passive RF
antenna assembly.
[0056] Considering only the Uplink signals from VA and VB, these
are essentially passed directly to the base station; the first
processing stage of a base station being the RF to Baseband
Down-conversion stages (20). FIG. 4 depicts a simple schematic of
an RF to Baseband processing stage for the purposes of this
embodiment, but it should be noted that such processing stage can
take many other forms. The two differentially phased Uplink signals
are down-converted to digital complex baseband; signal VA emerges
as b1.sub.1 at complex baseband and signal VB emerges as b2.sub.1
at complex baseband. Complex baseband signal b1.sub.1 for example
will comprise of a real and imaginary baseband signal components.
Broadly the method processes the differentially phased signals or
differentially amplitude weighted signals into baseband signals via
a RF to baseband down conversion stage coupled to the first passive
RF antenna assembly. Please note: for clarity, FIG. 4 does not
depict complex baseband signals with separate real and imaginary
signals, instead shown as a single complex signal line. It can be
shown that the RF phase difference between VA and VB ports will
emerge as a baseband phase shift between b1.sub.1 and b1.sub.2,
according to digital beamforming theory previously discussed. The
two complex baseband signals are then processed by a Maximal Ratio
Combining (MRC) stage or other similar receive combining process
(12.sub.1) stage. Broadly, the method performs a vector summation
on the baseband signals via a receiver combining stage. The MRC
process (an algorithm e.g., implemented in a digital signal
processor (DSP) or simply a hardware processor) will aim to combine
the two baseband signals b1.sub.1 and b1.sub.2 in a coherent
fashion in order to maximize signal quality. The MRC process is
able to multiply each complex baseband signal by a complex weight,
so signal b1.sub.1 is multiplied by w1.sub.1 and b1.sub.2 is
multiplied by w1.sub.2 and then perform a complex summation of the
weighted complex baseband signals. Ideally the MRC process will
essentially co-phase the two complex baseband signals prior to
summation; this would be achieved by multiplying one of the complex
baseband signals by a complex weight with a value which has a phase
difference equal to and opposite to the phase difference between
the two complex baseband signals. The output from the MRC process
(12.sub.1) results in an optimally combined complex baseband signal
O1 which is transferred to higher abstraction layers within the
base station for further processing for extraction of the
information.
[0057] A base station clearly needs to communicate to and from
multiple mobile terminals at the same time, and for this FIG. 4
depicts multiple logically independent MRC processes for k Uplink
channels (12.sub.1 to 12.sub.k), producing multiple outputs
(O.sub.1 to O.sub.k) for higher layer processing within the base
station. Each Uplink channel associated with different terminals
may be communicating with the base station at the same time.
Different down-conversion and MRC architectures are employed with
different radio access schemes. For example, in GSM which employs
Time Division Multiple Access (TDMA) then only a single MRC process
is only required, changing its complex weights every time slot to
process different uplink users. In WCDMA/UMTS which employs Code
Division Multiple Access (CDMA) then the complex baseband signal
emerging after down-conversion could be replicated multiple times
and each copy then complex multiplied by a channelization code to
extract the specific data associated with different terminals and
then multiple, independent concurrent MRC processes are used, one
for each terminal. In LTE which uses Frequency Division Multiple
Access (FDMA) may require the down-conversion to be performed in
two stages; a first stage resolving down to say a 10 MHz channel
then a second down-conversion to resolve individual FDMA channels
within the 10 MHz channel, or use of FFT processing techniques. The
scope of the present invention is not to stipulate any specific
down-conversion techniques or receive branch combining techniques,
but to depict that logically there will be extraction of
independent information associated with different terminals down to
a complex baseband level.
[0058] The RF signals at VA and VB, and the corresponding
down-converted complex baseband signals b1.sub.1 and b1.sub.2 are
expected to be highly correlated in that they will fade together in
temporal terms as a mobile terminal moves around, so the complex
weights are expected to vary very slowly, and only changing in
response to perhaps the mobile terminal moving around its
environment such that its RF energy arrives at different angles,
e.g., a mobile terminal moving away from a cell centre to cell edge
radially. The above describes the scope of the present invention,
where a MRC process is used to perform co-phasing of a pair of
signals, whose phase difference is a function of the angle of
arrival in elevation terms.
[0059] FIG. 4 depicts a generic M-branch MRC process. In practice,
this may be a 4-branch MRC process to which the remaining two
branches can be connected to the same antenna, and down-conversion
arrangement for an orthogonal polarized antenna array, for example.
