U.S. patent application number 16/232409 was filed with the patent office on 2019-05-02 for transmission and reception of a random access preamble signal.
The applicant listed for this patent is Telefonaktiebolaget LM Ericsson (publ). Invention is credited to Hakan Andersson, Mattias Frenne, Johan Furuskog, Peter Naucler, Henrik Sahlin.
Application Number | 20190132886 16/232409 |
Document ID | / |
Family ID | 50982897 |
Filed Date | 2019-05-02 |
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United States Patent
Application |
20190132886 |
Kind Code |
A1 |
Sahlin; Henrik ; et
al. |
May 2, 2019 |
Transmission and Reception of a Random Access Preamble Signal
Abstract
A method performed in a preamble transmitter for transmitting a
preamble sequence, the method comprising the steps of generating
S11 a short sequence s(n), the short sequence having the same time
duration as an OFDM symbol used for carrying data traffic in a
radio access network of the preamble transmitter, constructing S12
a preamble sequence by concatenating a plurality of said short
sequences in time, and transmitting S13 the constructed preamble
sequence as a radio signal to a preamble receiver.
Inventors: |
Sahlin; Henrik; (Molnlycke,
SE) ; Andersson; Hakan; (Linkoping, SE) ;
Frenne; Mattias; (Uppsala, SE) ; Furuskog; Johan;
(Stockholm, SE) ; Naucler; Peter; (Knivsta,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Telefonaktiebolaget LM Ericsson (publ) |
Stockholm |
|
SE |
|
|
Family ID: |
50982897 |
Appl. No.: |
16/232409 |
Filed: |
December 26, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15125652 |
Sep 13, 2016 |
10201018 |
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PCT/EP2014/062383 |
Jun 13, 2014 |
|
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16232409 |
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61969912 |
Mar 25, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04J 13/0062 20130101;
H04J 13/107 20130101; H04J 13/0003 20130101; H04W 74/0833 20130101;
H04L 27/265 20130101; H04L 27/2613 20130101; H04W 74/004
20130101 |
International
Class: |
H04W 74/08 20060101
H04W074/08; H04L 27/26 20060101 H04L027/26; H04J 13/10 20060101
H04J013/10; H04J 13/00 20060101 H04J013/00; H04W 74/00 20060101
H04W074/00 |
Claims
1. A user equipment comprising: processing circuitry configured to:
generate a time-continuous short sequence; construct a preamble
sequence by concatenating a plurality of the short sequences in
time, wherein the short sequence has a same time duration as an
orthogonal frequency division multiplexing (OFDM) symbol used for
carrying data traffic in a radio access network of the user
equipment; a transmitter circuit configured to transmit the
generated preamble sequence as a radio signal.
2. The user equipment of claim 1, wherein the transmitter circuit
is configured to transmit the preamble sequence over a Physical
Random Access Channel (PRACH) of the radio access network.
3. The user equipment of claim 1, wherein the short sequence used
to construct the preamble sequence is a cyclic prefix for
neighboring short sequences.
4. The user equipment of claim 1, wherein the short sequence
comprises a Zadoff-Chu sequence.
5. A method, performed in a user equipment, for transmitting a
preamble sequence, the method comprising: generating a short
sequence, the short sequence having a same time duration as an
Orthogonal Frequency-Division Multiplexing (OFDM) symbol used for
carrying data traffic in a radio access network of the user
equipment; constructing a preamble sequence by concatenating a
plurality of the short sequences in time; and transmitting the
constructed preamble sequence as a radio signal to a preamble base
station.
6. A computer program product stored in a non-transitory computer
readable medium for controlling a user equipment to transmit a
preamble sequence, the computer program product comprising software
instructions that, when run on processing circuitry of the user
equipment, causes the user equipment to: generate a short sequence,
the short sequence having a same time duration as an Orthogonal
Frequency-Division Multiplexing (OFDM) symbol used for carrying
data traffic in a radio access network of the user equipment;
construct a preamble sequence by concatenating a plurality of the
short sequences in time; and transmit the constructed preamble
sequence as a radio signal to a preamble base station.
Description
[0001] This application is a Divisional Application of U.S. patent
application Ser. No. 15/125,652, filed 13 Sep. 2016, which is a U.
S. National Phase Application of International Application No.
PCT/EP2014/062383 filed 13 Jun. 2014, which claims the benefit of
U.S. Provisional Application Ser. No. 61/969,912, filed 25 Mar.
2014, the disclosures of each of which are incorporated herein by
reference in their entirety.
TECHNICAL FIELD
[0002] The present disclosure relates to random access in wireless
communication systems, and in particular to a transmitter, a
receiver, and to methods for transmitting and receiving random
access preamble signals.
BACKGROUND
[0003] The fourth generation, 4G, wireless access within the 3rd
generation partnership project, 3GPP, long-term evolution, LTE, is
based on orthogonal frequency-division multiplexing, OFDM, in
downlink and discrete Fourier transform, DFT, spread OFDM, also
known as single carrier frequency division multiple access,
SC-FDMA, in uplink. Here, the uplink consists of the physical
channels PUSCH, PUCCH, and PRACH and of the physical signals DMRS
and SRS. According to the 3GPP specification, see, e.g., 3GPP TS
36.211 V11.3.0, the PUSCH, PUCCH, DMRS, and SRS all use an IFFT of
size 2048 in the transmitter, with a sampling rate of 30.72 MHz.
The same size of 2048 can be used for the FFT in the receiver.
Dedicated hardware is commonly used for these FFTs. With another
sampling rate than 30.72 MHz, the IFFT and FFT size will change
accordingly.
[0004] The Physical Random-Access Channel, i.e., the PRACH, is used
for initial access for a wireless device into the radio access
network and also for timing offset estimation, i.e., estimation of
timing offset between wireless device transmissions and reception
at a base station. A description of this procedure is given in 3GPP
TS 36.213 V11.3.0. An illustration 100 of PRACH, as specified for
LTE, see, e.g., 3GPP TS 36.211 V11.3.0, is given in FIG. 1. Here
five different formats, referred to in FIG. 1 as Format 0-Format 4,
are specified where a PRACH preamble 101, 101' consists of one 101
or two 101' sequences, each of length 24 576 samples. The preambles
have a cyclic prefix 102, CP, of length between 3 168 and 21 024
samples for formats 0 to 3.
[0005] Several methods have been proposed for how to detect the
PRACH preambles transmitted by the UE, see e.g., S. Sesia. I.
Toufik. M Baker "LTE, The UMTS Long Term Evolution, From Theory to
Practice", Second Edition, John Wiley & Sons Ltd., 2011, where
both a full frequency domain and a hybrid time-frequency approach
are presented. In the full frequency domain approach the received
signal is processed with an FFT corresponding to the length of the
preamble. Hence, as shown in FIG. 2, an FFT 203 of length 24 576 is
thus required for each antenna. Dedicated hardware is commonly used
for this PRACH FFT. After this large FFT, the PRACH bandwidth is
extracted, which is a subset of the output from this large FFT.
[0006] In the hybrid time-frequency approach, a low-pass filter is
first used in the time domain in order to extract the PRACH
bandwidth. This lowpass filter is followed by an FFT of a size much
smaller than 24 576. However, one such low-pass filter has to be
applied to each antenna signal.
[0007] Consequently, as illustrated by FIGS. 1 and 2, the PRACH
preamble as specified in LTE Release 8 covers a time interval which
is much longer than the length of OFDM symbols used for other
transmissions such as user data symbols. Current PRACH preamble
receivers are thus designed under the assumption that propagation
conditions are not varying significantly during the length of the
preamble. This may be problematic, since assumptions, or
constraints, are placed on the communication system. These
constraints include expectations on low UE speed, i.e., Doppler
spread, low frequency errors and low Doppler shifts, and also low
phase noise in transmitters and receivers.
