U.S. patent application number 12/124290 was filed with the patent office on 2008-09-11 for system and method for transmitting datain a multiple-branch transmitter-diversity orthogonal frequency-division multiplexing (ofdm) system.
This patent application is currently assigned to MEDIATEK INC.. Invention is credited to Chao-Ming Chang, Hung-Kun Chen, Mao-Ching Chiu, Charles Huang, Kuo-Hui Li.
Application Number | 20080219148 12/124290 |
Document ID | / |
Family ID | 31190810 |
Filed Date | 2008-09-11 |
United States Patent
Application |
20080219148 |
Kind Code |
A1 |
Li; Kuo-Hui ; et
al. |
September 11, 2008 |
SYSTEM AND METHOD FOR TRANSMITTING DATAIN A MULTIPLE-BRANCH
TRANSMITTER-DIVERSITY ORTHOGONAL FREQUENCY-DIVISION MULTIPLEXING
(OFDM) SYSTEM
Abstract
A system and method for transmitting data in multiple-branch
transmitter-diversity OFDM systems is presented. In one embodiment,
an approach is taken where an inverse Fourier transform (IFT) is
performed on data prior to encoding the data for transmission in
the multiple-branch transmitter-diversity system. In another
embodiment an IFT is performed on data prior to encoding the data
using a space-time block code (STBC) algorithm.
Inventors: |
Li; Kuo-Hui; (Hsinchu,
TW) ; Huang; Charles; (Hsinchu, TW) ; Chiu;
Mao-Ching; (Hsinchu, TW) ; Chen; Hung-Kun;
(Hsinchu, TW) ; Chang; Chao-Ming; (Hsinchu,
TW) |
Correspondence
Address: |
THOMAS, KAYDEN, HORSTEMEYER & RISLEY, LLP
600 GALLERIA PARKWAY, S.E., STE 1500
ATLANTA
GA
30339-5994
US
|
Assignee: |
MEDIATEK INC.
Hsin-Chu
TW
|
Family ID: |
31190810 |
Appl. No.: |
12/124290 |
Filed: |
May 21, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10301046 |
Nov 21, 2002 |
7394754 |
|
|
12124290 |
|
|
|
|
60400888 |
Aug 1, 2002 |
|
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Current U.S.
Class: |
370/210 |
Current CPC
Class: |
H04L 1/0618 20130101;
H04L 27/2626 20130101 |
Class at
Publication: |
370/210 |
International
Class: |
H04J 11/00 20060101
H04J011/00 |
Claims
1. A transmitter comprising: an inverse Fourier transform (IFT)
circuit performing_an IFT on a data block to produce an
inverse-Fourier-transformed data block; and a space-time block-code
(STBC) encoder encoding the inverse-Fourier-transformed data block
for transmission in a multiple-branch transmitter-diversity
system.
2. The system of claim 1, wherein the STBC encoder comprises
circular-shift operation logic performing a circular shift
operation on the inverse-Fourier-transformed data block.
3. The system of claim 1, wherein the STBC encoder encodes the
inverse-Fourier-transformed data block using a STBC algorithm.
4. The system of claim 1, further comprising a wireless
communication device, the wireless communication device housing the
transmitter.
5. In a wireless OFDM communication device employing
multiple-branch transmitter diversity, a method comprising:
performing an inverse Fourier transform on a data block to produce
an inverse-Fourier-transformed data block; and encoding the
inverse-Fourier-transformed data block by using a space-time
block-code algorithm for transmission in a multiple-branch
transmitter-diversity system.
6. The method of claim 5, further comprising: performing a circular
shift operation on the inverse-Fourier-transformed data block.
7. A wireless OFDM communication device employing multiple-branch
transmitter diversity, the wireless communication device
comprising: means for performing an inverse Fourier transform on a
data block to produce an inverse-Fourier-transformed data block;
and means for encoding the inverse-Fourier-transformed data block
by using a space-time block-code algorithm for transmission in a
multiple-branch transmitter-diversity system.