This is not shown for clarity. The second antenna array is designed
to create de-correlation to the first antenna array, and as such
provide conventional receive diversity. In essence, FIG. 4 with a
4-branch MRC process would depict conventional 2-way de-correlated
diversity combining plus adaptive elevation beamforming
(tilting).
[0060] The Downlink channel is shown only as a single Downlink
channel in FIG. 4, and therefore assuming two antenna arrays, this
could represent up to two Downlink channels in a 2T4R base station
configuration. The Downlink signal can also be transmitted from the
same antenna assembly. In FIG. 4, the Downlink signal is duplexed
onto one of the feed lines between base station and antenna
assembly, and at the antenna assembly the duplexed signal is
un-duplexed using duplexing filter 43, whereupon the Downlink
signal is processed independently of the Uplink channel; a splitter
44 divides the Downlink signal into two branches, and one branch is
connected to a variable phase shifter to create a phase difference
between the two Downlink signals, which are then re-combined with
the Uplink signals at duplexing filters 47A and 47B prior to
connection to the VA and VB ports of the Beamforming Network. The
Downlink signal will be processed by the beamforming network in
accordance to the phase difference of the Downlink signal at the VA
and VB ports, resulting in a coherent beam with variable beam tilt.
An operation of FIG. 4 would adjust the Downlink tilt to be set at
the minimum tilt angle of the Beamforming array, for example assume
this is 2.degree. tilt. The Uplink "per terminal" adaptive
Beamforming range (adaptive because of the MRC process) may produce
coherent beams for independent Uplink channels ranging from
2.degree. to say 8.degree.. In this case we are ensuring that
minimum inter-cell Uplink interference occurs between cell sites.
However, another operation might be to set the downlink tilt at say
5.degree. tilt (middle of the Beamforming range of the Beamforming
network). In this case the Uplink adaptive Beamforming range
remains as 2.degree. to 8.degree. and as such allows Uplink
channels to achieve an improved link budget at the cell edges. If
different Beamforming ranges are required, then the phase shifters
(64.sub.1-64.sub.1N) allow the Beamforming range to be moved by
imposing a bulk phase slope across the array face of the antenna.
For example, it would be possible to create a Beamforming range of
5.degree. to 11.degree..
4.2 Embodiment 2
[0061] FIG. 5 depicts Uplink Elevation Beamforming using base
station MRC receive combining process with a passive antenna array
based on variable tilt in response to differential amplitude across
VA & VB Ports. A second embodiment of the current invention is
illustrated in FIG. 5, and is virtually identical to the first
embodiment but using an antenna assembly based upon an adaptive
beamforming network (50) designed for processing differential
amplitude signals at the ports of the beamforming network (VA and
VB). Such a beamforming network (50) suitable for operation
includes that based upon U.S. Pat. No. 6,864,837B. Assuming an
Uplink channel, the RF energy arriving at the antenna array is
processed via the beamforming Network (50) which results in signals
at the ports VA and VB of the beamforming Network. The signals,
associated with terminal T.sub.1 at VA and VB will be substantially
of equal phase, but will have an amplitude differential; the phase
difference being a function of the angle of arrival in elevation,
assuming that the angle of arrival falls within the valid
Beamforming range of the beamforming network. The signals at VA and
VB are connected to Duplexing filters, 47A and 47B, respectively,
which are designed to accommodate a Downlink signal for the antenna
array, which will be discussed later in this embodiment.
[0062] Considering only the Uplink signals from VA and VB, these
are essentially passed directly to the base station; the first
processing stage of a base station being the RF to Baseband
Down-conversion stages (20), and similar to that already described
for the first embodiment. The two differential amplitude Uplink
signals are down-converted to digital complex baseband; signal VA
emerges as b1.sub.1 at complex baseband and signal VB emerges as
b2.sub.1 at complex baseband. Complex baseband signal b1.sub.1 for
example will consist of a real and imaginary baseband signal
components. It can be shown that the RF amplitude difference
between VA and VB ports will emerge as a baseband amplitude
difference between b1.sub.1 and b1.sub.2, according to digital
Beamforming theory previously discussed. The two complex baseband
signals are then processed by a Maximal Ratio Combining (MRC) or
other similar receive combining process (121). The MRC process
(e.g., an algorithm deployed in a DSP) will aim to combine the two
baseband signals b1.sub.1 and b1.sub.2 in such a manner as to
maximize signal quality. The MRC process is able to multiply each
complex baseband signal by a complex weight, so signal b1.sub.1 is
multiplied by w1.sub.1 and b1.sub.2 is multiplied by w1.sub.2 and
then perform a complex summation of the weighted complex baseband
signals. Ideally the MRC process will essentially multiply each
complex baseband signal according to the S/N of each signal prior
to summation. There ought to be no phase difference between VA and
VB, and hence no phase difference at complex baseband between
b1.sub.1 and b1.sub.2 and as such no co-phasing would be necessary.