[0008] Thus, there is a need for an improved PRACH signaling
technique, i.e., a preamble transmitter and receiver, which does
not place or otherwise imply the above mentioned constraints on the
communication system.
[0009] US 2010/172423 discloses that as preamble sequences are
increased, processing complexity and resources required at the
receiver end are increased substantially. Preamble sequences are
suggested where null periods are placed within or among transmitted
samples, where least two sets of samples and at least two null
periods are transmitted within one OFDM symbol.
[0010] With currently emerging technologies, such as 5G
communication systems, the use of many antenna elements is of great
interest. As illustrated in FIG. 3, the antenna signals can come
from several antenna polarizations 304. Here, the antenna signals
305 are first received in a Radio Unit, RU, 306. The signals are
then sampled and quantized in an Analog-to-Digital Converter, ADC,
307. A transformation from time to frequency domain is done using
an FFT module 308, or, alternatively by a DFT module not shown in
FIG. 3, after which a PRACH receiver 309 is applied to detect a
preamble comprised in the received radio, i.e., antenna, signals.
Here, an FFT is typically calculated for each antenna or for each
subset of antennas, such that different users and channels in
different sub-bands of the received signal can be extracted before
further signal processing.
[0011] FIG. 3 illustrates current PRACH receivers having multiple
antennas. FIG. 3 visualizes that with a large number of receiver
antennas 310, the amount of FFT processing in the receiver is also
large, which is generally a drawback. With dedicated antenna-signal
processing only used for PRACH, a significant amount of special
hardware for PRACH must be included, which hardware causes
increased material cost as well as increased energy consumption.
Also, running PRACH-specific antenna-signal processing consumes
power and requires cooling capacity. Consequently, there is a need
for a PRACH receiver better suited for multiple-antenna
operation.
[0012] In order to increase received signal strength, a beamforming
procedure can be used in which several antenna signals are scaled,
phase shifted, and added before the PRACH receiver 309 is applied.
Beamforming aims at combining received signals from several
antennas such that more signal energy is received in specific
spatial directions. Several beams can be formed in order to
beamform towards different spatial directions. With two
polarizations, the antenna signals from each polarization are
typically beamformed separately. The same, or different,
beamforming can be applied to the different polarizations.
[0013] This beamforming 411 can be done in the frequency domain,
i.e., after the FFT 408, as illustrated in FIG. 4. After the FFT
408, the individual sub-carriers can be extracted such that
different physical channels and signals can be extracted. With
digital beamforming 411 in the frequency domain, the antenna
signals are first processed with an FFT 408 and then beamformed
411. In this manner, different sub-carriers can be beamformed
differently. This allows for different beamforming for different
physical channels and signals. Also, if several UEs are multiplexed
in frequency, then these can be processed with individual
beamforming.
[0014] However, with digital beamforming, a specific PRACH FFT has
to be calculated for each receiver antenna, before extracting the
PRACH bandwidth and beamforming into a smaller amount of signals.
This is potentially a drawback due to the added signal processing
required.
[0015] Alternatively, the beamforming can be done in the time
domain 511b, as shown in FIG. 5. Here, the beamforming is done on a
digital signal, i.e., after analog-to-digital conversion by the ADC
507. However, since the FFT 508 is calculated after the beamforming
511b, all sub-carriers are beamformed in the same spatial
direction, which is a potential drawback in some scenarios, e.g.,
where UEs are spread out over a large area.
[0016] An alternative time-domain beamforming 611c is illustrated
in FIG. 6, where the beamforming 611c is done before ADC 607. Here,
the beamforming is done on an analog signal, i.e., before
analog-to-digital conversion by the ADC 607.
[0017] Combinations of analog and digital beamforming and time- and
frequency-domain beamforming are also possible.
[0018] With analog beamforming, such as the beamforming illustrated
in FIG. 6, the number of spatial directions for PRACH is limited by
the number of analog beamformers. In LTE release 8, the PRACH
preamble, and thus also the PRACH FFT, spans almost a whole
sub-frame. The analog beamforming must therefore be fixed during a
whole sub-frame which limits the number of beamforming
directions.
[0019] Hence, present solutions for receiving PRACH and performing
UE initial access and timing offset estimation are costly in terms
of extra hardware and design effort, as well as in increased energy
consumption and signal processing resources. Furthermore,
improvements in the interworking between PRACH reception and
beamforming in multiple antenna systems are preferred in order to
reduce complexity of implementation.
[0020] It is an object of the present disclosure to provide
solutions to, or at least mitigate, the above mentioned
deficiencies in the art.
SUMMARY
[0021] An object of the present disclosure is to provide at least a
transmitter, a receiver, and methods for transmitting and receiving
random access preamble signals, which seeks to mitigate, alleviate,
or eliminate one or more of the above-identified deficiencies in
the art and disadvantages singly or in any combination.
[0022] This object is obtained by a preamble transmitter
comprising: [0023] a short sequence generator arranged to generate
a short sequence s(n), and [0024] a preamble sequence generator
adapted to construct a preamble sequence by concatenating a
plurality of said short sequences in time, as well as [0025] a
transmitter unit arranged to transmit the generated preamble
sequence as a radio signal.
[0026] The short sequence s(n) has the same time duration as an
OFDM symbol used for carrying data traffic in a radio access
network of the preamble transmitter.
[0027] Thus, by the present technique, there is no need for a
special FFT used for receiving preambles in an uplink receiver of
the transmitted radio signal.
[0028] According to an aspect, the preamble transmitter is further
arranged to transmit the preamble sequence over a Physical Random
Access Channel, PRACH, of a radio access network.
[0029] Thus, by the present technique, there is no need for a
special PRACH FFT in the uplink receiver of the transmitted radio
signal.
[0030] The object is also obtained by a preamble receiver arranged
to receive radio signals comprising a preamble sequence. The
preamble receiver comprises: [0031] at least one antenna element
and corresponding radio unit configured to receive a radio signal,
and [0032] at least one analog to digital converter, ADC,
configured to perform analog to digital conversion of the received
radio signal, as well as [0033] at least one FFT module arranged to
determine a Fast Fourier Transform of the analog to digital
converted signal, and also [0034] at least one detector adapted to
detect the preamble sequence based on the determined FFT.
[0035] The preamble sequence comprises a concatenation in time of a
plurality of short sequences s(n), where each such short sequence
s(n) has the same time duration as an OFDM symbol used for carrying
data traffic in a radio access network of the preamble receiver.
Also, the size of the FFT used for detecting the preamble signal is
of the same size as one used for detecting an OFDM symbol carrying
data traffic in a radio access network.
[0036] Thus, by the present technique, there is no need for a
special FFT used for receiving preambles in the uplink receiver.
This is especially important if FFT operations are performed for a
large number of receiver antennas, as will become apparent from the
present disclosure.
[0037] According to an aspect, the preamble receiver is further
arranged to use one FFT hardware resource and FFT configuration
both for detecting OFDM symbols carrying data, and also for
detecting preamble sequences.
[0038] Thus, by the present technique, there is no special PRACH
FFT in the preamble receiver.
[0039] According to one aspect, the preamble receiver comprises a
preamble detector arranged to determine a plurality of FFTs from a
plurality of FFT windows, and to non-coherently combine the FFT
results into a combined received preamble signal.
[0040] According to another aspect, the preamble receiver comprises
a preamble detector arranged to determine a plurality of FFTs from
a plurality of FFT windows, and to coherently combine the FFT
results into a combined received preamble signal.
[0041] Thus, by the feature of the preamble detector arranged to
determine a plurality of FFTs from a plurality of FFT windows, and
to coherently combine FFT results, there is provided a preamble
receiver which is robust to high UE speeds, i.e., large Doppler
spreads, and also to large frequency errors and high phase
noise.
[0042] According to an aspect, the preamble receiver is arranged
for beamforming. The beamforming weights are configured to change
between FFT windows such that the number of spatial directions for
which preamble detection is done is increased.
