U.S. patent application number 11/403469 was filed with the patent office on 2006-08-17 for soft handoff for ofdm.
This patent application is currently assigned to NORTEL NETWORKS LIMITED. Invention is credited to Ming Jia, Jianglei Ma, Wen Tong, Dong-Sheng Yu, Peiying Zhu.
Application Number | 20060182063 11/403469 |
Document ID | / |
Family ID | 28452380 |
Filed Date | 2006-08-17 |
United States Patent
Application |
20060182063 |
Kind Code |
A1 |
Ma; Jianglei ; et
al. |
August 17, 2006 |
Soft handoff for OFDM
Abstract
The present invention relates to soft handoffs in an OFDM
system. Each mobile terminal measures pilot signal strengths of
transmissions from adjacent base stations. If the pilot signal
strength for a base station exceeds the defined threshold, that
base station is added to an active set list. Each mobile terminal
notifies the base stations of their active set lists. By providing
the set list to the base station controller and the servicing base
station, the mobile terminal identifies the sole servicing base
station or triggers a soft handoff mode when multiple base stations
appear on the active set list. The soft handoff mode uses a
combination of scheduling and space-time coding to affect efficient
and reliable handoffs.
Inventors: |
Ma; Jianglei; (Kanata,
CA) ; Jia; Ming; (Kanata, CA) ; Zhu;
Peiying; (Kanata, CA) ; Tong; Wen; (Ottawa,
CA) ; Yu; Dong-Sheng; (Ottawa, CA) |
Correspondence
Address: |
WITHROW & TERRANOVA, P.L.L.C.
P.O. BOX 1287
CARY
NC
27512
US
|
Assignee: |
NORTEL NETWORKS LIMITED
St. Laurent
CA
|
Family ID: |
28452380 |
Appl. No.: |
11/403469 |
Filed: |
April 13, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10104399 |
Mar 22, 2002 |
7042858 |
|
|
11403469 |
Apr 13, 2006 |
|
|
|
Current U.S.
Class: |
370/331 ;
370/208; 370/395.4 |
Current CPC
Class: |
H04L 5/0044 20130101;
H04L 5/0048 20130101; H04L 27/2613 20130101; H04L 25/0232 20130101;
H04L 5/0037 20130101; H04B 7/022 20130101; H04L 1/0618 20130101;
H04W 36/18 20130101; H04L 5/0023 20130101; H04L 27/2655 20130101;
H04L 27/2647 20130101; H04L 5/0007 20130101; H04L 5/0025 20130101;
H04L 27/2626 20130101 |
Class at
Publication: |
370/331 ;
370/208; 370/395.4 |
International
Class: |
H04L 12/56 20060101
H04L012/56; H04Q 7/00 20060101 H04Q007/00; H04J 11/00 20060101
H04J011/00; H04L 12/28 20060101 H04L012/28 |
Claims
1. An orthogonal frequency division multiplexing (OFDM) system
comprising: a) a base station controller adapted to schedule data
for a mobile terminal during a soft handoff mode and deliver at
least a portion of scheduled data for the mobile terminal to at
least one of a plurality of base stations; and b) the plurality of
base stations operatively associated with the base station
controller, each base station participating in the soft handoff
adapted to: i) receive the scheduled data for delivery to the
mobile terminal; ii) provide space-time coding for the scheduled
data to generate a plurality of space-time coded signals; iii)
perform an Inverse Fourier Transform (IFT) on each of the plurality
of space-time coded signals to generate a plurality of OFDM
signals, each of the plurality of space-time coded signals
configured to result in corresponding OFDM signals mapped into
defined sub-bands in an OFDM spectrum of sub-bands, the defined
sub-bands not used by other ones of the base stations participating
in the soft handoff; and iv) transmit the plurality of OFDM signals
with spatial diversity for reception by the mobile terminal.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to wireless communications,
and in particular to facilitating soft handoffs in orthogonal
frequency division multiplexing (OFDM) systems.
BACKGROUND OF THE INVENTION
[0002] Wireless communication systems divide areas of coverage into
cells, each of which is served by a base station. A mobile terminal
will continuously monitor the signal strengths of the servicing
base station for the current cell as well as for adjacent cells.
The mobile terminal will send the signal strength information to
the network. As the mobile terminal moves toward the edge of the
current cell, the servicing base station will determine that the
mobile terminal's signal strength is diminishing, while an adjacent
base station will determine the signal strength is increasing. The
two base stations coordinate with each other through the network,
and when the signal strength of the adjacent base station surpasses
that of the current base station, control of the communications is
switched to the adjacent base station from the current base
station. The switching of control from one base station to another
is referred to as a handoff.
[0003] A hard handoff is a handoff that completely and
instantaneously transitions from a first to a second base station.
Hard handoffs have proven problematic and often result in dropped
calls. CDMA systems incorporate a soft handoff, wherein when the
mobile terminal moves from a first to a second cell, the handoff
process happens in multiple steps. First, the mobile terminal
recognizes the viability of the second base station, and the
network allows both the current and adjacent base stations to carry
the call. As the mobile terminal move closer to the second base
station and away from the first base station, the signal strength
from the first base station will eventually drop below a useful
level. At this point, the mobile terminal will inform the network,
which will instruct the first base station to drop the call and let
the second base station continue servicing the call. Accordingly, a
soft handoff is characterized by commencing communications with a
new base station before terminating communications with the old
base station. Soft handoffs in CDMA systems have proven very
reliable.
[0004] In the ever-continuing effort to increase data rates and
capacity of wireless networks, communication technologies evolve.
Multiple-input-multiple-output (MIMO) orthogonal frequency division
multiplexing (OFDM) systems represent an encouraging solution for
the next generation high-speed data downlink access. A benefit of
such systems is their high spectral efficiency wherein all of the
allocated spectrum can be used by all base stations. The systems
are generally considered to have a frequency reuse factor of one.
Unfortunately, these systems generate strong co-channel
interference, especially at cell borders. Basic frequency reuse-one
planning will lead to very low data rates and a poor quality of
service for mobile terminals at cell borders. Even though data
repetition, re-transmission techniques, and fairness scheduling for
data transmission can be employed, it is difficult to equalize data
rate distribution across the cell. Accordingly, high-speed data
service is severely limited.
[0005] In other technologies, such as CDMA, soft handoffs are used
to enhance service at cell borders. However, a straightforward
extension of soft handoff techniques developed for CDMA systems is
not applicable to the MIMO-OFDM systems, since the separation of
the interference for the OFDM waveform is virtually impossible.
Because different spreading code masking is not available in OFDM
transmission, the destructive interferences between base stations
transmitting the same signal can cause significant degradation of
performance. Accordingly, there is a need for an efficient soft
handoff technique for OFDM systems as well as a need to increase
data rates and reduce interference at cell borders.
SUMMARY OF THE INVENTION
[0006] The present invention relates to soft handoffs in an OFDM
system. In downlink communications, each mobile terminal constantly
measures all of the possible pilot signal strengths of
transmissions from adjacent base stations, identifies the strongest
pilot signals, and compares them against a defined threshold. If
the pilot signal strength for a base station exceeds the defined
threshold, that base station is added to an active set list. Each
mobile terminal will notify the base stations of their active set
lists. If there is only one base station in the active set list,
that base station is singled out to service the mobile terminal. If
there is more than one base station on the active set list, a soft
handoff is enabled between those base stations. The soft handoff
condition will continue until only one base station is on the
active set list, wherein the lone base station will continue to
serve the mobile terminal. The soft handoff can be initiated by the
mobile terminal, which will report the active set list to the base
station controller via the servicing base station. The base station
controller will alert the base stations on the active set list of
the soft handoff. Notably, the base station controller can select a
sub-set of the base stations from the active set list to establish
the soft hand off. During soft handoff, all base stations on the
active set list will facilitate communications with the mobile
terminal as defined below. Preferably, the base station controller
keeps track of all of the active set lists for the respective
mobile terminals. The mobile terminals will keep track of their
individual set lists.
