U.S. patent application number 10/744351 was filed with the patent office on 2007-10-11 for apparatus and method for improved performance in mc-cdma radio telecommunication systems that use pulse-shaping filters.
Invention is credited to Mohammad J. Borran, Panayiotis Papadimitriou, Prabodh Varshney, Hannu Vilpponen.
Application Number | 20070237067 10/744351 |
Document ID | / |
Family ID | 31188395 |
Filed Date | 2007-10-11 |
United States Patent
Application |
20070237067 |
Kind Code |
A9 |
Borran; Mohammad J. ; et
al. |
October 11, 2007 |
Apparatus and method for improved performance in MC-CDMA radio
telecommunication systems that use pulse-shaping filters
Abstract
A apparatus and method for improved performance in radio
telecommunications systems, and in particular multi-carrier code
division multiple access (MC-CDMA) networks that employ
pulse-shaping filters on the transmit side of a radio link. In
order to more accurately transmit a radio signal bearing a symbol
sequence, the modulated and spread information stream is upsampled
using a technique that involves inserting zeros in the frequency
domain. A corresponding downsampling procedure on the receive side
permits reconstruction of the transmitted symbols. A new channel
estimation algorithm may also be used, and the improved channel
estimation advantageously employed to obtain more faithful symbol
detection.
Inventors: |
Borran; Mohammad J.;
(Houston, TX) ; Varshney; Prabodh; (Coppell,
TX) ; Vilpponen; Hannu; (Oulu, FI) ;
Papadimitriou; Panayiotis; (Euless, TX) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20050018598 A1 |
January 27, 2005 |
|
|
Family ID: |
31188395 |
Appl. No.: |
10/744351 |
Filed: |
December 22, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/US03/23135 |
Jul 24, 2003 |
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10744351 |
Dec 22, 2003 |
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60398418 |
Jul 25, 2002 |
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Current U.S.
Class: |
370/208 |
Current CPC
Class: |
H04L 25/0204 20130101;
H04L 27/265 20130101; H04L 25/03834 20130101; H04L 5/026 20130101;
H04L 25/0246 20130101; H04L 25/025 20130101; H04L 25/022 20130101;
H04L 27/2644 20130101; H04L 25/0212 20130101; H04L 25/0224
20130101; H04L 27/2602 20130101 |
Class at
Publication: |
370/208 |
International
Class: |
H04J 11/00 20060101
H04J011/00 |
Claims
1. An apparatus for use in a multi-carrier code division multiple
access (MC-CDMA) telecommunication network, said apparatus
comprising: a transmitter for transmitting a radio signal, wherein
the transmitter comprises: an orthogonal frequency division
multiplexing (OFDM) modulator that divides an input vector X of
length N.sub.b into two parts, inserts N.sub.x zeros between them,
and takes an inverse fast Fourier transform (IFFT) of size
N.sub.bN.sub.x to obtain a time-domain signal; and a pulse-shaping
filter coupled to the OFDM modulator for processing the time-domain
signal prior to transmission; and a receiver for receiving the
transmitted time-domain signal, wherein the receiver comprises an
OFDM demodulator for taking a fast Fourier transform (FFT) of size
N.sub.bN.sub.x, of the received signal and removes the zeros
inserted in the transmitter from the resulting frequency-domain
signal.
2. The apparatus of claim 1, wherein the time-domain signal has a
sampling rate of Ns samples per chip, and wherein N.sub.x,
=N.sub.b(N.sub.s-1).
3. The apparatus of claim 1, wherein the pulse-shaping filter is a
raised-cosine filter.
4. The apparatus of claim 1, wherein the inserted zeros are higher
frequency components than the input vector X.
5. The apparatus of claim 1, wherein the receiver further comprises
a filter matched to the pulse-shaping filter of the transmitter for
filtering the received signal and providing the filtered signal to
the OFDM demodulator.