8. The wireless communication device of claim 7, further
comprising: means for performing a circular shift operation on the
inverse-Fourier-transformed data block.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation Application of U.S.
patent application Ser. No. 10/301,046, filed Nov. 21, 2002 and
entitled "SYSTEM AND METHOD FOR TRANSMITTING DATA IN A MULTIPLE
BRANCH TRANSMITTER DIVERSITY OTRHOGONAL FREQUENCY DIVISION
MULTIPLEXING (OFDM) SYSTEM", which claims the benefit of U.S.
provisional patent application Ser. No. 60/400,888, entitled
"System implementation of space-time-coded OFDM," filed on Aug. 1,
2002, which is incorporated herein by reference in its
entirety.
FIELD OF INVENTION
[0002] The present invention relates generally to data
communications and, more particularly, to a system and method for
transmitting data in a multiple-branch transmitter-diversity
orthogonal frequency-division multiplexing (OFDM) system.
BACKGROUND
[0003] Wireless communication is characterized as transmitting
signals between two locations without the use of a connection
cable. In some instances, the two locations may be rather distant
from each other. Wireless communication may provide a convenient
option for communication between such distant locations.
[0004] Equipment for the wireless communication system has
typically been rather expensive. However, recently, equipment costs
have declined due to recent developments in semiconductor
fabrication, wireless communication technology, and digital signal
processing. Thus, applications related to wireless communications
are increasing. For example, wireless local area networks (LAN) has
recently emerged as a viable alternative to traditional
wired-LAN.
[0005] In typical wireless communication systems, data or
information is converted into a radiofrequency (RF) signal prior to
transmission to a remote receiver. The remote receiver receives the
RF signal and processes the received RF signal to recover the
originally transmitted data or information. Many approaches have
been proposed to assure proper transmission and recovery of the
data or information. Systems and methods relating to space-time
block code (STBC) are included among these approaches.
[0006] STBC has been proposed to provide transmit diversity gain to
a receiver with a very simple maximum-likelihood decoding
algorithm. FIG. 5 is a schematic illustration showing the operating
principle of STBC. As shown in FIG. 5, an STBC coder 500 is
associated with two transmission antennas 502, 504 to achieve
transmitter diversity. Once signals have been transmitted from the
two transmission antennas 502, 504, a receiving antenna 506
receives the substantially simultaneously transmitted signals. In
FIG. 5, the signals, channel impulse responses, and noise are shown
in the time domain, which are conventionally denoted by lower-case
letters. An input complex-symbol stream of the STBC encoder 500 is
grouped into pairs of signals, which are denoted as x.sub.1 and
x.sub.2. Each symbol has a symbol period of T, with x.sub.1 being
the first symbol and x.sub.2 being the second symbol that follows
x.sub.1. After receiving x.sub.1 and x.sub.2, the STBC encoder 500
performs a two-step calculation, with each calculation outputting
one symbol at each of the two output ports of the STBC encoder 500.
At the first step of the calculation, the STBC encoder 500 simply
relays x.sub.1 and x.sub.2 to its first parallel output port 512
and second parallel output port 514, respectively. Then x.sub.1 and
x.sub.2 are directed to individual RF transmitters (not shown) to
be converted to RF signals for transmission by antennas 502, 504.
At the second step of the calculation, -x*.sub.2 (which is the
negative of the complex conjugate of x.sub.2) and x*.sub.1 (which
is the complex conjugate of x.sub.1) are generated by the STBC
encoder 500 at the first parallel output port 512 and second
parallel output port 514, respectively. It is presumed, in the STBC
model, that the gain h.sub.1 of the channel between the first
antenna 502 and the receiving antenna 506 is independent of the
gain h.sub.2 of the channel between the second antenna 504 and the
receiving antenna 506. It is further presumed that both channels
are static across two consecutive transmit symbols. In other words,
it is presumed that the channel gain for the second symbol period
is the same as the channel gain for the first symbol period. Thus,
if y.sub.1 and y.sub.2 represent the received symbols at the first
and second symbol periods, respectively, then y.sub.1 and y.sub.2
may be expressed as:
y.sub.1=h.sub.1x.sub.1+h.sub.2x.sub.2+z.sub.1 [Eq. 1],
and:
y.sub.2=h.sub.1(-x*.sub.2)+h.sub.2x*.sub.1+z.sub.2 [Eq. 2],
[0007] where z.sub.1 and z.sub.2 are independent additive noise
terms. If {tilde over (h)}.sub.1 and {tilde over (h)}.sub.2 are the
estimated channel gains of h.sub.1 and h.sub.2, respectively, then
approximations (within a multiplicative factor) of the two transmit
signals {circumflex over (X)}.sub.1 and {circumflex over (X)}.sub.2
can be approximately recovered in terms of {tilde over (h)}.sub.1
and {tilde over (h)}.sub.2 as:
{circumflex over (x)}.sub.1={tilde over (h)}*.sub.1y.sub.1+{tilde
over (h)}.sub.2y*.sub.2 [Eq. 3],
and:
{circumflex over (x)}.sub.2={tilde over (h)}*.sub.2y.sub.1-{tilde
over (h)}.sub.1y*.sub.2 [Eq. 4].