[0007] Accordingly, by providing the set list to the base station
controller and the servicing base station, the mobile terminal
identifies the sole servicing base station or triggers a soft
handoff (SHO) mode when multiple base stations appear on the active
set list. The SHO mode uses a combination of scheduling and STC
coding to affect efficient and reliable handoffs. During a SHO
mode, the base station controller either multicasts or
non-multicasts data packets intended for the mobile terminal to
each of the base stations on the active set list. Multicasting
indicates that each data packet is sent to each base station on the
active set list for transmission to the mobile terminal.
Non-multicasting indicates that data packets are divided into to
sub-packets in some manner and each sub-packet is sent to one of
the base stations on the active set list for transmission to the
mobile terminal. Unlike multicasting, redundant information is not
transmitted from each base station on the active set list.
[0008] In either multicasting or non-multicasting embodiments, the
base stations in the active set can partition the time and
frequency resources of the OFDM signal. Accordingly, each base
station transmits part of the OFDM signal sub-band. Preferably, a
boost in transmit power is associated with sub-bands being used.
The base stations provide STC encoding of the transmitted data and
the mobile terminals provide corresponding STC decoding to recover
the transmitted data. The STC coding may be either
space-time-transmit diversity (STTD) or V-BLAST-type coding. STTD
coding encodes data into multiple formats and simultaneously
transmits the multiple formats with spatial diversity (i.e. from
antennas at different locations). V-BLAST t-type coding separates
data into different groups and separately encodes and
simultaneously transmits each group. Other coding will be
recognized by those skilled in the art. The mobile terminal will
separately demodulate and decode the transmitted data from each
base station, and then combine the decoded data from each base
station to recover the original data.
[0009] Prior OFDM handoffs were hard handoffs, and the servicing
base station handled scheduling of data for transmission for any
given mobile terminal autonomously. Since only one base station
served a mobile terminal at any one time, there was no need to
employ joint scheduling. In contrast, the present invention employs
joint scheduling for base stations on the active set list of a
mobile terminal. As such, the base station controller or like
scheduling device is used to schedule data packets for transmission
during the SHO mode for each mobile terminal. Although the base
station controller may provide all scheduling for associated base
stations, the preferred embodiment of the present invention
delegates scheduling of data for mobile terminals that are not in
the SHO mode to the servicing base station. In this arrangement, a
scheduler is employed at the base station controller to assign data
to a time slot for the base stations on the active set list. The
base stations perform joint base station space-time coding. The
time slots not assigned by the base station controller scheduler
are used for data of mobile terminals not participating in a soft
handoff.
[0010] Those skilled in the art will appreciate the scope of the
present invention and realize additional aspects thereof after
reading the following detailed description of the preferred
embodiments in association with the accompanying drawing
figures.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0011] The accompanying drawing figures incorporated in and forming
a part of this specification illustrate several aspects of the
invention, and together with the description serve to explain the
principles of the invention.
[0012] FIG. 1 is a block representation of a cellular communication
system.
[0013] FIG. 2 is a block representation of a base station according
to one embodiment of the present invention.
[0014] FIG. 3 is a block representation of a mobile terminal
according to one embodiment of the present invention.
[0015] FIG. 4 is a logical breakdown of an OFDM transmitter
architecture according to one embodiment of the present
invention.
[0016] FIG. 5 is a logical breakdown of an OFDM receiver
architecture according to one embodiment of the present
invention.
[0017] FIG. 6 is a table illustrating an active set list table
according to one embodiment of the present invention.
[0018] FIG. 7A is a table illustrating round robin scheduling.
[0019] FIG. 7B is a table illustrating flexible scheduling.
[0020] FIGS. 8A-8C are a flow diagram outlining an exemplary
operation of the present invention.
[0021] FIG. 9 is a block representation of a cellular communication
system constructed according to one embodiment of the present
invention.
[0022] FIG. 10 is a diagram of frequency sub-band usage according
to the embodiment of FIG. 9.
[0023] FIG. 11 is a block representation of a cellular
communication system constructed according to one embodiment of the
present invention.
[0024] FIG. 12 is a diagram of frequency sub-band usage according
to the embodiment of FIG. 11.
[0025] FIG. 13 is a diagram illustrating a technique for boosting
the power associated with pilot signals while minimizing co-channel
interference according to one embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] The embodiments set forth below represent the necessary
information to enable those skilled in the art to practice the
invention and illustrate the best mode of practicing the invention.
Upon reading the following description in light of the accompanying
drawing figures, those skilled in the art will understand the
concepts of the invention and will recognize applications of these
concepts not particularly addressed herein. It should be understood
that these concepts and applications fall within the scope of the
disclosure and the accompanying claims.
[0027] With reference to FIG. 1, a base station controller (BSC) 10
controls wireless communications within multiple cells 12, which
are served by corresponding base stations (BS) 14. In general, each
base station 14 will facilitate communications with mobile
terminals 16, which are within the cell 12 associated with the
corresponding base station 14. As a mobile terminal 16 moves from a
first cell 12 to a second cell 12, communications with the mobile
terminal 16 transition from one base station 14 to another. The
term "handoff" is generally used to refer to techniques for
switching from one base station 14 to another during a
communication session with a mobile terminal 16. The base stations
14 cooperate with the base station controller 10 to ensure that
handoffs are properly orchestrated, and that data intended for the
mobile terminal 16 is provided to the appropriate base station 14
currently supporting communications with the mobile terminal
16.
[0028] Handoffs are generally characterized as either hard or soft.
Hard handoffs refer to handoffs where the transition from one base
station 14 to another is characterized by the first base station 14
stopping communications with the mobile terminal 16 at the precise
time when the second base station 14 begins communications with the
mobile terminal 16. Unfortunately, hard handoffs are prone to
dropping communications, and have proven to be sufficiently
unreliable. Soft handoffs are characterized by multiple base
stations 14 simultaneously communicating with a mobile terminal 16
during a handoff period. Typically, the same information is
transmitted to the mobile terminal 16 from different base stations
14, and the mobile terminal 16 attempts to receive signals from
both base stations 14 until the base station 14 to which the mobile
terminal 16 is transitioning is deemed capable of taking over
communications with the mobile terminal 16.
[0029] FIG. 1, a handoff area 18 is illustrated at the junction of
three cells 12, wherein a mobile terminal 16 is at the edge of any
one of the three cells 12 and could potentially be supported by any
of the base stations 14 within those cells 12. The present
invention provides a method and architecture for facilitating soft
handoff in an orthogonal frequency division multiplexing (OFDM)
wireless communication environment.
[0030] A high level overview of the mobile terminals 16 and base
stations 14 of the present invention is provided prior to delving
into the structural and functional details of the preferred
embodiments. With reference to FIG. 2, a base station 14 configured
according to one embodiment of the present invention is
illustrated. The base station 14 generally includes a control
system 20, a baseband processor 22, transmit circuitry 24, receive
circuitry 26, multiple antennas 28, and a network interface 30. The
receive circuitry 26 receives radio frequency signals bearing
information from one or more remote transmitters provided by mobile
terminals 16 (illustrated in FIG. 3). Preferably, a low noise
amplifier and a filter (not shown) cooperate to amplify and remove
broadband interference from the signal for processing. Down
conversion and digitization circuitry (not shown) will then
downconvert the filtered, received signal to an intermediate or
baseband frequency signal, which is then digitized into one or more
digital streams.
[0031] The baseband processor 22 processes the digitized received
signal to extract the information or data bits conveyed in the
received signal. This processing typically comprises demodulation,
decoding, and error correction operations. As such, the baseband
processor 22 is generally implemented in one or more digital signal
processors (DSPs). The received information is then sent across a
wireless network via the network interface 30 or transmitted to
another mobile terminal 16 serviced by the base station 14. The
network interface 30 will typically interact with the base station
controller 10 and a circuit-switched network forming a part of a
wireless network, which may be coupled to the public switched
telephone network (PSTN).