[0043] According to an aspect, the preamble receiver comprises
hardware support for more than one simultaneous analog beamforming
configuration, the preamble receiver being adapted for switching at
a first switching rate between spatial directions with one
beamforming configuration, and for switching at a second switching
rate between spatial directions with another beamforming
configuration, the first rate being different from the second
rate.
[0044] Consequently, there is herein provided support for an
increased number of beamforming directions if switching beamforming
between FFT windows.
[0045] There is also provided herein a combined fast beamforming
switching and slow beamforming switching. This means that both UEs
with high and low SNR can be detected, albeit the latter with a
larger delay.
[0046] According to an aspect, a single IFFT is applied per beam
direction and polarization.
[0047] According to an aspect, the preamble receiver is arranged to
perform simultaneous beam-forming and matched filtering by a
comprised joint filter.
[0048] Thus, the present technique provides for low computational
complexity in the receiver, since coherent accumulation of the
matched filter outputs from different FFT windows is possible.
[0049] There is furthermore provided a technique which enables
using only one IFFT per beam direction and polarization in a
beamforming system, as well as simultaneous beam-forming and
matched filtering.
[0050] There is also disclosed a preamble transmitter and receiver
system, comprising at least one preamble transmitter and at least
one preamble receiver as disclosed herein.
[0051] The object is also obtained by a network node comprising the
preamble receiver according to any of the aspects disclosed
herein.
[0052] The object is further obtained by a method performed in a
preamble transmitter for transmitting a preamble sequence. The
method comprises the steps of [0053] generating a short sequence
s(n), the short sequence having the same time duration as an OFDM
symbol used for carrying data traffic in a radio access network of
the preamble transmitter, [0054] constructing a preamble sequence
by concatenating a plurality of said short sequences in time, and
[0055] transmitting the constructed preamble sequence as a radio
signal to a preamble receiver.
[0056] The object is additionally obtained by a method performed in
a preamble receiver for receiving a radio signal and detecting a
preamble sequence comprised in the radio signal. The method
comprises the steps of [0057] receiving a radio signal comprising a
preamble signal constructed from a plurality of short sequences
s(n) via at least one antenna element and radio unit, the short
sequence s(n) having the same time duration as an OFDM symbol used
for carrying data traffic in a radio access network of the preamble
transmitter, [0058] performing analog to digital conversion of the
radio signal by an ADC comprised in the preamble receiver, and
[0059] determining a Fast Fourier Transform, FFT, of the analog to
digital converted signal, wherein the size of the FFT used for
detecting the preamble signal is of the same size as for detecting
an OFDM symbol used for carrying data traffic in a radio access
network, as well as [0060] detecting the preamble sequence based on
the determined FFT.
[0061] According to an aspect, the step of determining an FFT
further comprises determining an FFT having a single configuration
for detecting OFDM symbols carrying data, and also for detecting
preamble sequences.
[0062] According to an aspect, the step of detecting comprises
determining a plurality of FFTs from a plurality of FFT windows,
and also non-coherently combining the FFT results into a combined
received preamble signal.
[0063] According to an aspect, the step of detecting comprises
determining a plurality of FFTs from a plurality of FFT windows,
and also coherently combining the FFT results into a combined
received preamble signal.
[0064] There is also provided a computer program comprising
computer program code which, when executed in a preamble
transmitter, causes the preamble transmitter to execute a method
according to aspects disclosed herein.
[0065] There is further provided a computer program comprising
computer program code which, when executed in a preamble receiver,
causes the preamble receiver to execute a method according to
aspects disclosed herein.
[0066] The computer programs and the methods display advantages
corresponding to the advantages already described in relation to
the preamble transmitter and receiver.
BRIEF DESCRIPTION OF THE DRAWINGS
[0067] Further objects, features, and advantages of the present
disclosure will appear from the following detailed description,
wherein some aspects of the disclosure will be described in more
detail with reference to the accompanying drawings, in which:
[0068] FIGS. 1-2 are signaling diagrams illustrating exchange of
signals in embodiments of a network according to prior art.
[0069] FIGS. 3-6 are block diagrams illustrating embodiments of a
receiver system according to prior art.
[0070] FIG. 7 is a signaling diagram illustrating exchange of
signals in an embodiment of a network.
[0071] FIG. 8 is a flowchart illustrating embodiments of method
steps performed in a preamble transmitter.
[0072] FIG. 9 is a signaling diagram illustrating exchange of
signals in an embodiment of a network.
[0073] FIG. 10 is a flowchart illustrating embodiments of method
steps performed in a preamble receiver.
[0074] FIGS. 11-13 are signaling diagrams illustrating exchange of
signals in embodiments of a network.
[0075] FIGS. 14-25 are block diagrams illustrating embodiments of a
receiver system.
[0076] FIG. 26 is a block diagram illustrating embodiments of a
transmitter system.
[0077] FIG. 27 is a block diagram illustrating embodiments of a
receiver system.
[0078] FIGS. 28-30 are flowcharts illustrating embodiments of
method steps.
DETAILED DESCRIPTION
[0079] Aspects of the present disclosure will be described more
fully hereinafter with reference to the accompanying drawings. The
apparatus, computer program and methods disclosed herein can,
however, be realized in many different forms and should not be
construed as being limited to the aspects set forth herein. Like
numbers in the drawings refer to like elements throughout, except
for prefix digits in the number which represent the figure in which
the element is to be found.
[0080] The terminology used herein is for the purpose of describing
particular aspects of the disclosure only, and is not intended to
limit the invention. As used herein, the singular forms "a", "an"
and "the" are intended to include the plural forms as well, unless
the context clearly indicates otherwise.
Abbreviations
3GPP 3rd Generation Partnership Project
4G Fourth Generation
5G Fifth Generation
ADC Analog-to-Digital Converter
[0081] BF Beam forming
DFT Discrete Fourier Transform
DL Downlink
[0082] DM-RS Demodulation reference signal
FDD Frequency Division Duplex
FFT Fast Fourier Transform
IDFT Inverse Discrete Fourier Transform
IFFT Inverse Fast Fourier Transform
LTE Long Term Evolution
MF Matched Filter
[0083] OFDM Orthogonal frequency-division multiplexing PBCH
Physical broadcast channel PRACH Physical random access channel
PRB Physical Resource Block
PSS Primary Synchronization Signal
[0084] PUCCH Physical uplink control channel PUSCH Physical uplink
shared channel
RB Resource Block
RBS Radio Base Station
RU Radio Unit
SC Sub-Carrier
SC-FDMA Single Carrier-Frequency Division Multiple Access
SNR Signal to Noise Ratio
SRS Sounding Reference Signal
SSS Secondary Synchronization Signal
TDD Time Division Duplex
UE User Equipment
UL Uplink
ZC Zadoff-Chu
[0085] A preamble receiver, e.g., for receiving signals on the
PRACH, is proposed herein in which FFTs of the same size as for
other uplink channels and signals are used. The preamble receiver
discussed herein constitutes part of a receiver in a wireless
communication system, such as an LTE or 5G RBS.
[0086] In other words, the preamble receiver 2741, 1548 disclosed
herein is arranged to use one FFT hardware resource and FFT
configuration both for detecting OFDM symbols carrying data, and
also for detecting preamble sequences.
[0087] The preamble sequence discussed herein can be used for a
variety of purposes, including but not limited to initial access,
handover, scheduling requests, and resynchronization.
[0088] The PRACH preamble used is based on several concatenated
short sequences, where each short sequence has the same length as
the length of the OFDM symbols used for all other physical
channels. The preamble sequence is constructed by repeating the
short sequence a number of times to make the preamble sequence.
Consequently, the short sequence used to construct the preamble
sequence works as a cyclic prefix to its neighbors, as will be
further detailed below.
[0089] Further, in the preamble detector proposed herein, several
received signals from different FFT windows can be combined.