Therefore, this embodiment might be more suitable for MRC
processing based on post-detection MRC architectures since phase
information would be lost in post-detection schemes.
[0063] The RF signals at VA and VB, and the corresponding
down-converted complex baseband signals b1.sub.1 and b1.sub.2 are
expected to be highly correlated in that they will fade together in
temporal terms as a mobile terminal moves around, so the complex
weights are expected to vary very slowly, and only changing in
response to perhaps the mobile terminal moving around its
environment such that its RF energy arrives at different angles,
e.g., a mobile terminal moving away from a cell centre to cell edge
radially. The above describes the scope of the present invention,
where a simple MRC process is used to perform co-weighting of a
pair of signals, whose amplitude difference is a function of the
angle of arrival in elevation terms.
[0064] FIG. 5 depicts a generic M-branch MRC process. In practice,
this may be a 4-branch MRC process to which the remaining two
branches can be connected to the same antenna, and down-conversion
arrangement for an orthogonal polarized antenna array, for example.
This is not shown for clarity. The second antenna array is designed
to create de-correlation to the first antenna array, and as such
provide conventional receive diversity. In essence, FIG. 5 with a
4-branch MRC process would depict conventional 2-way de-correlated
diversity combining plus adaptive elevation beamforming
(tilting).
[0065] The Downlink channel is shown only as a single Downlink
channel in FIG. 5, and therefore assuming two antenna arrays, this
could represent up to two Downlink channels in a 1T4R or 2T4R base
station configuration. The Downlink signal can also be transmitted
from the same antenna assembly. In FIG. 5, the Downlink signal is
duplexed onto one of the feed lines between base station and
antenna assembly, and at the antenna assembly the duplexed signal
is un-duplexed using duplexing filter 43, whereupon the Downlink
signal is processed independently of the Uplink channel; a splitter
(44) divides the Downlink signal into two branches, and one branch
is connected to a variable phase shifter (46) to create a phase
difference between the two Downlink signal branches, which are then
connected to the input ports of a 180.degree. Hybrid Coupler (45);
the outputs of the Hybrid coupler (45) being re-combined with the
Uplink signals at duplexing filters 47A and 47B prior to connection
to the VA and VB ports of the Beamforming Network (50). The Hybrid
Coupler (45) converts phase difference to a complimentary power
difference, to which the Beamformer network (50) can create
different beams (tilts). The Downlink signal will be processed by
the Beamforming network in accordance to the amplitude difference
(created by the phase difference imposed by phase shifter (46)) of
the Downlink signals at the VA and VB ports, resulting in a
coherent beam with variable beam tilt. An operation of FIG. 5 would
adjust the Downlink tilt to be set at the minimum tilt angle of the
beamforming array, for example assume this is 2.degree. tilt. The
Uplink "per terminal" adaptive Beamforming range (adaptive because
of the MRC process) may produce coherent beams for independent
Uplink channels ranging from 2.degree. to say 8.degree.. In this
case we are ensuring that minimum inter-cell Uplink interference
occurs between cell sites. However, another operation might be to
set the downlink tilt at say 5.degree. tilt (middle of the
beamforming range of the Beamforming network). In this case the
Uplink adaptive beamforming range remains as 2.degree. to 8.degree.
and as such allows Uplink channels to achieve an improved link
budget at the cell edges. If different beamforming ranges are
required, then the phase shifters (64.sub.1-64.sub.1N) allow the
beamforming range to be moved by imposing a bulk phase slope across
the array face of the antenna. For example, it would be possible to
create a beamforming range of 5.degree. to 11.degree..
4.3 Embodiment 3
[0066] FIG. 6 depicts Uplink Elevation Beamforming using 2T4R base
station with MRC receive combining process with a passive antenna
array based on variable tilt in response to differential phase
across VA & VB Ports. A third embodiment of the current
invention is illustrated in FIG. 6, and is virtually identical to
the first embodiment using an antenna assembly based upon an
adaptive beamforming network (40) designed for processing
differentially phased signals at the ports of the beamforming
network (VA and VB). Such a beamforming network (40) suitable for
operation includes that based upon U.S. Pat. No. 7,450,066, U.S.