6. The apparatus of claim 1, wherein the transmitter further
comprises: an encoder for encoding information to be transmitted;
and a modulator for modulating the encoded information.
7. The apparatus of claim 1, wherein the OFDM demodulator
reconstructs as output vector Z the transmitted signal by
concatenating the remaining portions of the frequency-domain signal
after removing the zeros.
8. The apparatus of claim 7, wherein the receiver further comprises
a detector for detecting received symbols after the transmitted
signal has been converted to the frequency domain.
9. The apparatus of claim 8, wherein the receiver further performs
channel estimation by analyzing received pilot symbols that were
inserted in the frequency domain by the transmitter.
10. The apparatus of claim 9, wherein the frequency response of the
pulse-shaping filter is known, and wherein performing the channel
estimation applies the known pulse-shaping filter frequency
response.
11. The apparatus of claim 10, wherein the receiver performs the
channel estimation using a least squares (LS) analysis.
12. The apparatus of claim 9, wherein the detector applies the
channel estimation to the detection of the received symbols.
13. For use in a telecommunication system operable according to an
MC-CDMA protocol, an improved receiver for processing a received
time-domain signal that was upsampled by taking the IFFT of a
divided input signal into which zeros have been inserted in the
frequency domain, said receiver comprising: an OFDM demodulator for
taking an FFT of the received time-domain signal and removing the
inserted zeros.
14. The receiver of claim 13, wherein the OFDM demodulator
comprises a lowpass filter for removing the zeros.
15. A method for processing an encoded and modulated
information-bearing signal to be transmitted over an air interface
in an MC-CDMA communication network; said method comprising the
steps of: dividing the modulated information-bearing signal into a
plurality of streams; spreading each of the plurality of streams
with a spreading code; summing the spread streams into a symbol
sequence of length N.sub.b; dividing the symbol sequence into two
portions; inserting zeros between the two portions; taking an IFFT
of the resulting stream to obtain a time-domain signal; providing
the time-domain signal to a pulse-shaping filter; and transmitting
the signal.
16. The method of claim 15, wherein the zeros are inserted as high
frequency components.
17. The method of claim 15, further comprising the steps of:
receiving the transmitted time-domain signal; taking an FFT of the
received signal to obtain a frequency-domain signal; and removing
the previously inserted zeros from the frequency-domain signal.
18. The method of claim 17, further comprising the step of
estimating the frequency response of the transmission channel.
19. The method of claim 18, wherein the transmitted signal is
transmitted by a transmitter having a pulse-shaping filter with a
known frequency response, and wherein the step of estimating
comprises determining a maximum likelihood estimate by solving a LS
problem that applies the known pulse-shaping filter frequency
response.
20. The method of claim 18, further comprising the step of
detecting the received symbols using the channel estimate.
Description
CLAIM OF BENEFIT (35 U.S.C. .sctn. 119(e))
[0001] This Application claims the benefit of U.S. Provisional
Application No. 60/398,418, filed Jul. 25, 2002, which is
incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present invention relates generally to radio telephony,
and more specifically to a method and apparatus for using improved
sampling and channel estimation techniques to improve the
performance of wideband MC-CDMA radio telecommunication systems
that use pulse-shaping filters.
BACKGROUND OF THE INVENTION
[0003] Radio telephony, generally speaking, involves the use of
portable radios for telephone communication by a user through a
radio telecommunication network. The network connects a large
number of network subscribers with each other and, usually, with
subscribers of other networks as well. Connections between calling
and called parties are made using a network infrastructure that
includes information channels and switching devices to route calls
to the appropriate destination. Connected subscribers may engage in
voice conversations or exchange text messages, email, or other
forms of data.