[0032] On the transmit side, the baseband processor 22 receives
digitized data, which may represent voice, data, or control
information, from the network interface 30 under the control of
control system 20, which encodes the data for transmission. The
encoded data is output to the transmit circuitry 24, where it is
modulated by a carrier signal having a desired transmit frequency
or frequencies. A power amplifier (not shown) will amplify the
modulated carrier signal to a level appropriate for transmission,
and deliver the modulated carrier signal to the antennas 28 through
a matching network (not shown). Modulation and processing details
are described in greater detail below.
[0033] With reference to FIG. 3, a mobile terminal 16 configured
according to one embodiment of the present invention is
illustrated. Similarly to the base station 14, the mobile terminal
16 will include a control system 32, a baseband processor 34,
transmit circuitry 36, receive circuitry 38, multiple antennas 40,
and user interface circuitry 42. The receive circuitry 38 receives
radio frequency signals bearing information from one or more base
stations 14. Preferably, a low noise amplifier and a filter (not
shown) cooperate to amplify and remove broadband interference from
the signal for processing. Downconversion and digitization
circuitry (not shown) will then downconvert the filtered, received
signal to an intermediate or baseband frequency signal, which is
then digitized into one or more digital streams.
[0034] The baseband processor 34 processes the digitized received
signal to extract the information or data bits conveyed in the
received signal. This processing typically comprises demodulation,
decoding, and error correction operations, as will be discussed on
greater detail below. The baseband processor 34 is generally
implemented in one or more digital signal processors (DSPs) and
application specific integrated circuit (ASIC).
[0035] For transmission, the baseband processor 34 receives
digitized data, which may represent voice, data, or control
information, from the control system 32, which it encodes for
transmission. The encoded data is output to the transmit circuitry
36, where it is used by a modulator to modulate a carrier signal
that is at a desired transmit frequency or frequencies. A power
amplifier (not shown) will amplify the modulated carrier signal to
a level appropriate for transmission, and deliver the modulated
carrier signal to the antennas 40 through a matching network (not
shown). Various modulation and processing techniques available to
those skilled in the art are applicable to the present
invention.
[0036] In OFDM modulation, the transmission band is divided into
multiple, orthogonal carrier waves. Each carrier wave is modulated
according to the digital data to be transmitted. Because OFDM
divides the transmission band into multiple carriers, the bandwidth
per carrier decreases and the modulation time per carrier
increases. Since the multiple carriers are transmitted in parallel,
the transmission rate for the digital data, or symbols, on any
given carrier is lower than when a single carrier is used.
[0037] OFDM modulation requires the performance of an Inverse Fast
Fourier Transform (IFFT) on the information to be transmitted. For
demodulation, the performance of a Fast Fourier Transform (FFT) on
the received signal is required to recover the transmitted
information. In practice, the Inverse Discrete Fourier Transform
(IDFT) and Discrete Fourier Transform (DFT) are implemented using
digital signal processing for modulation and demodulation,
respectively.
[0038] Accordingly, the characterizing feature of OFDM modulation
is that orthogonal carrier waves are generated for multiple bands
within a transmission channel. The modulated signals are digital
signals having a relatively low transmission rate and capable of
staying within their respective bands. The individual carrier waves
are not modulated directly by the digital signals. Instead, all
carrier waves are modulated at once by IFFT processing.
[0039] In the preferred embodiment, OFDM is used at least for the
downlink transmission from the base stations 14 to the mobile
terminals 16. Further, the base stations 14 are synchronized to a
common clock via GPS signaling and coordinate communications via
the base station controller 10. Each base station 14 is equipped
with n transmit antennas 28, and each mobile terminal 16 is
equipped with m receive antennas 40. Notably, the respective
antennas can be used for reception and transmission using
appropriate duplexers or switches and are so labeled only for
clarity.
[0040] With reference to FIG. 4, a logical OFDM transmission
architecture is provided according to one embodiment. Initially,
the base station controller 10 sends data 44 to be transmitted to a
mobile terminal 16 to the base station 14. The data, which is a
stream of bits, is scrambled in a manner reducing the
peak-to-average power ratio associated with the data using data
scrambling logic 46. A cyclic redundancy check (CRC) for the
scrambled data is determined and appended to the scrambled data
using CRC logic 48. Next, channel coding is performed using channel
encoder logic 50 to effectively add redundancy to the data to
facilitate recovery and error correction at the mobile terminal 16.
The channel encoder logic 50 uses known Turbo encoding techniques
in one embodiment. The encoded data is then processed by rate
matching logic 52 to compensate for the data expansion associated
with encoding.
[0041] Bit interleaver logic 54 systematically reorders the bits in
the encoded data to minimize the loss of consecutive data bits is
provided by. The resultant data bits are systematically mapped into
corresponding symbols depending on the chosen baseband modulation
by mapping logic 56. Preferably, Quadrature Amplitude Modulation
(QAM) or Quadrature Phase Shift Key (QPSK) modulation is used. The
symbols may be systematically reordered to further bolster the
immunity of the transmitted signal to periodic data loss caused by
frequency selective fading using symbol interleaver logic 58.
[0042] At this point, groups of bits have been mapped into symbols
representing locations in an amplitude and phase constellation.
Blocks of symbols are then processed by space-time block code (STC)
encoder logic 60, which modifies the symbols in a fashion making
the transmitted signals more resistant to interference and readily
decoded at a mobile terminal 16. The STC encoder logic 60 will
process the incoming symbols and provide n outputs corresponding to
the number of transmit antennas 28 for the base station 14. The
control system 20 and/or baseband processor 22 will provide a
mapping control signal to control STC encoding. Further detail
regarding the STC encoding is provided later in the description. At
this point, assume the symbols for the n outputs are representative
of the data to be transmitted and capable of being recovered by the
mobile terminal 16. See A. F. Naguib, N. Seshadri, and A. R.
Calderbank, "Applications of space-time codes and interference
suppression for high capacity and high data rate wireless systems,"
Thirty-Second Asilomar Conference on Signals, Systems &
Computers, Volume 2, pp. 1803-1810, 1998, which is incorporated
herein by reference in its entirety.
[0043] For the present example, assume the base station 14 has two
antennas 28 (n=2) and the STC encoder logic 60 provides two output
streams of symbols. Accordingly, each of the symbol streams output
by the STC encoder logic 60 is sent to a corresponding IFFT
processor 62, illustrated separately for ease of understanding.
Those skilled in the art will recognize that one or more processors
may be used to provide such digital signal processing alone or in
combination with other processing described herein. The IFFT
processors 62 will preferably operate on the respective symbols
using IDFT or like processing to effect an inverse Fourier
Transform. The output of the IFFT processors 62 provides symbols in
the time domain. The time domain symbols are grouped into frames,
which are associated with prefix and pilot headers by like
insertion logic 64. Each of the resultant signals is up-converted
in the digital domain to an intermediate frequency and converted to
an analog signal via the corresponding digital up-conversion (DUC)
and digital-to-analog (D/A) conversion circuitry 66. The resultant
(analog) signals are then simultaneously modulated at the desired
RF frequency, amplified, and transmitted via the RF circuitry 68
and antennas 28. Notably, the transmitted data is preceded by pilot
signals, which are known by the intended mobile terminal 16 and
implemented by modulating the pilot header and scattered pilot
sub-carriers. The mobile terminal 16, which is discussed in detail
below, will use the scattered pilot signals for channel estimation
and interference suppression and the header for identification of
the base station 14.
