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Claims  |
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What is claimed is:
1. A transmitter for modulating an information signal for transmission in a
spread spectrum communication system comprising:
means for demultiplexing said information signal into first and second
subsignals;
first means for combining said first subsignal with a first coset code and
for combining said second subsignal with a second coset code orthogonal to
said first coset code so as to produce a first composite coset-encoded
signal;
means for generating an orthogonal function signal; and
means for modulating said first composite coset-encoded signal with said
orthogonal function signal in order to provide a first modulated signal.
2. The transmitter of claim 1 further including:
means for generating a pseudorandom noise signal of predetermined PN code,
and
means for combining said first modulated signal with said pseudorandom
noise signal of predetermined PN code so as to provide a first output
signal.
3. The transmitter of claim 1 further including:
means for demultiplexing said information signal into third and fourth
subsignals,
second means for combining said third subsignal with a third coset code and
for combining said fourth subsignal with a fourth coset code so as to
produce a second composite coset-encoded signal, said first, second third
and fourth coset codes being mutually orthogonal, and
means for modulating said second composite coset-encoded signal with said
orthogonal function signal in order to provide a second modulated signal.
4. The transmitter of claim 3, said transmitter further including:
means for generating in-phase pseudorandom noise (PN.sub.I) and quadrature
phase pseudorandom noise (PN.sub.Q) signals of predetermined PN codes, and
means for combining said PN.sub.I signal with said first modulated signal
to provide an I output signal, and for combining said PN.sub.Q signal with
said second modulated signal to provide a Q output signal.
5. The transmitter of claim 4, said transmitter further including means for
modulating in-phase (I) and quadrature phase (Q) carrier signals of a
predefined phase relationship with said I and Q output signals,
respectively.
6. The transmitter of claim 1 wherein said first means for combining
includes:
first means for replicating said first subsignal into first and second
identical symbol streams,
first means for multiplying each of said symbol streams by a coset code
coefficient of said first coset code in order to provide first and second
intermediate sequences,
a first multiplexer for combining said first and second intermediate
sequences into a first coset-encoded signal,
second means for replicating said second subsignal into third and fourth
identical symbol streams,
second means for multiplying said third and fourth symbol streams by a
coset code coefficient of said second coset code in order to provide third
and fourth intermediate sequences,
a second multiplexer for combining said third and fourth intermediate
sequences into said first coset-encoded signal, and
means for combining said first and second coset-encoded signals into said
first composite-encoded signal.
7. The transmitter of claim 1 wherein said means for combining said first
and second coset-encoded signals includes means for converting said first
and second coset-encoded signals into integer values selected from a set
of integers including +1 and -1.
8. A transmitter for modulating a set of p information signals of
equivalent data rate for simultaneous transmission in a spread spectrum
communication system, comprising:
means for combining each of said information signals with one of a set of p
coset codes to produce a set of p coset-encoded signals;
means for combining said p coset-encoded signals and for generating a
composite coset-encoded signal;
means for generating an orthogonal function signal; and
means for modulating said composite coset-encoded signal with said
orthogonal function signal in order to provide a first modulated signal.
9. The transmitter of claim 8, said transmitter further including:
means for generating a pseudorandom noise signal of predetermined PN code,
and
means for combining said modulated signal with said pseudorandom noise
signal of predetermined PN code so as to provide a first output signal.
10. The transmitter of claim 8 wherein said means for combining said
information signals with said coset codes includes:
means for replicating a first of said information signals into a set of p
identical symbol streams,
means for multiplying each of said symbol streams by one of a set of p
coset code coefficients included within a first of said coset codes in
order to provide a set of p intermediate sequences, and
a multiplexer for combining said p intermediate sequences into a first of
said coset-encoded signals.
11. The transmitter of claim 5, said transmitter further including means
for transmitting said I-modulated and said Q-modulated carrier signals
over I and Q communication channels, respectively.