[0008] Though the above STBC is described in space-time domain, it
is clear to one of ordinary skill in the art that the STBC can also
be applied to space-frequency domain. Since the principles
underlying STBC are well known, further discussion of STBC
principles is omitted.
[0009] STBC is optimally designed for systems that employ channels
which are flat-fading and time-invariant over two consecutive
symbol durations. Since time-invariant flat-fading channels have a
negligible variation in channel gain, systems employing flat-fading
channels may be modeled as constant-gain systems. Unfortunately, if
STBC is employed in a wide-band system, the constant-gain system
model is no longer accurate because the channel gain within the
bandwidth may vary considerably. Hence, for wide-band systems,
inter-symbol interference (ISI) mitigation techniques, such as
frequency-selective channels, are typically used.
[0010] In systems employing frequency-selective channels, each
channel may be viewed as having a constant gain due to the division
of the wide frequency band into multiple narrower frequency
channels. Thus, while inter-channel distortions may vary across the
wide frequency band, intra-channel distortions within the narrower
frequency channels may be presumed to be constant. Thus, the ISI
may be independently determined for each channel.
[0011] In mitigating ISI problems, orthogonal frequency division
multiplexing (OFDM) techniques have shown promise. OFDM techniques
have only recently gained popularity, due in part to advances in
signal processing and microelectronics. OFDM splits data streams
into N parallel data streams of reduced data rate, and transmits
each of the N parallel data streams on a separate sub-carrier. The
sub-carriers are made orthogonal to each other by appropriately
choosing the frequency spacing between the sub-carriers. Therefore,
since the orthogonality of the OFDM sub-carriers typically ensures
that the receiver can separate the OFDM sub-carriers, spectral
overlapping among sub-carriers may be permitted.
[0012] OFDM operation block diagrams are shown in FIG. 6. Following
convention, the signals, the channel impulse responses, and the
noise in the frequency domain are denoted by capital letters while
the signals, the impulse responses, and the noise in time domain
are denoted by a lower-case channel impulse responses, and the
noise in time domain are denoted by a lower-case letters. As shown
in FIG. 6, the serial-to-parallel (S/P) converter 600 collects
frequency-domain serial input signals X(k), where k={0, . . . ,
N-1}, and N is the number of OFDM sub-carriers. The collected
frequency-domain serial input signals X(k) are converted into
frequency-domain parallel signals. The frequency-domain parallel
signals are then directed to an inverse discrete Fourier transform
(IDFT) circuit 602. The IDFT circuit 602 transforms the
frequency-domain parallel signals into time-domain parallel
signals. The time-domain parallel signals from the IDFT circuit 602
are then converted into time-domain serial signals, x(n), where
n={0, . . . , N-1}, by a parallel-to-serial (P/S) converter 604.
Then, a cyclic-prefixed time-domain signal x.sub.CP(n) is generated
in the CP unit 606 by adding a cyclic prefix (CP) with a guard
period of G. The length of the CP is a value longer than the
channel delay spreading. Thus:
x ( n ) = 1 N k = 0 N - 1 X ( k ) j 2 .pi. kn N , n = 0 , , N - 1 ,
and [ Eq . 5 ] x CP ( n ) = x ( ( n + N - G ) mod N ) , [ Eq . 6 ]
##EQU00001##
[0013] with CP being a part of the original packet. Since N+G
signals are actually transmitted, the signal block becomes
x.sub.CP(n), where n={0, . . . , N+G-1}. RF unit 608 converts the
signal block x.sub.CP into the RF signal, which is then transmitted
by antenna 610. Antenna 612 at the receiver side subsequently
receives the signal. After being converted by the RF unit 614, the
received signal y.sub.CP(n) becomes:
y.sub.CP(n)=x.sub.CP(n){circle around (X)} h(n)+z(n),n=0, . . . ,
N+G-1 [Eq. 7]
[0014] where {circle around (X)} denotes convolution operator, h(n)
is the time domain channel impulse response of the channel from the
antenna 610 to the antenna 612, and z(n) represents the noise.