[0044] Reference is now made to FIG. 5 to illustrate reception of
the transmitted signals by a mobile terminal 16. Upon arrival of
the transmitted signals at each of the antennas 40 of the mobile
terminal 16, the respective signals are demodulated and amplified
by corresponding RF circuitry 70. For the sake of conciseness and
clarity, only one of the two receive paths is described and
illustrated in detail. Analog-to-digital (A/D) converter and
down-conversion circuitry 72 digitizes and downconverts the analog
signal for digital processing. The resultant digitized signal may
be used by automatic gain control circuitry (AGC) 74 to control the
gain of the amplifiers in the RF circuitry 70 based on the received
signal level.
[0045] Preferably, each transmitted frame has a defined structure
having two identical headers. Framing acquisition is based on the
repetition of these identical headers. Initially, the digitized
signal is provided to synchronization logic 76, which includes
coarse synchronization logic 78, which buffers several OFDM symbols
and calculates an auto-correlation between the two successive OFDM
symbols. A resultant time index corresponding to the maximum of the
correlation result determines a fine synchronization search window,
which is used by the fine synchronization logic 80 to determine a
precise framing starting position based on the headers. The output
of the fine synchronization logic 80 facilitates frame acquisition
by the frame alignment logic 84. Proper framing alignment is
important so that subsequent FFT processing provides an accurate
conversion from the time to the frequency domain. The fine
synchronization algorithm is based on the correlation between the
received pilot signals carried by the headers and a local copy of
the known pilot data. Once frame alignment acquisition occurs, the
prefix of the OFDM symbol is removed with prefix removal logic 86
and a resultant samples are sent to frequency offset and Doppler
correction logic 88, which compensates for the system frequency
offset caused by the unmatched local oscillators in the transmitter
and the receiver and Doppler effects imposed on the transmitted
signals. Preferably, the synchronization logic 76 includes
frequency offset, Doppler, and clock estimation logic, which is
based on the headers to help estimate such effects on the
transmitted signal and provide those estimations to the correction
logic 88 to properly process OFDM symbols.
[0046] At this point, the OFDM symbols in the time domain are ready
for conversion to the frequency domain using the FFT processing
logic 90. The results are frequency domain symbols, which are sent
to processing logic 92. The processing logic 92 extracts the
scattered pilot signal using scattered pilot extraction logic 94,
determines a channel estimate based on the extracted pilot signal
using channel estimation logic 96, and provides channel responses
for all sub-carriers using channel reconstruction logic 98. The
frequency domain symbols and channel reconstruction information for
each receive path are provided to an STC decoder 100, which
provides STC decoding on both received paths to recover the
transmitted symbols. The channel reconstruction information
provides the STC decoder 100 sufficient information to process the
respective frequency domain symbols to remove the effects of the
transmission channel.
[0047] The recovered symbols are placed back in order using the
symbol de-interleaver logic 102, which corresponds to the symbol
interleaver logic 58 of the transmitter. The de-interleaved symbols
are then demodulated or de-mapped to a corresponding bitstream
using de-mapping logic 104. The bits are then de-interleaved using
bit de-interleaver logic 106, which corresponds to the bit
interleaver logic 54 of the transmitter architecture. The
de-interleaved bits are then processed by rate de-matching logic
108 and presented to channel decoder logic 110 to recover the
initially scrambled data and the CRC checksum. Accordingly, CRC
logic 112 removes the CRC checksum, checks the scrambled data in
traditional fashion, and provides it to the de-scrambling logic 114
for de-scrambling using the known base station de-scrambling code
to recover the originally transmitted data.
[0048] Since OFDM is a parallel transmission technology, the entire
useful bandwidth is divided into many-sub-carriers, which are
modulated independently. A common synchronization channel, a pilot
channel, and a broadcasting channel are multiplexed into the header
of the OFDM symbol in the frequency domain based on the sub-carrier
position. The common synchronization channel is used for initial
acquisition for timing synchronization, frequency and Doppler
estimation, and initial channel estimation.
[0049] In one embodiment, 256 common synchronization sub-carriers
are further divided between the respective transmission paths
wherein each path is associated with 128 common synchronization
sub-carriers, respectively. A common complex PN code of size 256,
which is shared by both transmit paths, is used to modulate the
sub-carriers reserved for the common synchronization channels.
[0050] The pilot channel is used for synchronization, initial
channel estimation, base station identification, and
carrier-to-interference ratio (CIR) measurements for cell (or base
station) selection. In one embodiment, 256 sub-carriers are
reserved for dedicated pilots wherein each transmission path has
128 pilot sub-carriers. A unique complex PN code with length 256 is
assigned to each base station 14 and mapped to these dedicated
pilots. The orthogonality of the PN codes assigned to the different
base stations 14 provides for base station identification and
initial interference measurement.
[0051] In one embodiment, the frame structure has two identical
header symbols at the beginning of every 10 ms frame. The framing
acquisition is based on the repeated headers. When turned on, the
mobile terminal 16 will start the time domain coarse
synchronization processing. A running buffer is used to buffer
several OFDM symbols, and then calculate the auto-correlation
between two successful OFDM symbols. The coarse synchronization
position is the time index corresponding to the maximum output of
the auto-correlations.
[0052] After framing acquisition, only the rough range of the
location of the starting position of the first header symbol is
known. To perform OFDM modulation in the frequency domain, the
starting location of OFDM symbol must be exact so the FFT can
transfer the signals from the time domain to the frequency domain.
Accordingly, the location of the first sample in the first header
of the OFDM symbol is determined. Fine synchronization is based on
the correlation between the pilot data in the headers of the
received signals and a local copy of the known pilot data.
[0053] With regard to channel estimation, each sub-band, which is
represented by a modulated sub-carrier, only covers a small
fraction of the entire channel bandwidth. The frequency response
over each individual sub-band is relatively flat, which makes
coherent demodulation relatively easy. Since the transmission
channel corrupts the transmitted signal in amplitude and phase,
reliable channel knowledge is required to perform coherent
detection. As noted, one embodiment uses a pilot signal for channel
parameter estimation to keep track of channel characteristics
caused by the movement of the mobile terminal 16. Accordingly,
scattered pilot signals are inserted in a regular pattern. The
pilot signals are periodically interpolated to obtain current
channel information required for STC decoding.
[0054] Based on the above, system access is characterized as
follows. Initially, coarse synchronization correlation is performed
based on the preamble header in the time domain to determine a
coarse synchronization location. At the coarse synchronization
location, a fine synchronization search window is identified. An
FFT is computed, and the system switches to the common
synchronization channel to perform fine synchronization within the
fine synchronization search window. Next, the strongest correlation
peaks are identified, and the relevant time index are used as the
candidate timing synchronization positions. An FFT is computed at
each candidate timing synchronization position, and the system
switches to the pilot channel.
[0055] The PN sequences for all base stations 14 are correlated,
and correlation peaks are selected to define an index corresponding
to all candidate timing synchronization positions. The CIRs for
these base stations 14 are identified. The base station with
highest CIR is selected as the serving base station, and the base
stations 14 with CIRs greater than a given threshold are also
selected for the active set list. If more than one base station 14
is on the active set list, the soft handoff procedures of the
present invention are initiated. The FFT is then computed and the
fine synchronization is provided using the PN code for each of the
selected base station(s) 14.
[0056] During operation, the transmitter architecture of the mobile
terminal 16 will facilitate system access as follows. In general,
downlink communications from a base station 14 to a mobile terminal
16 are initiated by the mobile terminal 16. Each mobile terminal 16
constantly measures all of the possible pilot signal strengths of
transmissions from adjacent base stations 14, identifies the
strongest pilot signals, and compares them against a defined
threshold. If the pilot signal strength for a base station 14
exceeds the defined threshold, that base station 14 is added to an
active set list. Each mobile terminal 16 will notify the base
stations 14 of their active set lists. If there is only one base
station 14 in the active set list, that base station 14 is singled
out to service the mobile terminal 16. If there is more than one
base station 14 on the active set list, a soft handoff is enabled
between those base stations 14. The soft handoff condition will
continue until only one base station 14 is on the active set list,
wherein the lone base station 14 will continue to serve the mobile
terminal 16. During soft handoff, all base stations 14 on the
active set list will facilitate communications with the mobile
terminal 16 as defined below. Preferably, the base station
controller 10 keeps track of all of the active set lists for the
respective mobile terminals 16. The mobile terminals 16 will keep
track of their individual set lists.