[0004] In a radio telecommunication network, the only actual
radio-frequency connection may be that between the subscribers'
radios and the network infrastructure (although this is not
necessarily true). Infrastructure nodes are often fixed in location
and interconnected using wires, cables, or optical fibers so that
they can transfer large amounts of information. The radio
connection to the subscribers is important, however, because it
gives them mobility. In ideal circumstances, a subscriber can make
a call and maintain the connection even when traveling over dozens,
or even hundreds of miles.
[0005] In order for such communications to occur, radio
telecommunication networks typically employ a large number of fixed
base stations spread over a wide geographic area, sometimes
referred to as the network coverage area. Each base station employs
one or more antennas for communicating with nearby mobile stations,
and of course is also connected to the remainder of the
infrastructure as well.
[0006] FIG. 1 is a simplified block diagram illustrating selected
components of a typical radio telecommunication network 100. Base
stations 105-110 are each shown to be connected with an antenna
111-116. Each antenna is intended to handle communications within a
selected area, sometimes referred to as a cell. (For this reason
the portable subscriber radios used in such a network are often
called "cellular" or simply "cell" phones.) For example, in FIG. 1
cell phones 11, 12, and 13 are shown to be in communication with
antennas 111, 112, and 113, via radio channels 1, 2, and 3,
respectively.
[0007] The broken lines in FIG. 1 represent cell boundaries. These
boundaries do not represent the precise range of their associated
antennae, of course, and are not always regular in shape or
consistent in size. And although only six cells are delineated,
there are typically many more in the network coverage area. Cell
phones may and often do move from cell to cell, and their network
communications are generally transferred from one network antenna
to another though a process called handover.
[0008] Base station controllers (BSCs) 120 and 125 are in
communication with, and generally control the operations of base
stations 105-107 and 108-110, respectively. The base station
controllers are in turn connected with a mobile switching center
(MSC) 130, which handles call routing and provides a connection to
other network MSCs (not shown) and gateway MSCs such as G-MSC 135,
which may provide a connection to another network. A visitor
location register, here VLR 140, maintains information relating to
cell phones in the area services by the associated MSC. (A home
location register (HLR) (not shown), may be provided to track the
location and other information related to all network
subscribers.)
[0009] Note that while cellular telephones have traditionally been
used for voice communication, advances in technology have permitted
the introduction and growing use of such instruments for other
applications including sending of text messages, instant messages,
data transfer, and Web surfing. Some have even incorporated
functions previously performed by personal digital assistants
(PDAs), such as appointment calendaring. For this reason the wide
variety of such devices capable of communicating through a radio
telecommunication will be referred to simply as "mobile
stations".
[0010] In radio telecommunication networks, the cellular
architecture provides a number of advantages. For one, in many
networks channelization for individual subscriber or control
communications is implemented by assigning different frequencies to
each channel. By controlling the range of these communications,
assigned frequencies may be reused in non-adjacent cells without
creating interference between users. In addition, the close
proximity of base stations with which to communicate means that
mobile stations can communicate with lower transmission power than
if they had to reach distant antennas. Conservation of power is, of
course, an important objective of battery-operated devices.
[0011] Frequency channelization in the mobile context is frequently
referred to as frequency division multiple access (FDMA). Each
mobile station is assigned one or more frequencies within the
overall operational bandwidth of communicating with the base
station. In some systems, each communication frequency is also
divided into time slots, a scheme referred to as time division
multiple access (TDMA). In TDMA, each mobile station is assigned
one or more of these slots and transmits a portion of its
information in turn. Naturally, the slots are of sufficient
duration and frequency so that each user perceives their own
conversation as continuous.
[0012] Another type of multiple access scheme is called
code-division multiple access (CDMA). CDMA operates somewhat
differently; rather than divide the available transmission
bandwidth into individual channels, many individual transmissions
are spread over a frequency band using a spreading code.
Transmissions intended for a particular receiver (i.e. mobile
station) are spread with spreading code assigned to the mobile
station, which decodes only that information intended for it and
ignores the differently-coded transmissions intended for others.