After removing cyclic prefix at the CP unit 616, the resulting
signal y(n) becomes:
y(n)=y.sub.CP((n+G)mod N), n=0, . . . , N-1 [Eq. 8]
[0015] Thereafter, y(n) is directed through a S/P converter 618.
The resulting signals are fed into a discrete Fourier transform
(DFT) circuit 220, which transforms time-domain signals into
frequency domain signals. The resulting output Y(k) of the DFT
circuit 620, where k={0, . . . , N-1} is:
Y(k)=X(k)H(k)+Z(k) [Eq. 9]
[0016] where H(k) and Z(k) are the N-dimensional DFT of h(n) and
z(n), respectively. The parallel output of the DFT circuit 620 is
then connected to a P/S converter 622, which generates serial
signals.
[0017] Since the principles underlying OFDM technology is well
known, further discussion of OFDM principles is omitted.
[0018] While STBC-OFDM technology has been proposed for wireless
communication systems, the complexity and cost impediments of
implementing STBC-OFDM to wireless communication systems is still
relatively high.
[0019] In typical OFDM systems with two-branch transmitter
diversity using STBC, a data block X is encoded by the STBC encoder
into two parallel signal blocks X.sub.A and X.sub.B, each having N
data elements. X.sub.A and X.sub.B are then inverse Fourier
transformed to produce time-domain signals X.sub.A and X.sub.B,
respectively. Cyclic prefixes are then added to each time-domain
signal to produce cyclic-prefixed time-domain signals X.sub.A,cp
and X.sub.B,cp, respectively. Each of the cyclic-prefixed
time-domain signals forms an OFDM block. The OFDM blocks are
converted into RF signals and transmitted from their respective
transmitter antennas simultaneously. Since two transmitter antennas
are employed for the transmission of the signals, each of the
signals is altered by channel characteristics h.sub.A and h.sub.B
of their respective channels. As mentioned above, it is assumed
that the channel characteristics are time-invariant during the
period of an OFDM block. Thus, when a receiver antenna receives the
aggregate signal from both of the transmitters, the received
aggregate signal y(n) may be seen as:
y.sub.cp(n)=(h.sub.A(n){circle around (X)}
x.sub.A,cp(n))+(h.sub.B(n){circle around (X)} x.sub.A,cp(n))+z(n)
[Eq. 10],
[0020] where n is the discrete time index, {circle around (X)}
represents a convolution function, and z(n) represents noise in the
system. The received signal y(n) is produced upon removing the
cyclic prefix of y.sub.cp(n) according Eq. 6. The received signal
may be represented in the frequency domain as:
Y(k)=(H.sub.A(k)X.sub.A(k))+(H.sub.B(k)X.sub.B(k))+Z(k),k=0, . . .
, N-1 [Eq. 11],
[0021] where Y(k), H(k) and Z(k) are the N-dimensional DFT of y(n),
h(n) and z(n), respectively. Eq. 11 shows the received signal Y(k)
as a superposition of the two transmitter signals and the
noise.
[0022] Thus, for two-branch STBC OFDM systems, a first pair of
signals x.sub.A=x.sub.1 and x.sub.B=x.sub.2 are generated, and,
after appropriate processing, are transmitted from the first and
second antennas, respectively. Upon receiving these signals, a
receiver reconstructs the transmitted signals using well-known
algorithms, such as maximum-likelihood estimation algorithms or
minimum-mean-square algorithms. Since the reconstruction of STBC
OFDM signals are well known in the art, further discussion of
signal reconstruction is omitted herein. In such STBC OFDM systems,
digital data is encoded and transformed into analog RF signals
suitable for transmission.