[0057] Accordingly, by providing the set list to the base station
controller 10 and the servicing base station 14, the mobile
terminal 16 identifies the sole servicing base station 14 or
triggers a soft handoff (SHO) mode when multiple base stations
appear on the active set list. The SHO mode uses a combination of
scheduling and STC coding to affect efficient and reliable
handoffs. During a SHO mode, the base station controller 10 either
multicasts or non-multicasts data packets intended for the mobile
terminal 16 to each of the base stations 14 on the active set list.
Multicasting indicates that each data packet is sent to each base
station 14 on the active set list for transmission to the mobile
terminal 16. Non-multicasting indicates that data packets are
divided into to sub-packets in some manner and each sub-packet is
sent to one of the base stations 14 on the active set list for
transmission to the mobile terminal 16. Unlike multicasting,
redundant information is not transmitted from each base station 14
on the active set list.
[0058] In either multicasting or non-multicasting embodiments, the
base stations 14 provide STC encoding of the transmitted data and
the mobile terminals 16 provide corresponding STC decoding to
recover the transmitted data. The STC coding may be either
space-time-transmit diversity (STTD) or V-BLAST-type coding. STTD
coding encodes data into multiple formats and simultaneously
transmits the multiple formats with spatial diversity (i.e. from
antennas 28 at different locations). V-BLAST-type coding separates
data into different groups and separately encodes and
simultaneously transmits each group with spatial diversity. Other
coding will be recognized by those skilled in the art. The mobile
terminal 16 will separately demodulate and decode the transmitted
data from each base station 14, and then combine the decoded data
from each base station 14 to recover the original data.
[0059] The following illustrates an exemplary process for
identifying base stations 14 to place in the active set list,
scheduling of data at the base stations 14, and STC coding for
transmission of scheduled data from the base stations 14 to the
mobile terminals 16.
[0060] For a multiple-input-multiple-output (MIMO) OFDM system as
illustrated in FIG. 1, the pilot signal is embedded in the preamble
of each frame for each base station 14. The mobile terminal 16 can
identify each base station 14 based on the pseudo-noise sequence of
the pilot signal. The mobile terminal 16 is able to measure the
carrier-to-interference ratio (CIR) based on the pilot signal for
each adjacent base station 14. Based on the strength of the pilot
signal, the mobile terminal 16 can determine the active set list.
If more than one base station 14 is on the active set list, the
mobile terminal 16 will trigger SHO procedure through the uplink
signaling with the base station 14, which will communicate the same
to the base station controller 10.
[0061] With reference to FIG. 6, an exemplary active set list for a
communication environment is shown. Assume that a single base
station controller 10 controls the operation of nine base stations,
BS1-BS9. Further assume that there are fifteen mobile terminals 16
identified as mobile terminals A-O within the communication
environment, and that all of the mobile terminals (A-O) are in
handoff areas from which service may be provided by two or three of
the base stations BS1-BS9. The shaded areas of the active set list
tables identify the active set lists of base stations BS1-BS9 for
each of the mobile terminals A-O. In the present example, mobile
terminals A, B, F, G, K, and L are involved in a two-way SHO
procedure wherein two of the base stations BS1-BS9 are on the
corresponding mobile terminals' active set lists. Similarly, mobile
terminals C, D, E, H, I, J, M, N, and O are in a three-way SHO
procedure, wherein three of the base stations BS1-BS9 are on the
corresponding mobile terminals' active set lists. For example, the
active set list of mobile terminal B identifies base stations BS3
and BS5 and the active set list for mobile terminal H identifies
base stations BS1, BS6, and BS7. As noted, once these mobile
terminals A-O determine that there are multiple base stations
BS1-BS9 on the active set list, the mobile terminal 16 will trigger
a SHO procedure through uplink signaling with its currently
servicing base station 14. The base station 14 will alert the base
station controller 10, which will begin the SHO procedure.
[0062] Prior OFDM handoffs were hard handoffs, and the servicing
base station 14 handled scheduling of data for transmission for any
given mobile terminal 16 autonomously. Since only one base station
14 served a mobile terminal 16 at any one time, there was no need
to employ joint scheduling. In contrast, the present invention
employs joint scheduling for base stations 14 on the active set
list of a mobile terminal 16. As such, the base station controller
10 and not the serving base station 14 is used to schedule data
packets for transmission during the SHO mode for each mobile
terminal 16. Although the base station controller 10 may provide
all scheduling for associated base stations 14, the preferred
embodiment of the present invention delegates scheduling of data
for mobile terminals 16 that are not in the SHO mode to the
servicing base station 14.
[0063] In order to minimize the complexity of the system, the base
station controller 10 classifies the active mobile terminals 16
into two categories: (1) SHO and (2) non-SHO. For a non-SHO mobile
terminal 16, each base station 14 will schedule packet
transmissions independently based on the channel quality reported
at that particular base station 14 by the mobile terminal 16. For
example, the scheduling may be based on maximum CIR scheduling,
round robin scheduling, or any other scheduling provision known in
the art. For a SHO mobile terminal 16, the base station controller
10 may use a simple round robin scheduler and may either multicast
or non-multicast the packets to the base stations 14 on the active
set list at a given time slot.
[0064] For multicast, each data packet is sent to each base station
14 on the active set list for transmission to the mobile terminal
16. For non-multicast, data packets are divided into to sub-packets
in some manner and each sub-packet is sent to one of the base
stations 14 on the active set list for transmission to the mobile
terminal 16. In the latter case, there is no redundancy among the
bases stations 14. Each base station 14 sends a unique piece of the
data being transmitted. When SHO-mode scheduling is not required,
the serving base stations 14 will schedule and transmit data to
mobile terminals 16 in the non-SHO mode. The round robin scheduling
provided by the base station controller 10 for a mobile terminal 16
in SHO mode can be determined by the ratio of the number of
SHO-mode mobile terminals to the non-SHO-mode mobile terminals 16.
Alternatively, the scheduling may be controlled to maximize
capacity, minimize delay, etc. The packet transmission for a SHO
mode can be signaled via fast downlink signaling.
[0065] An exemplary round robin scheduling technique for the base
station controller 10 is illustrated in FIG. 7A in light of the
active set list information provided in FIG. 6. As depicted,
communications between a base station 14 and a mobile terminal 16
are assigned to a given time slot in a scheduling period. The base
station controller 10 schedules communications for designated time
slots for mobile terminals 16 operating in a SHO mode and leaves
the shaded time slots open for traditional, non-SHO mode scheduling
at the respective base stations 14. Accordingly, the base station
controller 10 will schedule data to be sent to each of the base
stations 14 participating in a SHO mode with a given mobile
terminal 16 for a common time slot. For example, data to be
transmitted to mobile terminal I is scheduled for time slot 1 for
base stations BS1, BS6, and BS7. Data to be transmitted to mobile
terminal C is scheduled for time slot 1 and sent to base stations
BS3, BS4, and BS5. Similarly, data to be transmitted to mobile
terminal O is also scheduled for time slot 1 and delivered to base
stations BS2, BS8, and BS9 on its active set list. Thus, data to be
transmitted to a mobile terminal 16 in a SHO mode is scheduled for
a common time slot for each of the base stations 14 in the active
set list. To minimize the processing required for round robin
scheduling, the allocation of time slots for the various mobile
terminals 16 participating in the SHO mode are kept consistent from
one scheduling period to the next until there is a change in the
active set list for one or more of the mobile terminals 16. As
illustrated, the allocation of communications for the mobile
terminals 16 for time slot 1 and 13 are identical, and so on and so
forth. Once the base stations 14 provide the multicasting or
non-multicasting of the SHO mode data, the base stations 14 can
provide scheduling during the shaded time slots for mobile
terminals 16 that are not operating in the SHO mode.