12. A spread spectrum communication system for modulating an information
signal to be transmitted on in-phase (I) and quadrature phase (Q) using a
carrier signal and a replica of said carrier signal in phase quadrature
therewith, said system including a transmitter comprising:
means for demultiplexing said information signal into first and second sets
of subsignals;
means for combining said first set of subsignals with a first set of
orthogonal coset codes to produce a first composite coset-encoded signal,
and for combining said second set of subsignals with a second set of
orthogonal coset codes so as to produce a second composite coset-encoded
signal;
means for generating an orthogonal function signal;
means for generating in-phase pseudorandom noise (PN.sub.I) and quadrature
phase pseudorandom noise (PN.sub.Q) signals of predetermined PN codes; and
means for combining said PN.sub.I signal with said first composite
coset-encoded signal and said orthogonal function signal to provide an I
modulation signal, and for combining said PN.sub.Q signal with said second
composite coset-encoded signal and said orthogonal function signal to
provide a Q modulation signal.
13. The system of claim 12 further including:
means for modulating said carrier signal with said I modulation signal and
for modulating said replica of said carrier signal with said Q modulation
signal in order to provide I-modulated and Q-modulated carrier signals,
respectively, and
means for transmitting said I-modulated and said Q-modulated carrier
signals over said I and Q communication channels.
14. The system of claim 13 further including a receiver comprising:
means for producing an estimate of said information signal in accordance
with said I-modulated and Q-modulated carrier signals received over said I
and Q communication channels.
15. The communication system of claim 14 wherein said receiver further
includes means for producing intermediate received signals by demodulating
said received carrier signals using a replica of said orthogonal function
signal.
16. The communication system of claim 15 wherein said receiver further
includes:
means for generating a first despreading signal by replicating said
PN.sub.I signal, and
first means for correlating said intermediate received signals using said
first despreading signal in order to provide a first set of in-phase (I)
and quadrature phase (Q) projection signals.
17. The communication system of claim 16 further including:
means for combining said orthogonal function signal with a pilot signal in
order to provide a modulated pilot signal,
means for transmitting said modulated pilot signal over a pilot channel.
18. The communication system of claim 17 wherein said receiver further
includes:
means for demodulating said modulated pilot signal transmitted over said
pilot channel,
means for producing an estimate of said pilot signal transmitted over said
pilot channel,
first phase rotation means for generating said estimate of said first
information signal on the basis of said first set of said I and Q
projections and said estimate of said pilot carrier signal.
19. The communication system of claim 18 wherein said receiver further
includes:
means for generating a second despreading signal by replicating said
PN.sub.Q signal, and
second means for correlating said intermediate received signals using said
second despreading signal in order to provide a second set of in-phase (I)
and quadrature phase (Q) projection signals.
20. The communication system of claim 19 wherein said receiver further
includes second phase rotation means for generating an estimate of said
second information signal on the basis of said second set of I and Q
projections and said estimate of said transmitted pilot carrier signal.
21. The communication system of claim 18 wherein said receiver further
includes means for delaying said first set of I and Q projection signals.
22. A method for modulating an information signal for transmission in a
spread spectrum communication system, comprising the steps of:
demultiplexing said information signal into first and second subsignals;
combining said first subsignal with a first coset code and combining said
second subsignal with a second coset code orthogonal to said first coset
code so as to produce a first composite coset-encoded signal;
generating an orthogonal function signal; and
modulating said first composite coset-encoded signal with said orthogonal
function signal in order to provide a first modulated signal.
23. The method of claim 22 further including the steps of:
generating a pseudorandom noise signal of predetermined PN code, and
combining said first modulated signal with said pseudorandom noise signal
of predetermined PN code so as to provide a first output signal.
24. The method of claim 23 further including the steps of:
demultiplexing said information signal into third and fourth subsignals,
combining said third subsignal with a third coset code and combining said
fourth subsignal with a fourth coset code so as to produce a second
composite coset-encoded signal, said first, second third and fourth coset
codes being mutually orthogonal, and
modulating said second composite coset-encoded signal with said orthogonal
function signal in order to provide a second modulated signal.