[0023] An example of a prior-art STBC OFDM system is shown in FIG.
1. As shown in FIG. 1, the system has a serial-to-parallel
converter 110 that receives a frequency-domain serial data stream
105. The S/P converter 110 converts the frequency-domain serial
data stream 105 into an N-dimensional frequency-domain parallel
data block. The N-dimensional frequency-domain parallel data block
is supplied to a frequency-domain STBC encoder 115 as X.sub.1 and
X.sub.2 according to STBC convention. Upon receiving X.sub.1 and
X.sub.2, the STBC encoder 115 generates two parallel
frequency-domain digital signals X.sub.A 120 and X.sub.B 125 in a
two-step process, such that X.sub.A=X and X.sub.B=X.sub.2 for the
first process, and X.sub.A=-X*.sub.2 and X.sub.B=X*.sub.1 for the
second process.
[0024] Upon generating the frequency-domain digital signals 120,
125 (or the appropriate complex conjugate signals), an IDFT is
performed on the first frequency-domain digital signal X.sub.A 120
by an IDFT circuit 130. The IDFT of X.sub.A 120 produces a
time-domain digital signal x.sub.A. The time-domain digital signal
x.sub.A is converted to a time-domain serial data stream 150 by a
parallel-to-serial converter 140. Thereafter, a cyclic prefix is
added to the time-domain serial data 150 at a CP adder 160 to
produce a cyclic-prefixed time-domain data stream x.sub.Acp 170,
which is relatively immune to ISI effects. This cyclic-prefixed
time-domain data stream is converted to an analog RF signal for
transmission by a radio frequency (RF) transmitter 180.
[0025] Similarly, the second digital signal X.sub.B 125 follows a
similar path, and is cascaded through an IDFT circuit 135, a P/S
converter 145, and a CP adder 165 to produce another
cyclic-prefixed time-domain data stream x.sub.Bcp 175. This
cyclic-prefixed time-domain data stream is also converted to an
analog signal for transmission by an RF transmitter 185. The two
analog signals are transmitted substantially simultaneously from
both RF transmitters 180, 185.
[0026] As shown in FIG. 1, since the frequency-domain STBC encoder
115 generates two frequency-domain digital signals X.sub.A 120 and
X.sub.B 125 that are cascaded through two parallel paths, each
hardware component in one path must have an analogous component in
the other path. This duplication of hardware components results in
added circuitry for each path, and, concomitantly, results in added
computational complexity arising from the added circuitry.
[0027] The resulting complexity in computation and circuit give
rise to a heretofore unaddressed need in the industry.
SUMMARY
[0028] The present invention is directed to systems and methods for
transmitting data in multiple-branch transmitter-diversity OFDM
systems.
[0029] Briefly described, in architecture, one embodiment of the
system comprises an inverse Fourier transform (IFT) circuit adapted
to perform an IFT on a data block to produce an
inverse-Fourier-transformed data block, and a space-time block-code
(STBC) encoder adapted to encode the inverse-Fourier-transformed
data block for transmission in a multiple-branch
transmitter-diversity OFDM system.
[0030] The present disclosure also provides methods for
transmitting data in a multiple-branch transmitter-diversity OFDM
systems. In this regard, one embodiment of the method comprises the
steps of performing an inverse Fourier transform on a data block to
produce an inverse-Fourier-transformed data block, and encoding the
inverse-Fourier-transformed data block for transmission in a
multiple-branch transmitter-diversity OFDM system.
[0031] Other systems, methods, features, and advantages will be or
become apparent to one with skill in the art upon examination of
the following drawings and detailed description. It is intended
that all such additional systems, methods, features, and advantages
be included within this description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] Many aspects of the disclosure can be better understood with
reference to the following drawings. The components in the drawings
are not necessarily to scale, emphasis instead being placed upon
clearly illustrating the principles of the present disclosure.
Moreover, in the drawings, like reference numerals designate
corresponding parts throughout the several views.
[0033] FIG. 1 is a block diagram of a prior art space-time block
code (STBC) orthogonal frequency-division multiplexing (OFDM)
system having many duplicative components for signal
transmission.
[0034] FIG. 2 is a block diagram showing one embodiment of a system
having fewer duplicative components in the signal transmission path
than the system of FIG. 1.