Different combinations of these FFT windows are proposed depending
of the amount of phase noise, frequency errors, and UE speed.
[0090] The proposed technique is applicable in multi-antenna
systems implementing beamforming. For analog beamforming systems,
the beamforming weights are, according to an aspect, changed
between each FFT window such that the number of spatial directions
is increased for which preamble detection is done. With hardware
support from more than one simultaneous analog beamforming
resource, then one beamforming resource can be used for fast
switching between spatial directions while the other can have a
slow switching rate allowing more accumulated preamble energy in
each direction.
[0091] Thus, by the present technique, no special PRACH FFT is
necessary in the uplink receiver. This is especially important if
FFT operations are performed for a large number of receiver
antennas, due to the significant savings in, e.g., hardware
processing resources.
[0092] Furthermore, the present technique facilitates receiver
implementations with reduced computational complexity compared to
current PRACH receiver systems. For instance, [0093] Coherent
accumulation of matched filter outputs from different FFT windows
is possible, [0094] There is only one IFFT per beam direction and
polarization in a system with beamforming, [0095] It is possible to
perform simultaneous beam-forming and matched filtering by a joint
filter structure.
[0096] Another potential benefit of the present teaching is that a
preamble detector is provided which is robust towards high UE
speed, i.e. Doppler spread, large frequency errors, i.e., large
Doppler shifts, and severe phase noise.
[0097] Yet another potential advantage of the present technique is
an increased number of beamforming directions for analog
beamforming if switching beamforming between FFT windows, which is
especially beneficial for UEs with high SNR, e.g., those close to
the eNodeB.
[0098] Additionally, there is provided combined fast beamforming
switching and slow beamforming switching, wherein both UEs with
high and low SNR can be detected, albeit the latter with a larger
delay.
[0099] FIG. 7 shows a signaling diagram 712 illustrating timing in
a communication system implementing Time Division Duplex, TDD.
[0100] In a TDD system, the same frequency is used both for
downlink and uplink. Both the UE and the eNodeB must then switch
between transmitting and receiving, assuming that full duplex
operation is not possible.
[0101] The present teaching is focused on a TDD mode of operation.
However, the technique disclosed herein also directly applies to
FDD (Frequency-Division Duplex) systems. For FDD systems the
problem descriptions related to switch time between transmission
and receptions do not apply.
[0102] A dynamic TDD system is according to aspects configured with
a few sub-frames 713 that are fixed for downlink, i.e., they cannot
be used for uplink, see illustration in FIG. 7. These sub-frames
are used, e.g., for transmitting synchronization signals and
broadcasting control messages used for initial downlink
synchronization, continuous downlink synchronization, and call
setup. A dynamic TDD system can also be configured with fixed
uplink sub-frames 714. Such sub-frames can, e.g., be used for PRACH
to support initial access and uplink synchronization. Note that
FIG. 7 is based on a radio-frame of length 10 ms, which is divided
into 50 sub-frames, each of length 0.2 ms. This is in contrast to
LTE Release 8 where a radio-frame is split into 10 sub-frames, each
of length 1 ms.
[0103] Thus, according to an aspect, there is disclosed a method
for receiving the type of preamble sequence disclosed herein in
connection to the preamble transmitter, which method is not
necessarily limited to a specific radio frame length or sub-frame
division.
[0104] FIG. 8a shows a flow chart illustrating a procedure for
initial set-up of a UE in a radio access network, where PSS and SSS
are transmitted in subframe 0 and 25 in a dynamic TDD system.
[0105] FIG. 8b shows a flow chart where the steps in FIG. 8a have
been put into one example context, further discussed in connection
to FIG. 28 below.
[0106] The methods illustrated in FIGS. 8a and 8b are arranged to
be performed by a preamble transmitter 2636 which will be further
discussed in connection to FIG. 26 below.
[0107] This preamble transmitter comprises: [0108] a short sequence
generator 2650 arranged to generate a short sequence s(n), and
[0109] a preamble sequence generator 2637 adapted to construct a
preamble sequence by concatenating a plurality of said short
sequences in time, as well as [0110] a transmitter unit 2639a,b
arranged to transmit the generated preamble sequence as a radio
signal.
[0111] The short sequence s(n) has the same time duration as an
OFDM symbol used for carrying data traffic in a radio access
network of the preamble transmitter 2636.
[0112] Turning now to FIG. 28, which illustrates a method performed
in a preamble transmitter 2636 for transmitting a preamble
sequence, the method comprising the steps of [0113] generating S11
a short sequence s(n), the short sequence having the same time
duration as an OFDM symbol used for carrying data traffic in a
radio access network of the preamble transmitter 2636, [0114]
constructing S12 a preamble sequence by concatenating a plurality
of said short sequences in time, and [0115] transmitting S13 the
constructed preamble sequence as a radio signal to a preamble
receiver.
[0116] Thus, at initial setup, a UE starts by receiving and
synchronizing to downlink synchronization signals 815. As an
example, in LTE, the UE starts by detecting the PSS, or Primary
Synchronization Signal, after which the UE will attain a sub-frame
synchronization, OFDM symbol synchronization, and know the cell
identity, cell ID, group. Then the UE detects SSS, or Secondary
Synchronization Signal, after which the UE is frame synchronized
and knows the cell ID.
[0117] The UE, i.e., the preamble or PRACH transmitter, is then
according to some aspects configured by receiving and detecting
system information 816 carried by a broadcast signal. In LTE, this
broadcast information is carried by PBCH, or Physical Broadcast
Channel. This broadcast information can relate to time and
frequency allocation of PRACH, such that the UE knows when and
where it is allowed to transmit PRACH preambles. This is further
illustrated in FIG. 9, where the UE can transmit PRACH 918 in
sub-frame 5, which in this TDD system is a fixed allocation to
uplink transmissions. Also, the UE can be configured by broadcast
information or preconfigured with timing information of when within
a sub-frame it may transmit the preamble.
[0118] Based on broadcast information, or preconfigured in the UE
according to specification, a PRACH preamble signal is constructed
in the UE and transmitted 817.
[0119] A corresponding illustration for an eNodeB, i.e., the
preamble or PRACH receiver, is given in FIG. 10a.
[0120] FIG. 10b shows a flowchart where the steps in FIG. 10a have
been put into one example context, further discussed in connection
to FIG. 29 below.
[0121] The methods illustrated in FIGS. 10a and 10b are arranged to
be performed by a preamble receiver 2741, 1447 which will be
further discussed in connection to FIGS. 14 and 27 below. This
preamble receiver comprises: [0122] at least one antenna element
1410 and corresponding radio unit 1406, configured to receive a
radio signal, and [0123] at least one analog to digital converter,
ADC, 1407 configured to perform analog to digital conversion of the
received radio signal, as well as [0124] at least one FFT module
1408 arranged to determine a Fast Fourier Transform, of the analog
to digital converted signal, and also [0125] at least one detector
1428 adapted to detect the preamble sequence based on the
determined FFT.
[0126] Turning now to FIG. 29, which illustrates a method performed
in a preamble receiver 2741, 1447 for receiving a radio signal and
detecting a preamble sequence comprised in the radio signal, the
method comprising the steps of [0127] receiving S21 a radio signal
comprising a preamble signal constructed from a plurality of short
sequences s(n) via at least one antenna element 1410 and radio unit
1406, the short sequence s(n having the same time duration as an
OFDM symbol used for carrying data traffic in a radio access
network of the preamble transmitter 2636, [0128] performing S22
analog to digital conversion of the radio signal by an ADC 1407
comprised in the preamble receiver, and [0129] determining S23 a
Fast Fourier Transform, FFT, 1408 of the analog to digital
converted signal, wherein the size of the FFT used for detecting
the preamble signal is of the same size as for detecting an OFDM
symbol used for carrying data traffic in a radio access network, as
well as [0130] detecting S24 the preamble sequence based on the
determined FFT.