25. The method of claim 24 further including the steps of:
generating in-phase pseudorandom noise (PN.sub.I) and quadrature phase
pseudorandom noise (PN.sub.Q) signals of predetermined PN codes, and
combining said PN.sub.I signal with said first modulated signal to provide
an I output signal, and combining said PN.sub.Q signal with said second
modulated signal to provide a Q output signal.
26. A method for modulating a set of p information signals of equivalent
data rate for simultaneous transmission in a spread spectrum communication
system, comprising the steps of:
combining each of said information signals with one of a set of p coset
codes to produce a set of p coset-encoded signals;
combining said p coset-encoded signals and so as to generate a composite
coset-encoded signal;
modulating said composite coset-encoded signal with an orthogonal function
signal in order to provide a first modulated signal.
27. In a code division multiple access (CDMA) communication system, a
method for providing in-phase (I) and quadrature phase (Q) spread spectrum
communication channels over which is transmitted an information signal,
said method comprising the steps of:
demultiplexing said information signal into first and second sets of
subsignals;
combining said first set of subsignals with a first set of orthogonal coset
codes to produce a first composite coset-encoded signal, and combining
said second set of subsignals with a second set of orthogonal coset codes
so as to produce a second composite coset-encoded signal;
generating in-phase pseudorandom noise (PN.sub.I) and quadrature phase
pseudorandom noise (PN.sub.Q) signals of predetermined PN codes; and
combining said PN.sub.I signal with said first composite coset-encoded
signal and an orthogonal function signal to provide an I modulation
signal, and combining said PN.sub.Q signal with said second composite
coset-encoded signal and said orthogonal function signal to provide a Q
modulation signal.
28. The method of claim 27 further including the steps of:
modulating said carrier signal with said I modulation signal and modulating
said replica of said carrier signal with said Q modulation signal in order
to provide I-modulated and Q-modulated carrier signals, respectively, and
transmitting said I-modulated and said Q-modulated carrier signals over
said I and Q communication channels.
29. The method of claim 28 further including the step of receiving said
I-modulated and Q-modulated carrier signals transmitted over said I and Q
communication channels, and for producing an estimate of said information
signal in accordance therewith.
30. The method of claim 29 wherein said step of producing an estimate of
said information signal includes the step of demodulating said received
carrier signals using replicas of said orthogonal function signal, said
PN.sub.I signal and said PN.sub.Q signal. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communication systems utilizing spread
spectrum signals, and, more particularly, to a novel and improved method
and apparatus for communicating information in a spread spectrum
communication system.
II. Description of the Related Art
Communication systems have been developed to allow transmission of
information signals from a source location to a physically distinct user
destination. Both analog and digital methods have been used to transmit
such information signals over communication channels linking the source
and user locations. Digital methods tend to afford several advantages
relative to analog techniques, including, for example, improved immunity
to channel noise and interference, increased capacity, and improved
security of communication through the use of encryption.
In transmitting an information signal from a source location over a
communication channel, the information signal is first converted into a
form suitable for efficient transmission over the channel. Conversion, or
modulation, of the information signal involves varying a parameter of a
carrier wave on the basis of the information signal in such a way that the
spectrum of the resulting modulated carrier is confined within the channel
bandwidth. At the user location the original message signal is replicated
from a version of the modulated carrier received subsequent to propagation
over the channel. Such replication is generally achieved by using an
inverse of the modulation process employed by the source transmitter.
Modulation also facilitates multiplexing, i.e., the simultaneous
transmission of several signals over a common channel. Multiplexed
communication systems will generally include a plurality of remote
subscriber units requiring intermittent service of relatively short
duration rather than continuous access to the communication channel.
Systems designed to enable communication over brief periods of time with a
set of subscriber units have been termed multiple access communication
systems.
A particular type of multiple access communication system is known as a
spread spectrum system. In spread spectrum systems, the modulation
technique utilized results in a spreading of the transmitted signal over a
wide frequency band within the communication channel. One type of multiple
access spread spectrum system is a code division multiple access (CDMA)
modulation system. Other multiple access communication system techniques,
such as time division multiple access (TDMA), frequency division multiple
access (FDMA) and AM modulation schemes such as amplitude companded single
sideband are known in the art. However, the spread spectrum modulation
technique of CDMA has significant advantages over these modulation
techniques for multiple access communication systems. The use of CDMA
techniques in a multiple access communication system is disclosed in U.S.
Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM
MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL
REPEATERS", assigned to the assignee of the present invention.
In the above-referenced U.S. Pat. No. 4,901,307, a multiple access
technique is disclosed where a large number of mobile telephone system
users each having a transceiver communicate through satellite repeaters or
terrestrial base stations using CDMA spread spectrum communication
signals. In using CDMA communications, the frequency spectrum can be
reused multiple times thus permitting an increase in system user capacity.
The use of CDMA results in a much higher spectral efficiency than can be
achieved using other multiple access techniques.
More particularly, communication in a CDMA system between a pair of
locations is achieved by spreading each transmitted signal over the
channel bandwidth by using a unique user spreading code. Specific
transmitted signals are extracted from the communication channel by
despreading the composite signal energy in the communication channel with
the user spreading code associated with the transmitted signal to be
extracted.
In particular spread spectrum communication systems it has been desired to
allow various types of user channels (e.g., voice, facsimile, or
high-speed data) to operate at different data rates. These systems have
typically been designed to have channels operative at a nominal data rate,
and also to have reduced data rate traffic channels for providing more
traffic data capacity. However, increasing traffic capacity by using
reduced data rate channels lengthens the time required for data
transmission. Moreover, in certain spread spectrum communication systems
there is also a need for increased data rate traffic channels allowing for
transmission at data rates higher than the nominal rate.
In order to support data transmission at variable rates it has generally
been required to vary the rates of encoding, interleaving and modulation
in accordance with the input data rate. This rate variation has typically
required relatively sophisticated control of channel encoding and decoding
processes, thereby increasing system cost and complexity
Accordingly, it is an object of the invention to provide a spread spectrum
communication system in which communication channels are available for
data transmission at both higher and lower than the nominal system rate.
It is yet another object of the present invention to provide such a spread
spectrum communication system in which a common format is used for
encoding, interleaving and modulating data to be transmitted at various
rates.
It is yet another object of the present invention to provide a CDMA spread
spectrum communication system allowing for increases in traffic channel
capacity in the absence of corresponding reductions in data rate.
SUMMARY OF THE INVENTION
The implementation of CDMA techniques in spread spectrum communication
systems using orthogonal PN code sequences reduces mutual interference
between users, thereby allowing higher capacity and better performance.
The present invention provides an improved system and method for
communicating information over in-phase (I) and quadrature phase (Q)
communication channels in a CDMA spread spectrum communication system.
In an exemplary embodiment, an input information signal is transmitted over
either an I or Q communication channel using a direct sequence spread
spectrum communication signal. The information signal is initially divided
into first and second subsignals, which are respectively provided to first
and second coset-encoding networks. The first coset-encoding combines the
first subsignal with a first coset code, while the second coset-encoding
network combines the second subsignal with a second coset code orthogonal
to the first coset code. In this way the first and second coset-encoding
networks are operative to produce first and second coset-encoded signals,
respectively. A composite coset-encoded signal formed from the first and
second coset-encoded signals is then modulated by an orthogonal function
signal to provide a first modulated signal.
In-phase pseudorandom noise (PN.sub.I) and quadrature phase pseudorandom
noise (PN.sub.Q) signals of predetermined PN codes are used for spreading
the first modulated signal over either the I or Q communication channel,
respectively. For example, the PN.sub.I signal may be combined with the
first modulated signal to provide an I-channel modulation signal for
transmission to a receiver via the I communication channel.
In the exemplary embodiment the receiver is operative to produce an
estimate of the input information signal on the basis of the modulated
carrier signal received over either the I or Q communication channel. The
received signal is first demodulated using the orthogonal function signal.
The demodulated signal is then decorrelated using a despreading PN signal,
with the resultant projection signals being provided to a phase rotator.