[0035] FIG. 3 is a block diagram showing the time-domain STBC
encoder of FIG. 2 in greater detail.
[0036] FIG. 4 is a flowchart showing one embodiment of a method for
generating signals for transmission in a multiple-branch
transmitter-diversity OFDM system.
[0037] FIG. 5 is a block diagram showing a prior-art STBC
system.
[0038] FIG. 6 is a block diagram showing a prior-art OFDM
system.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0039] Reference is now made in detail to the description of the
embodiments as illustrated in the drawings. While several
embodiments are described in connection with these drawings, there
is no intent to limit the invention to the embodiment or
embodiments disclosed herein. On the contrary, the intent is to
cover all alternatives, modifications, and equivalents.
[0040] In a broad sense, systems and methods are presented in which
a number of duplicative components are reduced in multiple-branch
transmitter-diversity orthogonal frequency-division multiplexing
(OFDM) systems. In one embodiment, inverse Fourier transform (IFT)
calculations are performed on time-domain data, rather than on
frequency-domain data, thereby eliminating multiple calculations of
IFT. Since multiple-branch transmitter-diversity systems transmit
conjugate pairs of signals, systems and methods are also presented
in which a time-domain equivalent of frequency-domain conjugate
pairs of signals is generated. Since the IFT is a symmetrical
operation, an equivalent time-domain signal is transmitted by
replacing the frequency-domain conjugate pairs of signals by
time-domain circular-shifted signals. This is shown in greater
detail with reference to FIGS. 2 through 4.
[0041] FIG. 2 is a block diagram showing one embodiment of a
multi-branch transmitter-diversity OFDM system. As shown in FIG. 2,
a frequency-domain serial digital input 205 of duration T.sub.d is
received at a serial-to-parallel (S/P) converter 210. The S/P
converter 210 converts the frequency-domain serial digital input
205 into an N-dimensional frequency-domain parallel data block 215,
where N is the number of OFDM sub-carriers. The N-dimensional
frequency-domain parallel data block 215 is then input to an
inverse discrete Fourier transform (IDFT) circuit 220, which
performs an IDFT on the N-dimensional frequency-domain parallel
data block 215 to generate an N-dimensional time-domain parallel
data block 225. The N-dimensional time-domain parallel data block
225 is input to a parallel-to-serial (P/S) converter 230, which
converts the N-dimensional time-domain parallel data block 225 into
a serial time-domain signal 235, each having N data elements, of
duration T, where T=NT.sub.d. The serial time-domain signals are
arranged in pairs. As shown in FIG. 2, the first serial time-domain
signal in each pair is denoted by x.sub.1, and the second serial
time-domain signal in each pair is denoted by x.sub.2. The pair of
serial time-domain signals 235 is input to a time-domain space-time
block code (STBC) encoder 240, and the time-domain STBC encoder 240
converts the pair of serial time-domain signals 235 into two
parallel pairs of signals at output ports 250, 255. In other words
after receiving the pair of serial time-domain signals x.sub.1 and
x.sub.2, the time-domain STBC encoder 240 performs a two-step
calculation to generate two signals at a first output port 250 and
a second output port 255. At the first step of the calculation, the
time-domain STBC encoder 240 simply relays the pair of serial
time-domain signals x.sub.1 and x.sub.2 to the first output port
250 and the second output port 255, respectively. At the second
calculation step, the time-domain STBC encoder 240 generates
circular-shifted signals, -x'.sub.2 and x'.sub.1, which are
outputted at the first output port 250 and the second output port
255, respectively.
[0042] As one can see, the time-domain circular-shifted signals,
-x'.sub.2 and x'.sub.1, are Fourier counterparts to the
frequency-domain complex-conjugate signals, -x'.sub.2 and X*.sub.1
shown in the FIG. 1. Thus, the time-domain circular-shifted signals
-x'.sub.2 and x'.sub.1 at the output of the multiple-branch
transmitter-diversity OFDM system of FIG. 2 are ultimately
identical to the output of the STBC OFDM system of FIG. 1. Once the
signal, denoted by x.sub.A, at the first output port 250 is
produced, a cyclic prefix (CP) is added to the signal x.sub.A by a
CP adder 260 to produce a first cyclic-prefixed data stream 270.