[0131] Thus, the received preamble sequence comprises a
concatenation in time of a plurality of short sequences s(n). Each
such short sequence s(n) has the same time duration as an OFDM
symbol used for carrying data traffic in a radio access network of
the preamble receiver 2741, 1447. Thus, the size of the FFT used
for detecting the preamble signal is preferably of the same size as
one used for detecting an OFDM symbol carrying data traffic in a
radio access network.
[0132] Preamble Construction in UE
[0133] Turning now to the details of how a preamble is constructed
in the UE, i.e., in the preamble transmitter disclosed herein.
[0134] An example embodiment of a preamble transmitter 2636
arranged to perform the steps disclosed below is shown in FIG.
26.
[0135] The preamble sequence depends on the PRACH frequency
allocation, such that the number of sub-carriers, N.sub.seq,
allocated for PRACH equals the maximum number of symbols. For
example, with LTE nomenclature, 6 resource blocks are allocated to
PRACH, which correspond to 72 sub-carriers.
[0136] A short sequence can, e.g., be constructed by using
Zadoff-Chu sequences. The u:th root Zadoff-Chu sequence is defined
in 3GPP TS 36.211 V11.3.0 as
x u ( n ) = e - j .pi. un ( n + 1 ) N ZC , 0 .ltoreq. n .ltoreq. N
ZC - 1 ( 1 ) ##EQU00001##
where the length N.sub.ZC of the Zadoff-Chu sequence is a prime
number. For a PRACH allocation of 72 sub-carriers, the sequence
length can, e.g., be set to 71.
[0137] Thus, the short sequence, at times referred to herein as
s(n), comprises a Zadoff-Chu sequence.
A time-continuous short random-access signal s(t) is defined by
s short ( t ) = .beta. PRACH k = 0 N seq - 1 n = 0 N seq - 1 x u (
n ) e - 2 .pi. nk N seq e j 2 .pi. ( k + k 0 ) .DELTA. f t ( 2 )
##EQU00002##
where 0.ltoreq.t<T.sub.short, N.sub.seq=71, .beta..sub.PRACH is
an amplitude-scaling factor in order to conform to the transmit
power of PRACH,
k.sub.0=n.sub.PRB.sup.RAN.sub.sc.sup.RB-N.sub.RB.sup.ULN.sub.sc.su-
p.RB/2, and .DELTA.f is the sub-carrier spacing. The location in
the frequency domain is controlled by the parameter
n.sub.PRB.sup.RA; the resource block size in the frequency domain,
expressed as a number of subcarriers, is denoted by
N.sub.sc.sup.RB, and the uplink bandwidth configuration, expressed
in multiples of N.sub.sc.sup.RB, is denoted by N.sub.RB.sup.UL.
Using a Zadoff-Chu sequences implies that N.sub.seq=N.sub.ZC.
[0138] A short sequence of the same length as the OFDM symbol is
achieved by T.sub.short=1/.DELTA.f. For LTE Release 8, this
sub-carrier spacing equals .DELTA.f=15 kHz, see Table 6.2.3-1 in
3GPP 36.211 V11.3.0, such that the length of the short sequence
equals T.sub.short=66.6 .mu.s. With a change in subcarrier spacing
to, e.g., .DELTA.f=75 kHz, then the length of the short symbol
equals T.sub.short=13.3 .mu.s.
[0139] Thus, the short sequence s(n) has the same time duration as
an OFDM symbol used for carrying data traffic in a radio access
network of the preamble transmitter 2636.
[0140] The preamble to be transmitted is constructed by
s(t)=s.sub.short((t-T.sub.CP)mod(T.sub.short)) (3)
where 0.ltoreq.t<T.sub.SEQ, and T.sub.CP is the length of a
possible cyclic prefix.
[0141] FIG. 11 provides an illustration 1122 of the short sequences
s(n). By this repetition of the short sequence, each short sequence
will act as a cyclic prefix for the next short sequence. Here, the
short sequence is repeated 15 times, and succeeded by a smaller
part of the short sequence. This last part of the short sequence is
inserted in the end such that the preamble covers the whole length
of the last receiver FFT window.
[0142] Consequently, the short sequence used to construct the
preamble sequence is arranged as cyclic prefix for neighboring
short sequences.
[0143] A preamble sequence 1223 suited for a TDD (Time-Division
Duplex) system is illustrated in FIG. 12, see also FIG. 13. Here,
the preamble is shortened such that it begins later compared to the
case shown in FIG. 11.
[0144] Preamble Detector in eNodeB
[0145] Turning now to the details of how a preamble is detected in
an eNodeB, i.e., in the preamble receiver.
[0146] A network node 2740 comprising a preamble receiver 2741
arranged to perform the steps disclosed below is shown in FIG.
27.
[0147] A receiver structure for preamble detection is illustrated
in FIG. 14. Here, the radio signals 1405 from the antenna elements
1410 are received in radio units 1406, followed by an
Analog-to-Digital Conversion, ADC, 1407.
[0148] Model the radio transmission from mobile to base-station,
for receiver antenna number a by an L tap FIR filter h(m,a)
r ( n , a ) = n = 0 L - 1 h ( m , a ) x ( n - m - d ) + w ( n , a )
+ w ~ ( n , a ) ( 4 ) ##EQU00003##
[0149] Where x(n) is the transmitted sequence, w(n, a) is additive
white Gaussian noise with variance 2.sigma..sub.w.sup.2(a), {tilde
over (w)}(n, a) is interference, and d corresponds to a round-trip
delay for current mobile. This round-trip delay is limited by the
cell radius, i.e.,
0 .ltoreq. d .ltoreq. D - 1. ( 5 ) where D = 2 .times. R cell 3 10
5 F s ( 6 ) ##EQU00004##
and R.sub.cell is the cell radius in kilometers, F.sub.s is the
sampling rate, and .left brkt-bot.x.right brkt-bot. denotes
rounding towards nearest lower integer.
[0150] These time-domain signals are inputs to Fast Fourier
Transforms, FFT, 1408 as illustrated in FIG. 14. See also
illustrations in FIGS. 15 and 16 where the input signals to the FFT
processing, i.e., the FFT windows 1530, 1630 are illustrated. The
FFT window positions n.sub.s(p) correspond to the distance in time
between the start of the first short sequence and each SC-FDMA or
OFDM symbol in uplink, see illustration in FIG. 17. In this
illustration, the start of the first short sequence is placed at
the start of the subframe. For example, in LTE Release 8, the first
cyclic prefix in each slot is 160 samples, while the remaining
cyclic prefixes are 144 samples. Each SC-FDMA or OFDM symbol is
2048 samples such that the values of n.sub.s(p) as in Table 1 below
follow.
TABLE-US-00001 TABLE 1 Time shift in samples between sequential
OFDM (or SC-FDMA) symbols. p n.sub.s(p) [samples] 0 160 1 160 + 144
+ 2048 2 160 + 2*144 + 2*2048 3 160 + 3*144 + 3*2048 4 160 + 4*144
+ 4*2048 5 160 + 5*144 + 6*2048 6 160 + 6*144 + 6*2048 7 2*160 +
6*144 + 7*2048 8 2*160 + 7*144 + 8*2048 9 2*160 + 8*144 + 9*2048 10
2*160 + 9*144 + 10*2048 11 2*160 + 10*144 + 11*2048 12 2*160 +
11*144 + 12*2048 13 2*160 + 12*144 + 13*2048
[0151] Non-Coherent Antenna Accumulation
[0152] For each antenna a and FFT window p, calculate a DFT or FFT
over N.sub.FFT samples:
R ( k , p , a ) = 1 N FFT n = 0 N FFT - 1 r ( n + n s ( p ) , a ) e
- j 2 .pi. kn / N FFT ( 7 ) for k = 0 , , N FFT - 1 and a = 0 , , N
a - 1. ##EQU00005##
[0153] The PRACH preamble in the frequency domain is obtained by
extracting sub-carriers corresponding to those sub-carriers used
for PRACH, i.e. N.sub.seq samples, where
N.sub.seq.ltoreq.N.sub.IFFT
R.sub.PRACH(k,p,a)=R(k+k.sub.0,p,a), (8)
for k=0, . . . , N.sub.seq-1 and
k.sub.0=n.sub.PRB.sup.RAN.sub.sc.sup.RB-N.sub.RB.sup.ULN.sub.sc.sup.RB/2.