The phase rotator operates to provide an estimate of the composite
coset-encoded signal based on the projection signals and a received pilot
signal. Estimates of the first and second subsignals are made by
performing a further decorrelation based upon the orthogonality of the
first and second coset codes.
BRIEF DESCRIPTION OF THE DRAWINGS
Additional objects and features of the invention will be more readily
apparent frown the following detailed description and appended claims when
taken in conjunction with the drawings, in which:
FIG. 1 shows a block diagram of a conventional spread spectrum transmitter.
FIG. 2 shows a block diagram of a preferred embodiment of a spread spectrum
transmitter disposed to transmit I-channel and Q-channel information
signals.
FIG. 3 shows a block diagrammatic representation of an I-channel coset
encoding network operative to encode information signals in accordance
with the invention.
FIG. 4 is a block diagrammatic representation of a rate 1/p coset encoder
of a type suitable for inclusion in the coset encoding network of FIG. 3.
FIG. 5 shows a block diagrammatic representation of a pair of I-channel and
Q-channel coset encoding networks utilized in a preferred embodiment of
the invention to transmit data at four times a nominal data rate.
FIG. 6 shows a block diagrammatic representation of a pair of I-channel and
Q-channel 1/4 rate coset encoding networks utilized in a preferred
embodiment of the invention to transmit data at eight times the nominal
rate.
FIG. 7 shows a block diagrammatic representation of a coset encoding
network utilized in a preferred embodiment to transmit data at a rate
equivalent to one-half of the nominal rate.
FIG. 8 shows a block diagrammatic representation of a coset encoding
network utilized in a preferred embodiment to transmit data at a rate
equivalent to one-fourth of the nominal rate.
FIG. 9 depicts a pilot generation network for providing I and Q channel
pilot sequences.
FIG. 10 shows an exemplary implementation of an RF transmitter incorporated
within a preferred embodiment of the invention.
FIG. 12 is a block diagram of an exemplary diversity receiver disposed to
receive the RF signal energy transmitted over the I and Q communication
channels.
FIG. 13 is a block diagram of a receiver finger included within the
diversity receiver of FIG. 12 designed to process signal energy received
over a selected transmission path.
FIG. 14 provides a more detailed representation of the selected receiver
finger illustrated in FIG. 13.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown a spread spectrum transmitter such as
is described in U.S. Pat. No. 5,103,459, issued 1992, entitled "SYSTEM AND
METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE
SYSTEM", which is assigned to the assignee of the present invention, and
which is herein incorporated by reference. In the transmitter of FIG. 1,
data bits 100 consisting of, for example, voice converted to data by a
vocoder, are supplied to an encoder 102 where the bits are convolutional
encoded with code symbol repetition in accordance with the input data
rate. When the data bit rate is less than the bit processing rate of the
encoder 102, code symbol repetition dictates that encoder 102 repeat the
input data bits 100 in order to create a repetitive data stream at a bit
rate which matches the operative rate of encoder 102. The encoded data is
then provided to interleaver 104 where it is convolutional interleaved.
The interleaved symbol data is output from interleaver 104 at an exemplary
rate of 19.2 ksps to an input of exclusive-OR gate 106.
In the system of FIG. 1 the interleaved data symbols are scrambled to
provide greater security in transmissions over the channel. Scrambling of
the voice channel signals may be accomplished by pseudonoise (PN) coding
the interleaved data with a PN code specific to an intended recipient
subscriber unit. Such PN scrambling may be provided by the PN generator
108 using a suitable PN sequence or encryption scheme. The PN generator
108 will typically include a long PN generator for producing a unique PN
code at a fixed PN chip rate of 1.2288 MHz. This PN code is then passed
through a decimator, with the resulting 19.2 kilo-symbol per second (ksps)
scrambling sequence being supplied to the other input of exclusive-OR 106
in accordance with subscriber unit identification information provided
thereto. The output of exclusive-OR 106 is then provided to one input of
exclusive-OR 110.