Similarly a CP is added to the signal, denoted by x.sub.B, at the
second output port 255 by another CP adder 265 to produce a second
cyclic-prefixed data stream 275. RF transmitters 280, 285 convert
the cyclic-prefixed data streams to analog RF signals for
transmission. The two analog RF signals are transmitted by both RF
transmitters 280, 285 at substantially the same time.
[0043] Unlike the prior-art system of FIG. 1, which produced
complex conjugate signals 120, 125 in the frequency domain, the
embodiment of FIG. 2 produces circular-shifted signals 250, 255 in
the time domain, thereby reducing the number of IDFT calculations.
Thus, unlike prior art STBC OFDM systems that perform two IDFT
operation, the embodiment of FIG. 2 performs only one IDFT
operation on the N-dimensional frequency-domain parallel data block
215. By performing only one IDFT operation, the embodiment of FIG.
2 removes the need for many of the duplicative components that are
found in the prior art system of FIG. 1.
[0044] It should be appreciated that the computational burden
increases exponentially as the number of branches in a
multiple-branch transmitter-diversity system increases. Thus, much
of the computational burden may be reduced by performing the IDFT
operation only once in systems having two, three, four, or more
branches. In this regard, other embodiments of the invention may
include multiple-branch transmitter-diversity OFDM systems having
more than two transmission branches.
[0045] FIG. 3 is a block diagram showing the time-domain STBC
encoder 240 of FIG. 2 in greater detail. As described above with
reference to FIG. 2, the time-domain STBC encoder 240 receives
pairs of input signals x.sub.1 and x.sub.2 at its input port. In a
more general sense, each consecutive pair of input signals
x.sub.2n-1 and x.sub.2n, where n represents a data block index, are
received at the input port of the time-domain STBC encoder 240.
Upon receiving x.sub.2n-1 and x.sub.2n, the time-domain STBC
encoder 240 performs a two-step calculation to generate two pairs
of signals at each of the output ports 250, 255. At the first step
of the calculation, the first pair of outputs of the time-domain
STBC encoder 240 are x.sub.2n-1 on the first output port 250
(hereinafter "output port A"), and x.sub.2n on second output port
255 (hereinafter "output port B"). At the second step of the
calculation, the second pair of outputs of the time-domain STBC
encoder 240 are -x'.sub.2n on output port A 250 and x'.sub.2n-1 on
output port B 255, where -x'.sub.2n and x'.sub.2n-1 represent
circular-shifted signals of x.sub.2n and x.sub.2n-1, respectively.
Thus, the time-domain STBC encoder 240 comprises circular-shift
operation logic 320 and output-routing logic 330 to properly
manipulate and organize the signals.
[0046] The circular-shift operation logic 320 receives x.sub.2n-1
and x.sub.2n, and generates circular-shifted signals x'.sub.2n-1
and -x'.sub.2n, respectively, from x.sub.2n-1 and x.sub.2n, such
that:
x'.sub.2n-1(n)=x.sub.2n-1(N-1-n) [Eq. 12]
[0047] for n={0, . . . , N-1} where n represents the index of the
sub-element in x'.sub.2n-1 or -x'.sub.2n, and
-x'.sub.2n(n)=-x.sub.2n(N-1-n) [Eq. 13],
[0048] for n={0 . . . , N-1}. Thus, the circular-shifted signals
x'.sub.2n-1 and -x'.sub.2n are directed to the output-routing logic
330. The output-routing logic 330 receives x.sub.2n-1, x.sub.2n,
x'.sub.2n-1, and -x.sub.2n, and outputs x.sub.2n-1 at output port A
250 for each odd-indexed data block; outputs x.sub.2n at output
port B 255 for each odd-indexed data block; outputs -x'.sub.2n at
output port A 250 for each even-indexed data block; and outputs
x'.sub.2n-1 at output port B 255 for each even-indexed data block.
In other words, rather than generating frequency-domain complex
conjugates, the time-domain STBC encoder 240 generates time-domain
circular-shifted signals.