The number of mobile stations that can operate in a given area is
therefore limited by the number of unique encoding sequences
available, rather than the number of frequency bands. The operation
of a CDMA network is normally performed in accordance with a
protocol referred to as IS-95 (interim standard-95) or,
increasingly, according to its third generation (3G) successors,
such as those sometimes referred to as CDMA2000, 1xEV-DO, and
1xEV-DV, the latter of which provides for the transport of both
data and voice information.
[0013] In a more recently developed scheme, the use of CDMA
techniques is combined with orthogonal frequency division
multiplexing (OFDM). OFDM is a modulation method in which multiple
user symbols are transmitted in parallel using a large number of
different sub-carriers. These sub-carriers, sometimes called
frequency bins, are used to spread transmitted information with
respect to frequency rather than time (as with conventional CDMA).
This multiple access scheme is sometimes referred to as
multi-carrier CDMA (or MC-CDMA).
[0014] FIG. 2 is a simplified block diagram illustrating selected
components of a typical MC-CDMA telecommunication system 200. As
depicted in FIG. 2, system 200 has a transmit side 205 and a
receive side 210. On the transmit side information, which may be
either voice or data for transmission, is first encoded in encoder
215. The encoded information is passed to modulator 220 for
modulation according to one of several modulation schemes such as
QPSK or 16 QAM. The modulated symbols are then provided to an
MC-CDMA transmitter 225 for transmission over an air interface
radio channel 230.
[0015] The transmitted information is received on the receive side
by an MC-CDMA receiver 235, which processes the information and
presents it to detector 240 for symbol detection. Simply stated,
detector 240 attempts to faithfully reconstruct the transmitted
symbol stream by removing from the received signal the effects of
any distortion or noise added in transmission. In part, these
undesirable but unavoidable effects are removed, or at least
mitigated, by analyzing the quality of certain received symbols
called pilot symbols. These pilot symbols are not part of the
transmitted user information, but are inserted into it. Their
transmitted value is known to the receiver, which can estimate
channel effects from the condition in which they are received. The
detected symbol stream is then presented to decoder 245 for
decoding. The decoded information is stored or provided to a user
interface such as a speaker (not shown) so that it may be perceived
by the user.
[0016] Another form of interference that may distort transmitted
radio communications is referred to as inter-symbol interference
(ISI). IS arises largely from the multipath effect, a phenomenon
that occurs when a propagating radio signal fans out and
encounters, for example, different reflecting surfaces and
propagation media creating a number of `copies` of the same signal
that may each arrive at the receiver at slightly different times.
Transmit filters, such as pulse-shaping filters, that are used to
limit the frequency content of the transmitted signal can also
introduce ISI.
[0017] Channel equalizers in the receiver are often used to counter
ISI induced by the multipath-effect. For ISI caused by the transmit
filter, a matched filter may be implemented in the receiver to
create an ISI-free composite filter. This approach is not always
taken, however. For example, the pulse-shaping filters specified in
CDMA standards such as IS-95 and 1xEv-DV are not ISI-free. As a
result, the pulse-shaping filters in these systems introduce
unabated ISI that degrades the ability of the receive side to
accurately estimate the channel and detect the transmitted
symbols.
[0018] Needed, therefore, is a way to reduce or eliminate ISI
effects in MC-CDMA systems that use pulse-shaping filters in order
to improve system performance. The present invention provides just
such a solution.
SUMMARY OF THE INVENTION
[0019] In one aspect, the present invention is an improved
apparatus for use in a radio telecommunication system including a
transmitter and a receiver, the transmitter including pulse-shaping
filter having a known frequency response and an orthogonal
frequency division multiplexing (OFDM) modulator for taking an
inverse fast Fourier transform (IFFT) of the symbol stream to be
transmitted after the stream has been divided into sub-streams and
zeros inserted between the sub-streams. Preferably the stream is
divided into two sub-streams of equal length and the zeros are
inserted as high-frequency components. The apparatus may further
include a receiver to receive a transmitted radio signal, take a
fast Fourier transform (FFT), and remove the previously inserted
zeros. The apparatus may also apply the known pulse-shaping filter
frequency response in a least squares analysis to determine a
maximum likelihood estimate of the transmission channel frequency
response.