Using the same notation as in previous section, and with the use of
Zadoff-Chu sequences, then N.sub.seq=N.sub.ZC.
[0154] Thus, the preamble transmitter 2636 is according to an
aspect, arranged to transmit the preamble sequence over a Physical
Random Access Channel, PRACH, of a radio access network.
[0155] Multiply with a matched filter (of N.sub.seq coefficients)
in the frequency domain
C MF , v ( k , p , a ) = 1 N seq P v * ( k , p ) R PRACH ( k , p ,
a ) . ( 9 ) ##EQU00006##
[0156] This matched filter is constructed from the DFT of known
short sequence and the cyclic shift of this short sequence. The
cyclic shift corresponds to a frequency-domain rotation with the
shift n.sub.shift(p):
P v ( k , p ) = e j 2 .pi. kn shift ( p ) / N FFT 1 N seq n = 0 N
seq - 1 x u ( n ) e - j 2 .pi. kn / N seq . ( 10 ) ##EQU00007##
[0157] The output from the matched filters corresponding to the
same antenna, but from different FFT windows, can now be coherently
added as
C v ( k , a ) = p = p 0 p 0 + P - 1 C MF , v ( k , p , a ) ( 11 )
##EQU00008##
where p.sub.0 is the index of the first, out of P, FFT windows
included in the PRACH preamble detector. See, e.g., FIGS. 11 and 15
for which p.sub.0=1 and P=12. For the format in FIGS. 12 and 16,
only FFT window 2 to 12 are used such that p.sub.0=2 and P=11.
[0158] Thus, according to an aspect, the preamble receiver
comprises a preamble detector arranged to determine a plurality of
FFTs from a plurality of FFT windows, and to coherently combine the
FFT results into a combined received preamble signal.
[0159] Now, in order to detect preamble and estimate round-trip
time, the output from the IFFT will be transformed to the time
domain. Calculate an IDFT, of size N.sub.IFFT, resulting in a
correlation vector of length N.sub.IFFT:
c v ( m , a ) = 1 N IFFT k = 0 N seq - 1 C v ( k , a ) e j 2 .pi. k
m / N IFFT ( 12 ) ##EQU00009##
for m=0, . . . , N.sub.IFFT-1. Selecting N.sub.IFFT>N.sub.seq
corresponds to an interpolation, which can be done in order to
increase the resolution of the timing estimation. A simple
estimator of the noise variance {circumflex over
(.sigma.)}.sub.w.sup.2(a) can be formulated as
.sigma. ^ w 2 ( a ) = p = p 0 p 0 + P - 1 k = 0 N seq - 1 C MF , v
( k , p , a ) 2 . ( 13 ) ##EQU00010##
[0160] As decision variables, the absolute square for each value of
the cross-correlation vector is used, normalized with the estimated
noise variance {circumflex over (.sigma.)}.sub.w.sup.2(i),
.lamda. v ( m ) = a = 0 N a - 1 c v ( m , a ) 2 .sigma. ^ w 2 ( a )
( 14 ) ##EQU00011##
where a summation over antennas, including polarizations, is
included. A preamble detector and round-trip time estimator might
be formulated as searching for the maximum value in this vector of
normalized absolute squared correlations and comparing this maximum
value with a threshold.
[0161] Preamble number v is detected if the absolute squared value
of this autocorrelation exceeds a threshold
.lamda. v ( m ) = a = 0 N a - 1 c v ( m , a ) 2 .sigma. ^ w 2 ( a )
.gtoreq. .lamda. Threshold ( 15 ) ##EQU00012##
for at least one value of m, within the search window of size D. In
other words, the preamble with index v is detected if there is an
m.ANG.[0, D-1] such that
.lamda..sub.v(m).gtoreq..lamda..sub.threshold. This preamble
detector threshold .lamda..sub.threshold should be selected with
care such that the false detection rate is low without causing a
too low detection rate.
[0162] A timing estimate follows as the value of m which
corresponds to the maximum value of .lamda..sub.v(m) i.e.
m ^ = arg max m ( a = 0 N a - 1 c v ( m , a ) 2 .sigma. ^ w 2 ( a )
) ( 16 ) ##EQU00013##
such that the timing error in seconds equals
{circumflex over (T)}.sub.err={circumflex over
(m)}/(.DELTA.fN.sub.IFFT). (17)
[0163] Low-Coherence Case
[0164] The coherent addition of signals in (11) should not be done
when the coherence time is low. This coherence time is depending on
the rate of time variation of all distortions between baseband
transmitter and receiver. For example, a high Doppler spread will
lead to a fast time-varying channel which decreases the coherence
time. Also, large frequency errors or large phase noise leads to a
decreased coherence time such that the time should be reduced for
which the coherent addition is done.
[0165] Instead of adding all FFT windows coherently as in (11), a
smaller number of FFT windows might be added, i.e.,
C v ( k , a , c ) = p = p 0 p 0 + P - 1 W coh ( p , c ) C MF , v (
k , p , a ) ( 18 ) ##EQU00014##
where W.sub.coh(p, c), c=0, . . . , N.sub.c-1 is used to control
the coherence time. See for example FIGS. 18 and 19, where only two
FFT windows are coherently added
C v ( k , a , c ) = p = 1 + 2 c 2 + 2 c C MF , v ( k , p , a ) ( 19
) ##EQU00015##
before the IFFT
c v ( m , a , c ) = 1 N IFFT k = 0 N seq - 1 C v ( k , a , c ) e j
2 .pi. km / N IFFT . ( 20 ) ##EQU00016##
[0166] Note that FIG. 18 is an illustration where a few more
repetitions of the short sequence are used as compared to FIG. 19.
The decision variable can now be formulated as
.lamda. v ( m ) = c = 0 N c - 1 a = 0 N a - 1 c v ( m , a , c ) 2
.sigma. ^ w 2 ( a ) . ( 21 ) ##EQU00017##
[0167] Thus, according to an aspect, the preamble receiver
comprises a preamble detector arranged to determine a plurality of
FFTs from a plurality of FFT windows, and to non-coherently combine
the FFT results into a combined received preamble signal.
[0168] Frequency-Domain Beam-Forming
[0169] A beamforming gain can be achieved if several antenna
signals are coherently added with individual scaling and phase
shifts. For frequency-domain beamforming, these scaling and phase
shifts are applied after the FFT, see illustration in FIG. 20.
Here, the signals from the antennas are connected to a Radio Unit
(RU) followed by an ADC and FFT. Frequency-domain signals from many
antennas after the FFT are then combined in a Beamforming (BF). In
this way, the beamforming can be different for different
sub-carriers. For example, one or several PRACH specific
beamformings can be applied to those sub-carriers which are used
for PRACH. By these beamformings, the PRACH preamble detector is
sensitive in several spatial directions.