[0049] As seen from the systems of FIGS. 2 and 3, by placing the
IDFT circuit 220 before the STBC encoder 240, many of the
duplicative components of FIG. 1 can be eliminated. While FIGS. 2
and 3 show a system for transmitting data in multiple-branch
transmitter-diversity OFDM systems, another embodiment may be seen
as a method for transmitting data in multiple-branch
transmitter-diversity OFDM systems. One embodiment of such a method
is shown in FIG. 4.
[0050] FIG. 4 is a flowchart showing a method for generating STBC
OFDM signals. As shown in FIG. 4, data for transmission in an STBC
ODFM system is received (420) by an inverse Fourier transform (IFT)
circuit. An IFT is performed (430) on the received data, thereby
producing time-domain data from the received data. In one
embodiment, the IFT may be an inverse discrete Fourier transform
(IDFT) circuit 220 as shown in FIG. 2 or, more specifically, an
inverse fast Fourier transform (IFFT) circuit (not shown). Once the
IFT has been performed (430) on the data, the data is encoded (440)
according to a space-time block-coding (STBC) algorithm. In other
words, the time-domain data is encoded according to an STBC
algorithm such that x.sub.2n-1 is generated at a first output port
for each odd-indexed data block; x.sub.2n is generated at a second
output port for each odd-indexed data block; -x.sub.2n is generated
at the first output port for each even-indexed data block; and
x'.sub.2n-1 is generated at the second output port for each
even-indexed data block. In other words, rather than generating
frequency-domain complex conjugates, time-domain circular-shifted
signals are generated according to the method as shown in FIG.
4.
[0051] As shown from the embodiment of FIG. 4, by performing an IFT
on data prior to encoding the data for transmission in a
multiple-branch transmitter-diversity OFDM system, complexities
associated with performing multiple IFT operations is eliminated.
In one embodiment, the method steps of FIG. 4 may be performed by a
system as described in FIGS. 2 and 3. However, it will be clear to
one of ordinary skill in the art that other systems may perform the
method as outlined in FIG. 4, so long as the inverse Fourier
transform is performed prior to the space-time block-coding.
[0052] The IDFT circuit 220, the STBC encoder 240, and the CP
adders 260, 265 may be implemented in hardware, software, firmware,
or a combination thereof. In the preferred embodiment(s), the IDFT
circuit 220, the STBC encoder 240, and the CP adders 260, 265 are
implemented in hardware using any or a combination of the following
technologies, which are all well known in the art: a discrete logic
circuit(s) having logic gates for implementing logic functions upon
data signals, an application specific integrated circuit (ASIC)
having appropriate combinational logic gates, a programmable gate
array(s) (PGA), a field programmable gate array (FPGA), etc. In an
alternative embodiment, the IDFT circuit 220, the STBC encoder 240,
and the CP adders 260, 265 are implemented in software or firmware
that is stored in a memory and that is executed by a suitable
instruction execution system.
[0053] Although exemplary embodiments have been shown and
described, it will be clear to those of ordinary skill in the art
that a number of changes, modifications, or alterations to the
invention as described may be made. For example, while inverse
discrete Fourier transforms are described in the embodiments of
FIGS. 2 through 4, it will be clear to one of ordinary skill in the
art that an inverse fast Fourier transform (IFFT) may be performed
on the data if the number of data points is 2.sup.M. Additionally,
if the number of data points is not 2.sup.M, then it will be clear
to one of ordinary skill in the art that the data may be
zero-filled or interpolated to 2.sup.M data points to perform an
IFFT on the data points. Furthermore, since the Fourier transforms
(FT) and inverse Fourier transforms (IFT) are symmetrical
operations, it will be clear to one of ordinary skill in the art
that a scaled time-domain signal may be generated from the
frequency-domain signal by simply performing a FT on the data,
rather than performing an IFT. Moreover, one of ordinary skill in
the art will recognize that the signals transmitted from channel A
and channel B in FIG. 2 may be switched without adverse affect to
the signal transmission, so long as the correct conjugate pairs are
transmitted through their respective channels. Further, while a
two-branch transmitter-diversity system is shown in FIGS. 2 through
4 for simplicity of illustration, it will be clear to one of
ordinary skill in the art that the two-branch system may be readily
extended to multiple-branch transmitter-diversity systems having
additional branches. These changes, and other such modifications or
alterations, should therefore be seen as within the scope of the
disclosure.
* * * * *