[0020] In another aspect, the present invention is a receiver for
receiving a time-domain signal transmitted by a MC-CDMA transmitter
that upsamples a symbol stream and inserts zeros in the frequency
domain before converting the symbol stream into the time domain and
mapping the symbols into frequency bins for transmission, the
receiver including an OFDM demodulator that removes the zeros from
the symbol stream after converting it back to the frequency
domain.
[0021] In yet another aspect, the present invention is a method of
processing an MC-CDMA signal including the steps of encoding the
information, modulating the encoded signal onto a carrier, dividing
the modulated signal into a plurality of streams and spreading each
stream with a spreading code, summing the spread streams into a
symbol sequence of length N.sub.b, dividing the symbol sequence
into to a plurality of portions, inserting zeros between each
separate portion, taking an IFFT of the resulting stream to obtain
a time-domain signal, providing the time-domain signal to a
pulse-shaping filter, and transmitting the signal. The method may
further include the steps of receiving the transmitted signal and
removing the previously-inserted zeros. Finally, the method may
also include the step of applying the known pulse-shaping filter
frequency response in a least squares analysis to determine a
maximum likelihood estimate of the transmission channel frequency
response, and the step of applying the channel estimate thus
determined in a detector to detect the transmitted symbol
stream.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] For a more complete understanding of the present invention,
and the advantages thereof, reference is made to the following
drawings in the detailed description below:
[0023] FIG. 1 is functional block diagram illustrating the
relationship of selected components of a typical CDMA
telecommunication network, such as one that might advantageously
employ the system and method of the present invention.
[0024] FIG. 2 is a simplified block diagram illustrating selected
components of a typical MC-CDMA telecommunication system.
[0025] FIG. 3 is a flow diagram illustrating a method of radio
transmission in an MC-CDMA radio telecommunication system.
[0026] FIG. 4 is a functional block diagram illustrating selected
components of a radio telecommunication system operable according
to an embodiment of the present invention.
[0027] FIG. 5 is a flow diagram illustrating a method of channel
estimation according to an embodiment of the present invention.
[0028] FIG. 6 is a flow diagram a method of processing and
transmitting a radio signal according to an embodiment of the
present invention.
[0029] FIG. 7 is a flow diagram illustrating a method of receiving
and processing a radio signal according to an embodiment of the
present invention.
DETAILED DESCRIPTION
[0030] FIGS. 1 through 7, discussed herein, and the various
embodiments used to describe the present invention are by way of
illustration only, and should not be construed to limit the scope
of the invention. Those skilled in the art will understand the
principles of the present invention may be implemented in any
similar radio-communication device, in addition to those
specifically discussed herein.
[0031] The present invention is directed to a system and method for
communication in a radio telecommunication network, and is of
particular advantage when applied to a multi-carrier code division
multiple access (MC-CDMA) system that includes a pulse-shaping
filter on the transmit side. FIG. 3 is a flow diagram illustrating
a relevant portion of the process of signal transmission in such a
system. At START, it is assumed that the information to be
transmitted has been encoded and modulated. The modulated symbols
are then spread (step 305) using an appropriate spreading code and
provided to the OFDM modulator. (While one is discussed here, there
may be and often is more than one symbol stream.) The OFDM
modulator converts the signal to a time-domain signal (step 310) by
taking an inverse fast Fourier transform (IFFT) and provides this
signal to a pulse-shaping filter. After pulse-shaping (step 315),
the signal is transmitted (step 320) over an air interface.