Pat. No. 7,400,296, or U.S. Pat. No. 7,420,507. The difference
between the first and third embodiments is simply how the Downlink
signal is applied to the Beamforming Network (40). In FIG. 6, the
base station feed line which carries Downlink and Uplink channels
is applied to one input port of an 180.degree. Hybrid coupler (45).
This splits the Downlink signal into two equal and in-phase power
branches at the outputs of the Hybrid coupler (45). One output from
the Hybrid Coupler is connected to a variable RF phase shifter (46)
to impose a phase delay. This results in a differentially phased
Downlink signal which is applied to the VA and VB ports of the
beamforming network (40). However, the Uplink signal is processed
slightly differently to the first embodiment, because of the
presence of the Hybrid Coupler (45). As per the first embodiment,
assuming an Uplink channel, the RF energy arriving at the antenna
array is processed via the beamforming Network (40) which results
in signals at the ports VA and VB of the beamforming Network. The
signals, associated with terminal T.sub.1 at VA and VB will be
substantially of equal amplitude, but will have a phase
differential; the phase difference being a function of the angle of
arrival in elevation, assuming that the angle of arrival falls
within the valid beamforming range of the beamforming network.
Considering the Uplink signals at VA and VB, these are connected to
the Hybrid Coupler (45) ports 452 and 454 via RF phase shifter
(46), which will convert phase difference between the signal at
ports 452 and 454 into a signal pair at ports 451 and 453 as a
complimentary power difference, the power difference being a
function of the phase difference. Therefore the base station RF to
Baseband down-conversion and MRC processing treat the Uplink signal
as a complimentary or a differential amplitude pair, as per the
second embodiment. By converting a differentially phased Uplink
signal at ports VA and VB into differential amplitude of the Uplink
signal, the MRC process can be based on pre or post detection
implementations. A further advantage of this embodiment over the
first embodiment is that duplexing filters can be removed being
replaced by a simple Hybrid Coupler.
4.4 Embodiment 4
[0067] FIG. 7 depicts Uplink Elevation Beamforming using 2T4R base
station with MRC receive combining process with a passive antenna
array based on variable tilt in response to differential amplitude
across VA & VB Ports. A fourth embodiment of the current
invention is illustrated in FIG. 7, and is virtually identical to
the second embodiment using an antenna assembly based upon an
adaptive beamforming network (50) designed for processing
differential amplitude signals at the ports of the beamforming
network (VA and VB). Such a beamforming network (50) suitable for
operation includes that based upon U.S. Pat. No. 6,864,837B. The
difference between the second and fourth embodiments is simply how
the Downlink signal is applied to the Beamforming Network (50). In
FIG. 7, the base station feed line which carries Downlink and
Uplink channels is applied directly to one port of the beamforming
network, in this case port VA.
[0068] In this fourth embodiment, the entire Downlink signal is
applied to port VA and no Downlink power is applied to port VB.
This results in a fixed Downlink beam at minimum tilt or
Beamforming range producing a beam 10A to carry the Downlink
channel. The Uplink channel is processed exactly the same as the
second embodiment, where the Uplink signal from a terminal results
in differential power at ports VA and VB, which are directly
connected to the base station, down-converted to complex baseband,
and processed in the MRC receiver combining process. Again, an
advantage of this embodiment is that a post detection MRC scheme
can also be used, since only amplitude difference is processed (not
phase difference) with a post detection MRC scheme. The key
advantage of this fourth embodiment is that it does not require any
duplexing filters or additional Hybrid Couplers, but the
disadvantage being that the Downlink channel cannot be varied
within the range of the beamforming tilt range.
[0069] It would however be possible to derive many further
embodiments using the building blocks of Hybrid Couplers, an RF
Phase Shifter, Duplexing filters, etc. to arrive at similar
arrangements. For example, the schematic in FIG. 7 could include a
Hybrid Coupler and phase shifter applied between the base station
and the Beamforming Network (similar to that shown in embodiment
3). In this case it would be possible to vary Downlink beam tilt,
but would require pre-detection MRC to affect Uplink channel
Beamforming. Additionally, the above embodiments have assumed that
the access technology is based upon Frequency Division Duplexing
(FDD). To those versed in the art it is in the scope of the current
invention that the current invention can also be applied in Time
Division Duplex (TDD) systems arriving at numerous possible
configurations.