[0170] Denote the PRACH preamble in the frequency domain after
extracting sub-carriers corresponding to those sub-carriers used
for PRACH as
R.sub.PRACH(k,p,a)=R(k+k.sub.0,p,a). (22)
[0171] The beamformed signal for beam number b, with the
beamforming weights and phase shift factors denoted by W.sub.BF (a,
k, b) for sub-carrier k and antenna a, can be written as
R BF , PRACH ( k , p , b ) = a = 0 N a - 1 W BF ( a , k , b ) R
PRACH ( k , p , a ) . ( 23 ) ##EQU00018##
[0172] This beamformed signal is multiplied with a matched filter
(of N.sub.seq coefficients) in the frequency domain
C MF , v ( k , p , b ) = 1 N p P v * ( k , p ) R BF , PRACH ( k , p
, b ) . ( 24 ) ##EQU00019##
[0173] Here, the beamforming and the matched filtering can be done
simultaneously in a single multiplication, i.e.,
C BFMF , v ( k , p , b ) = a = 0 N a - 1 W BFMF ( k , p , a , b ) R
PRACH ( k , p , a ) . ( 25 ) where W BFMF , v ( k , p , a , b ) = 1
N p W BF ( a , k , b ) P v * ( k , p ) ( 26 ) ##EQU00020##
which can be precalculated and stored in memory 2744.
[0174] The output from the matched filters corresponding to the
same beamforming, but from different FFT windows, can now be
coherently added as
C BFMF , v ( k , b ) = p = p 0 p 0 + P - 1 C BFMF , v ( k , p , b )
( 27 ) ##EQU00021##
where p.sub.0 is the index of the first FFT window included in the
PRACH preamble detector.
[0175] Now, in order to detect preamble and estimate round-trip
time, the output from the IFFT will be transformed to the time
domain. Calculate an IDFT, of size N.sub.IFFT, resulting in an
correlation vector of length N.sub.IFFT
c BFMF , v ( m , b ) = 1 N IFFT k = 0 N seq - 1 C BFMF , v ( k , b
) e j 2 .pi. km / N IFFT . ( 28 ) ##EQU00022##
Selecting N.sub.IFFT>N.sub.p corresponds to an interpolation
which can be done in order to increase the resolution of the timing
estimation.
[0176] A simple noise variance {circumflex over
(.sigma.)}.sub.w.sup.2(b) can be estimated as
.sigma. ^ w 2 ( b ) = 1 PN seq p = p 0 p 0 + P - 1 k = 0 N seq - 1
C MF , v ( k , p , b ) 2 . ( 29 ) ##EQU00023##
[0177] As decision variable, an absolute square of each value of
the cross-correlation vector is used, normalized with the estimated
noise variance {circumflex over (.sigma.)}.sub.w.sup.2(b),
.lamda. v ( m , b ) = c v ( m , b ) 2 .sigma. ^ w 2 ( b ) . ( 30 )
##EQU00024##
[0178] Here several polarizations might be added into the decision
variable. A preamble detector and round-trip time estimator might
be formulated as searching for the maximum value in this vector of
normalized absolute squared correlations and comparing this maximum
value with a threshold.
[0179] Preamble number v is detected if the absolute squared value
of this autocorrelation exceeds a threshold
.lamda. v ( m , b ) = c v ( m , b ) 2 .sigma. ^ w 2 ( b ) .gtoreq.
.lamda. Threshold ( 31 ) ##EQU00025##
for at least one value of m, within the search window of size D. In
other words, the preamble with index v is detected if there is an
m.di-elect cons.[0, D-1] such that .lamda..sub.v(m,
b).gtoreq..lamda..sub.Threshold. This preamble detector threshold
.lamda..sub.Threshold should be selected with care such that the
false detection rate is low without causing a too low detection
rate.
[0180] Time-Domain Beam-Forming
[0181] For time-domain beamforming, the beamforming scaling and
phase shifts are applied 2131 before the FFT 2108, see illustration
in FIG. 21. Here, the signals 2105 from the antennas are connected
to a Radio Unit, RU, 2106 and an Analog-to-Digital Converter, ADC
2107 followed by a beamforming, BF, 2131, after which the output
from the beamforming is processed in an FFT 2108. Time-domain
signals from many antennas are thus combined in the beamforming. In
this way, the beamforming is the same for all sub-carriers. This
beamforming might be done on a digital signal, i.e., after the
analog-to-digital converter, ADC, 2107 as in FIG. 21 or on an
analog signal, i.e., before ADC 2207, as in FIG. 22.
[0182] At initial access, the eNodeB has limited knowledge of the
position of the UE. The PRACH receiver must therefore evaluate
several beamformings in order to be able to detect the PRACH
preamble. With time-domain beamforming, this requires one sequence
of processing from FFT to preamble detector per beamforming, see
FIG. 21 or 22. The beamforming and FFT support is costly in terms
of hardware support and power consumption.
[0183] An illustration 2332 is given in FIG. 23 of an approach in
which the beamforming is changed between each FFT window. Here the
outputs from each beamforming, followed by an FFT, are individually
processed in a matched filter, an IFFT, an absolute square
calculation, and finally a preamble detector. If the hardware
supports several simultaneous beamformings in the same time-window,
then several spatial directions can be processed, see FIG. 21 or
22. Each such beamforming is referred to as one baseband, BB,
port.
[0184] An alternative configuration is illustrated 2433 in FIG. 24.
Here, one BB port is used with a fixed time domain beamforming for
all time windows of a sub-frame. A second BB port is used to switch
beamformings between each window.
[0185] Typically, the number of FFT windows for which the analog
beamforming is constant equals the number of FFT windows which are
included in the same PRACH preamble detection. The number of
included FFT windows for a given beam improves the performance of
the PRACH preamble detection in terms of improved detection
rate.
[0186] For UEs with high SNR, i.e., typically located close to the
eNodeB, a reliable detection can thus be done with a small number
of FFT windows, while UEs with a low SNR, typically located further
away from the eNodeB, can in most cases only be done if many, or
all, FFT windows are included. By combining detectors with few FFT
windows included, i.e., with many different beamforming directions,
and detectors with many FFT windows but few beamforming directions,
a balance can be achieved between fast preamble detectors with high
SNR and slow detection for UEs with low SNR. That is, many PRACH
occasions might be needed for UEs with low SNR. This since the
baseband ports with many FFT windows included do not search all
PRACH directions during each PRACH occasion.
[0187] Hence, the present teaching facilitates utilizing the
multiple base band ports for different ranges of preamble
detection. In FIG. 25, an example is shown where port 0 uses a
single FFT window but scans all 12 beams in this example, which
point in unique directions in elevation and/or azimuth. Port 1 on
the other hand scans every other beam while using double FFT
windows. Port 2 scans every fourth beam using four FFT window
aggregations. If the beams are narrow, there may be a risk that a
UE is located between two scanned beams, which lead to a large SNR
loss which cannot be recovered by the doubling of FFT windows. So
there is a trade off in down selection of the number of used beams
in the preamble search procedure and the increased number of FFT
windows. To mitigate this in this example, port 3 uses also every
fourth beam and four FFT windows but where the beam pointing
directions are interlaced with the beams used for port 2. Hence it
is part of the present disclosure that different base band ports
scan interlaced beams.
[0188] Note that it is possible to use a single index to indicate
beams, whereas in reality the beam can in general be pointed in
both azimuth and elevation directions.
[0189] Consequently, according to an aspect, the preamble receiver
disclosed herein is arranged for beamforming, wherein the
beamforming weights are configured to change between FFT windows
such that the number of spatial directions for which preamble
detection is done is increased.
[0190] Also, the preamble receiver, according to aspects, comprises
hardware support for more than one simultaneous analog beamforming
configuration, the preamble receiver being adapted for switching at
a first switching rate between spatial directions with one
beamforming configuration, and for switching at a second switching
rate between spatial directions with another beamforming
configuration, the first rate being different from the second
rate.
[0191] Further, according to aspects, a single IFFT is applied per
beam direction and polarization, and the preamble receiver can also
be arranged to perform simultaneous beam-forming and matched
filtering by a comprised joint filter.
[0192] There is further disclosed herein a preamble transmitter and
receiver system, comprising at least one preamble transmitter
according to the present teaching, and at least one preamble
receiver according to the present teaching.