[0032] At the receiver, the transmitted signal is received (step
325) and converted from the time domain to the frequency domain
(step 330) in a demodulator applying a fast Fourier transform
(FFT). A channel estimate is made (step 335), and the demodulated
signal is provided to a detector for symbol detection (step 340).
(Note that the timing and regularity of channel estimation may vary
with system design.)
[0033] FIG. 4 is a simplified block diagram illustrating an
exemplary system 400 for sending information over an air interface
using MC-CDMA in accordance with an embodiment of the present
invention. The portion of the Figure above the broken line
represents a transmitter 401, such as one that might be found in a
telecommunication network base station, and below is illustrated a
receiver 451 for example one operating in a mobile station. The
broken line itself represents a multipath channel over the air
interface of the radio telecommunication network.
[0034] In transmitter 401, serial-to-parallel (S/P) converter 405
splits the modulated symbol streams (of all K users) into K blocks
of J streams (S.sub.0,0 to S.sub.K-1,J-1). Each of these streams s
is spread by multiplication with a Walsh-Hadamard code (c.sub.0 to
c.sub.j-1), and then presented to a summer (410.sub.0 . . .
410.sub.k . . . 410.sub.K-1), which sums the streams associated
with each block 0 to K-1 into a single spread stream (S.sub.0 to
S.sub.K-1). The spread streams S.sub.k are then passed through S/P
converters 415.sub.0 . . . 415.sub.k . . . 415.sub.K-1 before being
presented to interleaver 420 for block interleaving. The output of
the interleaver 420 is labeled X, which represents a symbol stream
of length N.sub.b. OFDM modulator (IFFT) 425 is coupled to
interleaver 420 and maps the interleaved signal into frequency bins
(sub-carriers), and may add a cyclic prefix.
[0035] In accordance with the present invention, prior to taking an
IFFT of the symbol stream X (here X.sub.0 . . . X.sub.k . . .
X.sub.K-1) OFDM modulator 425 first separates the stream into two
streams (of length N.sub.b/2 each). Between the two halves a group
of N.sub.b(N.sub.s-1) zeros at a high frequency are inserted,
creating a new vector of size N.sub.bN.sub.s. This means that the
IFFT will be larger (that is, of size N.sub.bN.sub.s) but the
resulting time-domain signal will have the desired sampling rate
N.sub.s without having modified the frequency content of the
signal. The resulting time-domain signal is then passed through a
pulse-shaping filter 430 and transmitted over a radio channel using
antenna 435.
[0036] Receiver 451 includes the antenna 453 for receiving the
transmitted radio signal. The received signal may first be passed
through a matched band-pass receive filter 455 to suppress
out-of-band noise and interference. Note that under certain
conditions, the matched filter may be unnecessary, as discussed
below. The filtered signal is then passed through an OFDM
demodulator (FFT) 460 and demodulated into frequency-domain signal
Z.sub.k (signals of other blocks may be present as well, but for
simplicity only one is shown). In accordance with the present
invention after taking the FFT (of size N.sub.bN.sub.s), the
previously added high-frequency components (zeros) are removed and
the two haves of the symbol stream are rejoined to form a single
stream of length N.sub.b. Deinterleaver 465 deinterleaves signal
Z.sub.k and is coupled to parallel-to-serial (P/S) converter
470.sub.k, which creates a bit stream Y.sub.k (again, there may be
one associated with each block, even though only one stream is
shown in FIG. 4). A detector 475.sub.k generates soft or hard
decision outputs for each original symbol or bit stream (S.sub.k,0
to S.sub.K,J-1).
[0037] As mentioned above, the present invention involves a new
upsampling and downsampling technique, the advantages of which will
now be described in greater detail. In accordance with an
embodiment of the present invention, the OFDM modulator 425 input
vector X (see FIG. 4) is divided into two parts of equal length
N.sub.b/2 by inserting between them N.sub.b(N.sub.s-1) zeros at
high frequency to form a vector of length N.sub.bN.sub.s. OFDM
modulator 425 then applies an IFFT of size N.sub.bN.sub.s to obtain
a time-domain signal that is then passed to pulse-shaping filter
430 for processing prior to transmission. Note that using this
method the time-domain signal achieves the desired sampling rate of
Ns samples per chip without modifying the frequency content of the
signal.