[0066] FIG. 7B illustrates an alternative scheduling arrangement,
wherein the scheduling for SHO mode and non-SHO mode mobile
terminals 16 is not repeated from one scheduling period to another,
but is recomputed and reassigned during each scheduling period.
During time slot 1, data to be transmitted to mobile terminal I is
sent to base stations BS1, BS6, and BS7, wherein data to be
transmitted to mobile terminal L is sent to base stations BS2 and
BS9. Base stations BS3, BS4, BS5, and BS8 are free to schedule data
to non-SHO mode mobile terminals 16. Corresponding time slot 13 in
the subsequent scheduling period does not parallel the allocations
of time slot 1. The base station controller 10 will compute a
different scheduling and slot allocation procedure for the
scheduling period, wherein mobile terminals J and O, which are
operating in the SHO mode, are scheduled to have data transmitted
to base stations BS1, BS6, and BS7, and base stations BS2, BS8, and
BS9, respectively. Those skilled in the art will recognize the
numerous ways to facilitate scheduling for SHO mode terminals via
the base station controller 10 while allocating time slots for the
base stations 14 to provide scheduling for mobile terminals not
operating in a SHO mode.
[0067] Regardless of scheduling techniques, each base station 14 on
the active set will perform the space-time coding at the same time
during the assigned time slot. Accordingly, the mobile terminal 16
will receive the entire space-time coded data packet transmitted
from the multiple base stations 14. The mobile terminal 16 will
separately demodulate and decode the transmitted data from each
base station 14, and then combine the decoded data from each base
station 14 to recover the original data.
[0068] With reference to FIGS. 8A-8C, an exemplary flow of an
active SHO process is described. Initially, a mobile terminal 16
will measure the pilot signal strength of each base station (step
200) and compute the carrier-to-interference ratio (CIR) using
equation 1 (step 202). CIR.sub.0=C/(I.sub.1+I.sub.2+I.sub.3+ . . .
+I.sub.N), Equation 1: wherein C is a measure of the pilot signal
strength of the servicing base station 14 and I.sub.1 through
I.sub.N are measures of the pilot signal strengths for adjacent
base stations 14 (BS1 through BSN). If the computed CIR is greater
than a threshold CIR (Th.sub.0) (step 204), the mobile terminal 16
will maintain the servicing base station 14 in the active set list,
and not add any of the adjacent base stations 14 to the active set
list. Thus, the mobile terminal 16 will receive communications only
from the servicing base station 14 and will not be in a SHO mode
(step 206). If the computed CIR is not greater than the threshold
CIR Th.sub.0, the mobile terminal 16 will compute another CIR using
equation 2 (step 208). CIR.sub.1=(C+I.sub.1)/(I.sub.2+I.sub.3+ . .
. +I.sub.N). Equation 2:
[0069] If CIR.sub.1 is greater than the threshold CIR (step 210),
the mobile terminal 16 will trigger a two-way SHO between the
servicing base station 14 and the adjacent base stations 14 from
which I.sub.1 was measured (step 212). If CIR.sub.1 was not greater
than the threshold CIR (step 210), then the mobile terminal 16
computes another CIR using equation 3 (step 214).
CIR.sub.2=(C+I.sub.1+I.sub.2)/(I.sub.3+ . . . +I.sub.N). Equation
3:
[0070] If CIR.sub.2 is greater than the threshold CIR (step 216),
the mobile terminal 16 will trigger a three-way SHO mode with the
servicing base station 14 and the adjacent base stations 14
associated with I.sub.1 and I.sub.2 (step 218). If CIR.sub.2 is not
greater than the threshold CIR (step 216), the mobile terminal 16
will compute a new CIR according to equation 4 (step 220),
CIR.sub.3=(C+I.sub.1+I.sub.2+I.sub.3)/(I.sub.4+ . . . +I.sub.N),
Equation 4: and the process will continue by adding an adjacent
interference component from adjacent base stations 14 until a
sufficient, combined CIR exceeds the threshold CIR Th.sub.0.
[0071] For the present example, assume that a two-way SHO procedure
was triggered wherein the flow moves to FIG. 8B. Once the mobile
terminal 16 achieves a CIR greater than the threshold CIR, it will
send information identifying the base stations 14 on the active set
list and the calculated CIR to the serving base station 14 (step
222). The serving base station 14 will report the active set list
and the calculated CIR to the base station controller 10 (step
224). The base station controller 10 grants the SHO mode for the
base stations 14 on the active set list or a subset thereof, and
establishes SHO procedure with the appropriate base stations 14
(step 226). The scheduler at the base station controller 10 will
assign time slots for the SHO mode as described above, and will
send data packets to the base stations 14 on the active set list or
a subset thereof (step 228). The base stations on the active set
list will perform the joint space-time coding and transmit data at
slots assigned by the scheduler of the base station controller 10
(step 230).
[0072] Next, the mobile terminal 16 will combine and decode the
signals from the base stations 14 on the active set list, and will
attempt to decode the transmitted data (step 232). The mobile
terminal 16 will then attempt to decode the data received from the
base stations 14 on the active set list (step 234). If the data is
properly decoded (step 236), the mobile terminal 16 will send an
acknowledgement (ACK) to the base stations 14 on the active set
list (step 238).
[0073] If the data is not properly decoded (step 236), the mobile
terminal 16 will send a negative-acknowledgement (NACK) to the base
stations 14 on the active set list (step 240). In response, the
base stations 14 on the active set list will perform joint
space-time coding and re-transmit the data (step 242). The mobile
terminal 16 may then perform an automatic repeat request (ARQ) or
hybrid ARQ (HARQ) soft combining (step 244), and the process will
repeat.
[0074] During the transition to a SHO mode, the servicing base
station 14 will have data that needs to be transmitted and will not
be able to be scheduled for multicast or non-multicast transmission
by the base station controller 10. Accordingly, the servicing base
station 14 must transmit the residual data to the mobile terminal
16 prior to fully entering the SHO mode. In one embodiment, a
single-cast technique is used where the servicing base station 14
transmits the residual data to the mobile terminal 16 and the other
base stations 14 on the active set list do not transmit information
in the channels or bands used by the servicing base station 14.
Additional information on single-casting is provided in greater
detail later in this specification. Referring again to FIG. 8B,
during transition to a SHO mode, the servicing base station 14 will
single-cast data to the mobile terminal 16 wherein the other base
stations on the active set list will not transmit (step 246).
Further, throughout the process of scheduling data for SHO mode
mobile terminals 16, each base station 14 will autonomously
schedule data for non-SHO mode mobile terminals 16 (step 248).
[0075] With reference to FIG. 8C, throughout the process, the
mobile terminal 16 will continue to measure the pilot signal
strength of all the adjacent base stations 14 (step 250) and
calculate CIRs. Accordingly, the mobile terminal 16 may compute the
CIR using equation 2 (step 252), and determine if the resultant CIR
is greater than the threshold CIR Th.sub.0 (step 254). If CIR.sub.1
is greater than threshold CIR Th.sub.0 (step 254), the mobile
terminal 16 will update and report the active set list to the
servicing base station 14 (step 256). Further, the base station
controller 10 will remove base station BS2 from the SHO mode for
the mobile terminal 16 (step 258). The base station BS2 is removed
because the CIR of the servicing base station 14 is sufficient
without use of base station BS2. Accordingly, the process will
continue with step 226 of FIG. 8B.