4.5 Embodiment 5
[0070] FIG. 8: Downlink Elevation Beamforming using 4T4R base
station (only 2 of 4 ports shown) connected to a passive antenna
array based on variable tilt in response to differential phase
across VA & VB Ports. A fifth and final embodiment of the
current invention is illustrated in FIG. 8. This embodiment uses an
antenna assembly based upon an adaptive Beamforming network (40)
designed for processing differential phased signals at the ports of
the Beamforming network (VA and VB). Such a Beamforming network
(40) suitable for operation includes those based upon U.S. Pat. No.
7,450,066, U.S. Pat. No. 7,400,296, or U.S. Pat. No. 7,420,507.
However, this embodiment includes both Uplink and Downlink channel
adaptive Beamforming in the elevation plane. For clarity we do not
show or describe the Uplink Beamforming process as this can be
considered identical or similar to that already described for
embodiments one and three. In FIG. 8, we show the Downlink channel
processing stages for an LTE base station. An LTE base station
might be more appropriate than a GSM or UMTS base station when
considering Downlink Beamforming, since 3GPP LTE specifications
include user specific Downlink reference/pilot symbols which can be
also applied through a beamforming process and hence for the user
terminal to make accurate radio channel estimation and
equalization. Furthermore, 3GPP LTE Release 9 specifications
include user specific pilot symbols on multiple layers to allow for
example 2.times.2 spatial multiplexing MIMO and 2-branch Transmit
Diversity with adaptive coherent beamforming; later releases of LTE
specifications promise additional spatial multiplexing layers and
higher order Tx diversity to be combined with coherent adaptive
Beamforming.
[0071] FIG. 8 depicts a high-level view of the Downlink channel
processing for a 3GPP LTE Release 9 base station operating in FDD
operation connected to an antenna array (40) based on differential
phase processing of the Beamforming network ports. In LTE there are
numerous multiple antenna techniques, configurations and
Transmission Modes some of which is discussed in the introduction
section including Spatial Multiplexing MIMO, Transmit Diversity and
Spatial Beamforming. The instance shown in FIG. 8 is where two
input lines or codewords (I1a and I1b), representing data carried
on the primary data channel in LTE, the Physical Downlink Shared
Channel (PDSCH) which is derived from higher abstraction layers of
the LTE protocol, are mapped to one terminal (T1). Each codeword is
processed by a process of scrambling (to randomize data) and symbol
modulation (e.g., QPSK, 16QAM or 64QAM modulation applied depending
upon the instantaneous Downlink channel conditions reported back by
the Terminal T.sub.1 on the Uplink--not shown). The scrambled and
modulated codewords are then processed through a Layer Mapping
stage, and is showing that processed (scrambled and symbol
modulated) codeword I.sub.1a is mapped to two Layers; the same
mapping applies for processing of codeword I.sub.1b. In this
embodiment, each processed codeword is split into two layers, which
are then pre-coded by applying complex weights; p1.sub.1 and
p2.sub.1 are the complex weights applied to each of the two
branches of the processed codeword I.sub.1a, resulting in complex
baseband signals c1.sub.1 and c2.sub.1; similarly p3.sub.1 and
p4.sub.1 are the complex weights applied to each of the two
branches of the processed codeword I.sub.1b, resulting in c3.sub.1
and c4.sub.1. The pre-coding stage in this case is designed to
achieve, through application of a weight vector, adaptive coherent
2-branch transmit Beamforming for each processed codeword, and
independent Beamforming for each processed codeword. The resulting
two pre-coded signals, associated with each codeword are then
mapped to LTE Physical Resources, e.g., frequency and time
resources (broadly an allocation stage) under the control of
traffic management scheduling algorithms (not shown), and
subsequently to OFDM Modulation (broadly a modulation stage for
modulating the pre-coded signals into modulated signals), which in
turn is up-converted from complex OFDM modulated baseband to RF,
and finally power amplified suitable for transmission (80) (broadly
a baseband to RF up-conversion stage up-converting the modulated
signals into RF signals). The two power amplified RF signals,
associated with codeword ha are then connected to the two ports (VA
and VB) of the Antenna Beamforming Network (40) which in turn
drives the array of antenna elements (62.sub.1-62.sub.N). In this
embodiment, adaptive elevation plane spatial beamforming for
Downlink data for terminal T.sub.1 carried on the PDSCH is achieved
by imposing phase a difference between complex baseband signals
c1.sub.1 and c2.sub.1. The resulting elevation beam tilt is a
function of the phase shift, and achieved by the pre-coding stage.