[0038] In the receiver 451, the OFDM demodulator 460 receives the
time domain signal and applies an FFT of size N.sub.bN.sub.s before
removing the previously-inserted N.sub.b(N.sub.s-1) high-frequency
components. A frequency domain signal of length N.sub.b comprising
the two low-frequency parts of the symbol stream may then be
de-interleaved and further processed.
[0039] Note that removing the previously-inserted high-frequency
components is, in effect, a lowpass filtering, and for this reason
may eliminate the need for a separate receive filter (such as
filter 455 shown in FIG. 4), so long as the bandwidth of the
received time-domain signal is not greater than N.sub.s/T.sub.c
Hz.
[0040] In accordance with the present invention, the performance of
the receiver is also enhanced by an improved channel estimation
technique. As mentioned above, channel estimation may be performed
by evaluating the condition of received pilot symbols. FIG. 5 is a
flow diagram illustrating a method 500 of channel estimation
according to an embodiment of the present invention. At START, it
is assumed that an MC-CDMA telecommunication system such as that
illustrated in FIG. 4 has been provided. It is also assumed that
the transmission channel is a multi-path channel of L.sub.c samples
in length, and that at least L.sub.c pilot symbols have been
equally spaced though the transmitted symbol stream. In this
embodiment, the pilot symbols are inserted in the frequency
domain.
[0041] First, the pilot symbols are collected from deinterleaved
symbol stream Y to form vector Y.sub.p (step 505). In similar
fashion, the vectors H.sub.cp and .sub.p and the matrix H.sub.tp
are formed (step 510) from the corresponding elements of H.sub.c, ,
and H.sub.t respectively (corresponding to the channel, the
additive noise, and the transmit pulse-shaping filter). These are
related as follows: Y.sub.p=H.sub.tpH.sub.cp+N.sub.p
[0042] If W is a matrix consisting of the first L.sub.c columns of
the OFDM demodulator FFT matrix, then H.sub.c is a diagonal matrix:
H.sub.c=diag(Wh.sub.c) and if W.sub.p is a matrix considering only
those rows including transmitted pilot signals:
H.sub.cp=W.sub.ph.sub.c. then:
Y.sub.p=H.sub.tpW.sub.ph.sub.c+N.sub.p.
[0043] A maximum-likelihood (ML) value for the channel impulse
response h.sub.c is then estimated (step 515). To obtain the ML
estimate for h.sub.c, the following log-likelihood function is
maximized: L(h.sub.c)=ln
p(Y.sub.p|h.sub.c)=A-B||Y.sub.p-H.sub.tpW.sub.ph.sub.c||.sup.2,
which since A and B are constant scalar quantities (with B>0),
is equivalent to the following optimization: h ^ c = arg .times.
.times. min h c .times. Y p - H tp .times. W p .times. h c 2
##EQU1##
[0044] This optimizing problem may be evaluated as a least squares
(LS) problem using the method of singular value decomposition
(SVD). If the SVD of H.sub.tpW.sub.p is given by
H.sub.tpW.sub.p=USV.sup.H, then:
h.sub.c=VS.sup.-1U.sup.HY.sub.p.
[0045] The channel estimate according to the present invention may
then be obtained by taking the FFT of h.sub.c (step 520) as
follows: H.sub.c=diag(Wh.sub.c)=diag(WVS.sup.-1U.sup.HY.sub.p)
Finally, since W, H.sub.t, V, S, and U are known, a matrix
L=WVS.sup.-1U.sup.H can be calculated (step 525) and the channel
estimation for each OFDM symbol may be expressed as
H.sub.c=diag(LY.sub.p). The method of the present invention thereby
advantageously applies the known transmit pulse-shaping filter
frequency response.