[0076] If the value of CIR.sub.1 was not greater than threshold CIR
Th.sub.0 (step 254), the mobile terminal 16 will compute CIR using
equation 3 (step 260). If the value of CIR.sub.2 is greater than
threshold CIR Th.sub.0 (step 262), the two-way SHO mode is still
necessary, and the process will continue at step 226 of FIG. 8B. If
the value of CIR.sub.2 is not greater than the threshold CIR
Th.sub.0 (step 262), the mobile terminal 16 will compute the value
of the CIR using equation 4 (step 264). Accordingly, if the value
of CIR.sub.3 is not greater than threshold CIR Th.sub.0 (step 266),
the mobile terminal 16 will compute the value of CIR.sub.4 (step
272), and so on and so forth until a sufficient number of base
stations 14 are added to the active set list to cause the value of
CIR to exceed the threshold CIR Th.sub.0 .
[0077] If the value of CIR.sub.3 is greater than the threshold CIR
Th.sub.0 (step 266), the mobile terminal 16 will update the active
set list to include the base station BS3 associated with I.sub.3
and report the updated active list to the service base station 14
(step 268). At this point, the base station controller 10 will add
the base station BS3 to the SHO mode (step 270), and the process
will continue at step 226 of FIG. 8B.
[0078] The data is transmitted from the base stations 14 to the
mobile terminals 16 using unique space-time coding schemes. The
following outlines two space-time-coding schemes involving
transmission division in the frequency domain at each base station
14. For each scheme, two embodiments are described. FIGS. 9 and 10
illustrate a MIMO-OFDM scheme for a mobile terminal 16 in a
SHO-mode involving three base stations 14 (BS1, BS2, and BS3).
Transmission division in the frequency domain is implemented in
combination with space-time coding at each base station 14. Such
transmission division involves segregating the available OFDM
frequency sub-bands among the participating base stations 14. Each
base station 14 only modulates the data it has been scheduled to
transmit on the corresponding sub-bands. FIG. 10 illustrates the
sub-band mapping among the three base stations 14 (BS1, BS2, and
BS3) for one path of a dual path implementing space-time coding for
a given period of time. The other path will use the same sub-bands,
but implement different coding. The mapping control signal (FIG. 4)
is used to control mapping of the sub-bands. The base stations 14
are coordinated via the base station controller 10 to select
different sub-bands for mapping control and STC encoding, as
described herein, and to control power boosting.
[0079] For the first base station 14 (BS1), the bottom third of the
sub-bands are used to modulate and transmit traffic data wherein
the remaining two-thirds of the sub-bands are unused. Notably, the
pilot signal is scattered throughout the traffic data, but not
throughout the unused sub-bands. For the second base station 14
(BS2), the middle third of the sub-bands are used to modulate and
transmit traffic data wherein the remaining two-thirds of the
sub-bands are unused. For the third base station 14 (BS3), the top
third of the sub-bands are used to modulate and transmit traffic
data wherein the remaining two-thirds of the sub-bands are unused.
For optimal performance, the power is boosted for the active
sub-bands to realize the full power transmission and cut for the
unused bands. Accordingly, the mobile terminal 16 will effectively
receive a different third of the frequency bands from each of the
base stations 14 (BS1, BS2, and BS3) and recover the corresponding
data based on the STC and scheduling parameters. Preferably, the
average power for the entire band remains within defined
limits.
[0080] For non-multicast scheduling, different subpackets are sent
to each base station 14 (BS1, BS2, and BS3), which will organize
the data to effect the frequency division mapping and provide the
space-time coding for two antennas as described above. Accordingly,
each base station 14 is transmitting unique data. Each active
sub-band is power boosted by 10log.sub.10(x) dB, where x is the
number of base stations 14 in SHO mode and is equal to three in
this example. The mobile terminal 16 receives the entire frequency
band, a portion from each base station 14, and performs space-time
decoding to retrieve the packet data.
[0081] For non-multicast scheduling, the same packets are sent to
each base station 14 (BS1, BS2, and BS3), which will organize the
data to effect the frequency division mapping and provide the
space-time coding for two antennas as described above. Accordingly,
each base station 14 is transmitting the same data at the same
time, albeit in different formats. Again, each active sub-band is
power boosted by 10log.sub.10(x) dB. The mobile terminal 16
receives the entire frequency band, a portion from each base
station 14, and performs space-time decoding and diversity combing
to retrieve the packet data. Both of the above options can achieve
SHO gain, which provides CIR improvement, by converting the
transmission power of a SHO base station 14 from interference into
a useful signal. The first option allows high data throughput, but
without macro-diversity combining gain, wherein the second option
yields a lower throughput, but provides macro-diversity gain. In
general, the number of participating base stations 14 in SHO made
can be reduced with the second option. Notably, there are several
possible designs for the sub-band division, which may include
interlacing and the like. Based on the teachings herein, those
skilled in the art will recognize the various combinations to
segregate the sub-bands among the participating base stations
14.
[0082] FIGS. 11 and 12 depict another MIMO-OFDM SHO scheme with
joint base station diversity. In this embodiment, each base station
14 (BS1, BS2, and BS3) is associated with two antennas 28 (.alpha.
and .beta.). Unique to this embodiment is that spatial diversity is
provided across base stations 14. As illustrated, the STC encoding
results in two STC data streams, which are respectively transmitted
from antennas at different base stations 14.
[0083] For non-multicast scheduling, a packet is divided into three
unique sub-packets and sent to the base stations 14 (BS1, BS2, and
BS3), respectively. Base station 14 (BS1) antenna .alpha. and Base
station 14 (BS2) antenna .beta. perform the space-time encoding for
the first sub-packet; base station 14 (BS2) antenna .alpha. and
base station 14 (BS3) antenna a perform the space-time encoding for
the second sub-packet; and base station 14 (BS3) antenna .beta. and
base station 14 (BS1) antenna .beta. perform the space-time
encoding for the third sub-packet. Each antenna pair transmits one
sub-packet, which is mapped onto one-third of the OFDM
time-frequency sub-bands. The remaining two-thirds of the sub-bands
are empty and not used for data transmission. Each transmitted
sub-band is power boosted by 10log.sub.10(x)dB, where x is the
number of base stations 14 in SHO mode and is equal to three in
this example. The mobile terminal 16 receives the entire frequency
band and performs space-time decoding to retrieve the packet
data.
[0084] For non-multicast scheduling, each packet is redundantly
sent to the three base stations 14 (BS1, BS2, and BS3). Base
station 14 (BS1) antenna .alpha. and Base station 14 (BS2) antenna
.beta. perform the space-time encoding for the packet; base station
14 (BS2) antenna .alpha. and base station 14 (BS3) antenna .alpha.
perform the space-time encoding for the packet; and base station 14
(BS3) antenna .beta. and base station 14 (BS1) antenna .beta.
perform the space-time encoding for the packet. Each antenna pair
transmits a copy of the packet, which is mapped onto one-third of
the OFDM time-frequency sub-bands. The remaining two-thirds of the
sub-bands are empty and not used for data transmission. Each
transmitted sub-band is power boosted by 10log.sub.10(x)dB. Again,
x is the number of base stations 14 in SHO mode and is equal to
three in this example. The mobile terminal 16 receives the entire
frequency band and performs space-time decoding to retrieve the
packet data.
[0085] The joint STC scheme of FIG. 11 provides additional
space-time coding gain over that provided in FIG. 9. The above
examples for the MIMO-OFDM SHO space-time coding arrangement can be
easily generalized into 2-way, 3-way and N-way SHO operation.
Because of the frequency division property of OFDM systems, part of
the band can be used for SHO while the remainder of the band is
used for transmitting the data packet to non-SHO users by each base
station 14. This provides more flexibility to the scheduling for
multi-users applications.