[0193] FIG. 26 shows a preamble transmitter 2636 arranged to
construct a preamble signal by a preamble sequence generator 2637
connected to a memory unit 2638, and also to transmit the generated
preamble signal via a communications interface 2639a, 2639b of the
preamble transmitter 2636. The preamble transmitter 2636 is,
according to an aspect, a UE in an LTE network.
[0194] FIG. 27 shows a network node 2740 comprising a preamble
receiver 2741 arranged to receive a radio signal from a preamble
transmitter and to detect a preamble signal comprised in the radio
signal. The preamble receiver 2741 is connected to a communications
interface 2742 comprised in the network node 2740, and to a
controller unit 2743 adapted to perform method steps of the present
teaching. The network node 2740 further comprises a memory unit
2744.
[0195] Also, the present disclosure comprises a network node 2740
comprising the preamble receiver 2741 according to the present
teaching.
[0196] FIGS. 28-30 are flowcharts illustrating embodiments of
method steps, which method steps will now be further detailed.
[0197] FIG. 28 illustrates a method performed in a preamble
transmitter 2636 for transmitting a preamble sequence, the method
comprising the steps of [0198] generating S11 a short sequence
s(n), the short sequence having the same time duration as an OFDM
symbol used for carrying data traffic in a radio access network of
the preamble transmitter 2636, [0199] constructing S12 a preamble
sequence by concatenating a plurality of said short sequences in
time, and [0200] transmitting S13 the constructed preamble sequence
as a radio signal to a preamble receiver.
[0201] FIG. 29 illustrates a method performed in a preamble
receiver 2741, 1447 for receiving a radio signal and detecting a
preamble sequence comprised in the radio signal, the method
comprising the steps of [0202] receiving S21 a radio signal
comprising a preamble signal constructed from a plurality of short
sequences s(n) via at least one antenna element 1410 and radio unit
1406, the short sequence s(n having the same time duration as an
OFDM symbol used for carrying data traffic in a radio access
network of the preamble transmitter 2636, [0203] performing S22
analog to digital conversion of the radio signal by an ADC 1407
comprised in the preamble receiver, and [0204] determining S23 a
Fast Fourier Transform, FFT, 1408 of the analog to digital
converted signal, wherein the size of the FFT used for detecting
the preamble signal is of the same size as for detecting an OFDM
symbol used for carrying data traffic in a radio access network, as
well as [0205] detecting S24 the preamble sequence based on the
determined FFT.
[0206] According to an aspect, the step of determining S23 an FFT
further comprises determining an FFT having a single configuration
for detecting OFDM symbols carrying data, and also for detecting
preamble sequences.
[0207] According to another aspect, the step of detecting S24
comprises determining a plurality of FFTs from a plurality of FFT
windows, and also non-coherently combining the FFT results into a
combined received preamble signal.
[0208] According to a further aspect, the step of detecting S24
comprises determining a plurality of FFTs from a plurality of FFT
windows, and also coherently combining the FFT results into a
combined received preamble signal.
[0209] In other words, there is disclosed herein:
A preamble transmitter 2636 comprising: [0210] a short sequence
generator 2650 arranged to generate a short sequence s(n), and
[0211] a preamble sequence generator 2637 adapted to construct a
preamble sequence by concatenating a plurality of said short
sequences in time, as well as [0212] a transmitter unit 2639a,b
arranged to transmit the generated preamble sequence as a radio
signal, the short sequence s(n) having the same time duration as an
OFDM symbol used for carrying data traffic in a radio access
network of the preamble transmitter 2636.
[0213] A preamble transmitter 2636 arranged to transmit the
preamble sequence over a Physical Random Access Channel, PRACH, of
a radio access network.
[0214] A preamble transmitter 2636, wherein a short sequence used
to construct the preamble sequence is arranged as cyclic prefix for
neighboring short sequences.
[0215] A preamble receiver 2741, 1447, arranged to receive radio
signals 1405 comprising a preamble sequence, the preamble receiver
2741, 1447 comprising: [0216] at least one antenna element 1410 and
corresponding radio unit 1406, configured to receive a radio
signal, and [0217] at least one analog to digital converter, ADC,
1407 configured to perform analog to digital conversion of the
received radio signal, as well as [0218] at least one FFT module
1408 arranged to determine a Fast Fourier Transform, of the analog
to digital converted signal, and also [0219] at least one detector
1428 adapted to detect the preamble sequence based on the
determined FFT, the preamble sequence comprising a concatenation in
time of a plurality of short sequences s(n), wherein each such
short sequence s(n) having the same time duration as an OFDM symbol
used for carrying data traffic in a radio access network of the
preamble receiver 2741, 1447, wherein the size of the FFT used for
detecting the preamble signal is of the same size as one used for
detecting an OFDM symbol carrying data traffic in a radio access
network.
[0220] A preamble receiver 2741, 1548, arranged to use one FFT
hardware resource and FFT configuration both for detecting OFDM
symbols carrying data, and also for detecting preamble
sequences.
[0221] A preamble receiver 2741, 1849, wherein the preamble
receiver comprises a preamble detector arranged to determine a
plurality of FFTs from a plurality of FFT windows, and to
non-coherently combine the FFT results into a combined received
preamble signal.
[0222] A preamble receiver 2741, 1548, wherein the preamble
receiver comprises a preamble detector arranged to determine a
plurality of FFTs from a plurality of FFT windows, and to
coherently combine the FFT results into a combined received
preamble signal.
[0223] A preamble receiver 2741, 1548, wherein a preamble sequence
is used for any of initial access, handover, scheduling request,
and resynchronization.
[0224] A preamble receiver arranged for beamforming, wherein the
beamforming weights are configured to change between FFT windows
such that the number of spatial directions for which preamble
detection is done is increased.
[0225] A preamble receiver, wherein the preamble receiver comprises
hardware support for more than one simultaneous analog beamforming
configuration, the preamble receiver being adapted for switching at
a first switching rate between spatial directions with one
beamforming configuration, and for switching at a second switching
rate between spatial directions with another beamforming
configuration, the first rate being different from the second
rate.
[0226] A preamble receiver, wherein a single IFFT is applied per
beam direction and polarization.
[0227] A preamble receiver, arranged to perform simultaneous
beam-forming and matched filtering by a comprised joint filter.
[0228] A preamble transmitter and receiver system, comprising at
least one preamble transmitter according to the present teaching,
and at least one preamble receiver according to the present
teaching.
[0229] A network node 2740 comprising the preamble receiver 2741
according to the present teaching.
[0230] A method performed in a preamble transmitter 2636 for
transmitting a preamble sequence, the method comprising the steps
of [0231] generating S11 a short sequence s(n), the short sequence
having the same time duration as an OFDM symbol used for carrying
data traffic in a radio access network of the preamble transmitter
2636, [0232] constructing S12 a preamble sequence by concatenating
a plurality of said short sequences in time, and [0233]
transmitting S13 the constructed preamble sequence as a radio
signal to a preamble receiver.
[0234] A method performed in a preamble receiver 2741, 1447 for
receiving a radio signal and detecting a preamble sequence
comprised in the radio signal, the method comprising the steps of
[0235] receiving S21 a radio signal comprising a preamble signal
constructed from a plurality of short sequences s(n via at least
one antenna element 1410 and radio unit 1406, the short sequence
s(n having the same time duration as an OFDM symbol used for
carrying data traffic in a radio access network of the preamble
transmitter 2636, [0236] performing S22 analog to digital
conversion of the radio signal by an ADC 1407 comprised in the
preamble receiver, and [0237] determining S23 a Fast Fourier
Transform, FFT, 1408 of the analog to digital converted signal,
wherein the size of the FFT used for detecting the preamble signal
is of the same size as for detecting an OFDM symbol used for
carrying data traffic in a radio access network, as well as [0238]
detecting S24 the preamble sequence based on the determined
FFT.
* * * * *