[0046] As mentioned above, in accordance with an embodiment of the
present invention the channel estimate is applied in detector
475.sub.k. Having calculated the channel estimate separately, it is
now combined with the (known) frequency response of the transmit
pulse-shaping filter H.sub.t to form a composite channel matrix
=H.sub.cH.sub.t. If the composite channel effect related to the
k-th transmit block is represented as .sub.k, and the corresponding
additive noise is .sub.k, then the input to the detector 475.sub.k
may be represented as: Y.sub.k=H.sub.kS.sub.k+N.sub.k. From these
new parameters the transmitted symbol stream may be reconstructed
by application of a variety of methods, for example by applying a
conventional matched-filter detector, or by using a maximum
likelihood detector.
[0047] FIG. 6 is a flow chart illustrating a method 600 of
transmitting a radio signal according to an embodiment of the
present invention. At START, it is presumed that a transmitter such
as transmitter 401 shown in FIG. 4 has been provided. The method
begins when information to be transmitted is encoded (step 605).
The encoded information is then modulated (step 610) according to
any one of several existing schemes. The modulated symbols are then
divided into K blocks of J streams each (step 615). Each stream
within a block is spread with a unique spreading code (step 620),
generally a Walsh-Hadamard code, and summed into a single stream
(step 625). This symbol stream is then re-divided into parallel
paths (step 630) in a serial-to-parallel converter for interleaving
(step 635) with the streams of the other blocks.
[0048] In accordance with the present invention, the interleaved
output X (in this illustration X.sub.0 through X.sub.K-1) is
divided into two streams (step 640) each of length N.sub.b/2. Then,
N.sub.b(N.sub.s-1) zeros are inserted between the two parts (step
645), forming a new vector of length N.sub.bN.sub.s. These signals
are then mapped into frequency bins (step 650) using an OFDM
modulator that takes an inverse fast Fourier transform (IFFT) of
size N.sub.bN.sub.s to obtain a time-domain signal of the same
length.
[0049] The result is a time-domain signal of sampling rate N, that
was obtained without modifying the signal's frequency content. The
time-domain signal is then presented to a pulse-shaping filter
(step 655) and then transmitted (step 660) over a radio
channel.
[0050] FIG. 7 is a flow chart illustrating a method 700 of
receiving a radio signal according to an embodiment of the present
invention. At START, it is presumed that a receiver such as
receiver 451 shown in FIG. 4 has been provided, and that a signal
according to the present invention has been transmitted. First the
transmitted time-domain radio signal is received (step 705). In
order to recover N.sub.b symbols at the output of the OFDM
demodulator, a fast Fourier transform (FFT) of size N.sub.bN.sub.s
is applied to the received signal (step 710), converting the signal
back to the frequency domain, and then the middle
N.sub.b(N.sub.S-1) high-frequency symbols are discarded (step
715).
[0051] The N.sub.b symbols output from the OFDM demodulator
(represented by the vector Z in FIG. 4), are then provided to a
deinterleaver for deinterleaving (step 720), and the originally
transmitted blocks are reconstructed (step 725). In order to
reproduce the transmitted symbol streams, a channel estimation is
performed (step 730), and using the estimated channel, each block
is presented to a detector for symbol detection (step 735).
[0052] The foregoing description therefore provides an improved
system and method for transmitting information in a MC-CDMA
telecommunication system that reduces or eliminates the ISI due to
typically used (non-ideal) pulses-shaping filters in the MC-CDMA
transmitters. An improved channel estimation technique is also
provided, and the improved estimate is advantageously applied in
symbol detection.
[0053] The preferred descriptions are of preferred examples for
implementing the invention, and the scope of the invention should
not necessarily be limited by this description. Rather, the scope
of the present invention is defined by the following claims.
* * * * *