[0086] During the transition from a non-SHO mode to a SHO mode, the
base stations 14 will have residual data, which needs to be
transmitted to the mobile terminals 16 and cannot be scheduled at
the base station controller 10. Accordingly, the present invention
uses a single-casting technique, wherein data delivery may be
orchestrated such that only one base station 14 transmits data
during SHO mode on select sub-bands while the other participating
base stations 14 avoid using the sub-bands used by the base station
14 to send the data. In this manner, interference associated with
the sub-bands of the other base stations 14 is minimized. During
single-casting, joint scheduling and processing associated with
combing data received in part or whole from multiple base stations
14 is unnecessary, since the entire data is sent from only one base
station 14. Again, boosting power for the active sub-carriers is
beneficial. Once the residual data has been transmitted to the
mobile terminals 16, the multicasting or non-multicasting for
mobile terminals 16 operating in a SHO mode takes over, wherein the
base station controller 10 schedules data, which is either
multicast or non-multicast, to the base stations 14 on the active
set list, and then transmitted to the mobile terminals 16.
[0087] As noted above, an important element for STC decoding is
accurate channel estimation. The scattered pilot patterns are
designed for the adjacent base station's pilot signal re-use
planning. A scattered pilot pattern can have cyclic layout on the
time-frequency plane. In order to achieve high quality channel
estimation for the space-time decoding, the interference from the
adjacent base stations 14 must be minimized. In one embodiment of
the present invention, power may be boosted for each base station's
scattered pilot singles, while for the same sub-carrier location of
the all the other base stations 14, these sub-carrier transmissions
should be turned off to create a power null as illustrated in FIG.
13. With this arrangement, the scattered pilot sub-carriers are
almost free from the co-channel interference.
[0088] Because the distances between mobile terminals 16 and base
stations 14 are different for each set, there is a relative
transmission delay between the signals from the different base
stations 14. During the base station identification and timing
synchronization stage, the mobile terminal 16 has already measured
the timing synchronization positions corresponding to different SHO
base stations 14 in the active set list. In the SHO mode, the
earliest arrival time from a particular base station 14 is used as
the synchronization position. As a result, only one base station 14
can be in perfect timing synchronization, while the others have
certain time offsets.
[0089] In general, an OFDM signal can tolerate time offsets up to
the difference of the prefix and the maximum channel delay. As long
as the time offset is within this tolerance, the orthogonality of
the sub-channel is preserved. However the time offset will cause an
additional phase rotation, which increases linearly with respect to
the sub-channel index. For non-coherent detection, no channel
information is needed, so the same STC decoding method as used in
the non-SHO mode can be applied by mobile terminal 16, if the
differential encoding direction is performed along time. However,
for coherent detection, accurate channel information is necessary.
The time offset may cause problems during channel
reconstruction.
[0090] Let X,Y,H represent the transmitted signal, received signal
and the channel response in a frequency domain, respectively and
ignore noise. For a 2.times.2 case (two transmit and receive
paths): Y(k)=H(k)X(k) where Y .function. ( k ) = [ Y 1 .function. (
k ) Y 2 .function. ( k ) ] , X .function. ( k ) = [ X 1 .function.
( k ) X 2 .function. ( k ) ] , H .function. ( k ) = [ h 11
.function. ( k ) h 21 .function. ( k ) h 12 .function. ( k ) h 22
.function. ( k ) ] , ##EQU1## and k is the sub-carriers index.
[0091] If there is a time offset, the above relation should be
modified as Y(k)=H'(k)X(k) where: H ' .function. ( k ) = [ h 11 '
.function. ( k ) h 21 ' .function. ( k ) h 12 ' .function. ( k ) h
22 ' .function. ( k ) ] , ##EQU2##
h.sub.ij(k)=h.sub.ij(k).phi..sub.i(k),.phi..sub.i(k)=exp(-i2.pi.k.delta.t-
.sup.i/N.sub.FFT), .phi..sub.i is the additional phase rotation
introduced by the time offset for i.sup.th transmit antenna, and
.delta.t.sup.(i) is the time offset in samples caused by the timing
synchronization error for the signals from i.sup.th transmit
antenna. .delta.t.sup.(i) is known during base station
identification and timing synchronization.
[0092] Theoretically the equivalent channel response H' can be
estimated and compensated with the help of pilot signals. However,
since the channel estimation is based on the scattered pilots, care
must be taken to compensate for relative transmission delay. The
design principle of the density of the scattered pilots is to allow
the reconstruction of the time and frequency varying channel
response. The spacing between pilots in time direction is
determined by the expected maximum Doppler frequency, while the
spacing between pilots in the frequency direction is determined by
the expected delay spread of the multi-path fading channel. The
grid density of the scattered pilot pattern can provide enough
sampling for the reconstruction of the propagation channel through
interpolation. On the other hand, .sigma. varies with the
sub-carrier index, and its variation frequency increases with the
increment of time offset. Therefore, the correlation bandwidth of
the total equivalent channel response H' is determined by both the
multi-path fading channel and the uncorrected time offset. As
mentioned above, there is a time offset for the signals from the
more distant base stations 14 because of the existence of the
relative transmission delay. For example, in a 2.times.2 MIMO-OFDM
system, 4 channels are needed for channel estimation. Two of them
may have relatively large time offsets, and as a result, a fast
additional phase rotation .sigma.. Notably, the time offset will
introduce fast phase rotation. When the variation of .sigma. is
much faster than that of H', the grid density of the scattered
pilots may not satisfy the sampling theorem; therefore, H' cannot
be obtained correctly by interpolation.
[0093] To obtain correct channel information for all the multiple
channels during SHO, a compensation method can be applied. The idea
is that only the propagation channel is interpolated, for the
variation of .sigma. is known. After FFT processing, the received
time domain samples are transferred to frequency domain components.
Then, h.sub.ij.sup.'(k) can be obtained at pilot sub-carriers k.
Before interpolation is used to obtain the channel response for all
the sub-carriers, the contribution from (p can be removed by
multiplying h.sub.ij.sup.'(k) with the conjugate of .phi..sub.i(k),
{overscore (h)}ij(k)=h.sub.ij.sup.'(k).phi..sub.i*(k) It should be
noted that only the channels related to the base station 14 with
time offset should be compensated. After interpolation, the channel
response, {overscore (h)}.sub.ij, of all the useful sub-carriers
are obtained. The total equivalent channel responses h.sub.ij.sup.'
of all the useful sub-carriers are obtained by multiplying
{overscore (h)}.sub.ij with .phi..sub.i.
[0094] In essence, the channel responses for each of the data
sub-carriers of the OFDM signal are compensated for transmission
delays associated with transmission from each of the multiple base
stations 14 participating in the OFDM soft handoff. In general, the
mobile terminal 16 will use the unique PN codes provided in the
preambles of each of the pilot signals from each of the base
stations 14 to determine the relative transmission delays from each
of the base stations 14 participating in the OFDM soft hand-off.
After a fast Fourier transform, the scattered pilot signals of the
OFDM signals are extracted in the frequency domain for each
receiver section. Channel responses for the scattered pilot signals
are estimated for each transmit channel. Any additional phase
rotation caused by the transmission delays from the estimated
channel responses are removed, preferably using the multiplication
techniques described above. At this point, the channel responses
for the scattered pilot signals are known, and are used to
interpolate the channel responses for the data sub-carriers in the
OFDM signal. Once the channel responses for the OFDM data
sub-carriers are estimated, the phase rotation caused by the
transmission delays are added to the channel responses for each of
the OFDM sub-carriers to provide the actual channel estimates to
use during receiving transmissions from the various base stations
14.
[0095] The present invention provides an efficient soft handoff
technique for OFDM systems and improves data rates while minimizing
interference associated with OFDM communications with mobile
terminals at cell borders. Those skilled in the art will recognize
improvements and modifications to the preferred embodiments of the
present invention. All such improvements and modifications are
considered within the scope of the concepts disclosed herein and
the claims that follow.
* * * * *