In FIG. 8, the Downlink Scrambling, Modulation, Pre-Coding and OFDM
Multiplexing processing of the PDSCH is shown for data associated
with one terminal, T1; this processing is shown collectively within
72.sub.1 (broadly described as a precoding stage). Broadly, the
precoding stage is for mapping of baseband data symbols associated
with a mobile terminal into two baseband signal branches and
pre-coding the branches via a multiplication of the signal branches
with complex pre-coding weights to impart a phase difference
between the two signal branches for generating pre-coded signals.
However, as LTE is a multiple access system there will be multiple
concurrent similar Scrambling, Modulation, Pre-Coding and OFDM
Multiplexing processes associated with PDSCH data associated for
other terminals which are denoted as 72.sub.2 to 72.sub.k for k
parallel processes. These other data channels will be mapped to
different OFDM sub-tones under the control of the Downlink traffic
scheduling algorithms. Different and independent data associated
with other terminals can be pre-coded independently and hence
result in independent beams in the elevation plane. Non PDSCH
channels such as common physical channels including the cell
broadcast channel are designed to be transmitted across the entire
cell, and as such do not need to be (and are not adaptively
Beamformed) via the pre-coding stage. Instead, these non-adaptive
Beamformed data channels can be split across the two transmission
lines with a fixed phase shift, or semi-static phase shift
resulting in a fixed/static beam tilt via the antenna assembly; the
phase shift for common channels can of course be changed
periodically to effectively deliver a conventional variable
electrical tilt function.
[0072] The final stages of processing of signals associated with
PDSCH codeword I.sub.1b are not shown for clarity, but would
undergo similar processing into a second antenna Beamforming
network, which might associated with an antenna array with an
orthogonal polarization to the first antenna array
(62.sub.1-62.sub.N), for example.
[0073] The pre-coding vector is expected to change very slowly as
the intention is to focus a coherent beam (tilt) toward the
terminal T.sub.1 for the Downlink channel, and the pre-coding
vector would be expected to change in response to significant
movement of travel of the terminal T.sub.1 through its radio
channel and environment.
[0074] It would however be possible to derive many further
embodiments using the building blocks of the aforementioned
embodiments to arrive at similar arrangements to achieve downlink
channel elevation plane spatial beamforming. For example, the
schematic in FIG. 8 could use an RF Antenna Assembly using a
beamforming network designed to vary a beam tilt in response to
differential amplitude (rather than differential phase) at its
ports, where the LTE Downlink pre-coding stages would in this case
need to apply complex weights to information symbols which would
create differential amplitude in order to effect variable beam tilt
in elevation. Additionally a Hybrid Coupler and phase shifter could
be applied between the base station and the Beamforming Network
(40) ports (i.e., similar to that described in embodiment 3) in
FIG. 8; in this case a common channel such as the broadcast channel
could be present on only one Tx/Rx line from the base station; the
Hybrid combiner and phase shifter performing a splitting and phase
shift function so that the broadcast channel information can be
applied across the VA and VB ports and deliver a particular beam
tilt for the common channels. In this case, information to be
adaptively beamformed in elevation, e.g., data carried on the PDSCH
would need to be pre-coded to create differential amplitude, which
would be converted to differential phase at the outputs of the
Hybrid combiner in order to be able to adaptively vary beam tilt.
Additionally, the above embodiments have assumed that the access
technology is based upon Frequency Division Duplexing (FDD). To
those versed in the art the scope of the current invention can also
be applied in Time Division Duplex (TDD) systems arriving at
numerous possible configurations too.
[0075] It should be noted that various stages of the present
invention can be implemented via a hardware processor and a
non-transitory memory (broadly a computer readable medium). For
example, a hardware system (not shown) may comprise a processor
element (e.g., a CPU), and a memory for performing the various
functions as described above.
[0076] It should be noted that the present disclosure can be
implemented in software and/or in a combination of software and
hardware, e.g., using application specific integrated circuits
(ASIC), a general purpose computer or any other hardware
equivalents, e.g., computer readable instructions pertaining to the
method(s) discussed above can be used to configure a hardware
processor to perform the steps, functions, stages and/or operations
of the above disclosed methods.
[0077] While various embodiments have been described above, it
should be understood that they have been presented by way of
example only, and not limitation. Thus, the breadth and scope of a
preferred embodiment should not be limited by any of the
above-described exemplary embodiments, but should be defined only
in accordance with the following claims and their equivalents.
* * * * *