Again referring to FIG. 1, the other input of exclusive-OR gate 110 is
connected to a Walsh waveform generator 112. Walsh generator 112 generates
a Walsh waveform assigned to the data channel over which information is
being transmitted. The Walsh waveform provided by generator 112 is
selected from a set of 64 Walsh waveforms, each having a length of length
64 Walsh chips. The 64 orthogonal waveforms correspond to entries within a
64 by 64 Hadamard matrix wherein a particular Walsh waveform is defined by
a row or column of the matrix. The scrambled symbol data and Walsh
waveform are exclusive-OR'ed by exclusive-OR gate 110 with the result
provided as an input to both of the exclusive-OR gates 114 and 116.
Exclusive-OR gate 114 also receives a PN.sub.I signal, while the other
input of exclusive-OR gate 116 receives a PN.sub.Q signal. The PN.sub.I
and PN.sub.Q signals are pseudorandom noise sequences typically
corresponding to a particular area, i.e., cell, covered by the CDMA system
and relate respectively to in-phase (I) and quadrature phase (Q)
communication channels. The PN.sub.I and PN.sub.Q signals are respectively
exclusive-OR'ed with the output of exclusive-OR gate 110 so as to further
spread the user data prior to transmission. The resulting I-channel code
spread sequence 122 and Q-channel code spread sequence 126 are used to
bi-phase modulate a quadrature pair of sinusoids. The modulated sinusoids
are summed, bandpass filtered, shifted to an RF frequency, and again
filtered and amplified prior to being radiated via an antenna to complete
transmission over the communication channel.
Conventional techniques for accommodating variable data rates within the
transmission system of FIG. 1 have generally required utilization of a
controller for varying the operating rates of the encoder 102, interleaver
104 and Walsh generator 112 in accordance with the input data rate. As is
described hereinafter, the present invention enables spread spectrum
transmission of an information signal at higher than the nominal rate, or
of transmission of a plurality of information signals at lower than the
nominal rate, using common encoding, interleaving and modulation rates.
FIG. 2 shows a block diagram of a preferred embodiment of a spread spectrum
transmitter 150 of the invention disposed to transmit an input information
signal S.sub.IN of data rate kR.sub.b, where k is an integer constant and
R.sub.b denotes a nominal transmitter data (i.e., bit) rate. As employed
herein the nominal data rate R.sub.b is defined as being equivalent to the
product of the PN chip rate and the convolutional encoding code rate,
divided by the number of Walsh chips per symbol of the Walsh waveform. In
an exemplary embodiment a nominal transmitter data rate R.sub.b of 9.6
kbps is set by utilizing a set of modulation parameters in which the PN
chip rate is selected to be 1.2288 MHz, the convolutional code rate is a
rate 1/2 code, and the Walsh waveform symbol length is set at 64. It is a
feature of the present invention that the transmitter 150 may be employed
to transmit information signals having data rates greater than or equal to
the nominal rate without adjustment of the values of the foregoing
modulation parameters. As is described hereinafter, the present invention
also provides a technique for transmitting a plurality of information
signals of data rates lower than the nominal rate without requiring
corresponding modulation parameter adjustment.
In particular applications the input information bit sequence S.sub.IN may
consist of, for example, voice converted to a stream of data bits by a
vocoder. As is indicated by FIG. 2, the input data stream is supplied to
an encoding and interleaving network 160. The network 160 convolutional
encodes the information bit sequence S.sub.IN, with the encoded data then
being interleaved and output from the network 160 as an encoded and
interleaved symbol stream S.sub.INT. Assuming rate 1/2 convolutional
encoding, the symbol stream S.sub.INT is supplied to a demultiplexer 170
at a symbol rate of 2 kR.sub.b. The demultiplexer 170 transforms the
symbol stream S.sub.INT into a set of k symbol substreams {A(1), A(2), . .
. A(k)} each at a rate of 2 R.sub.b, by routing successive symbols
S.sub.INT,i to successive ones of the substreams {A(1), A(2), . . . A(k)}.
The first k/2 symbol substreams are provided to an I-channel
coset-encoding network 180, while the remaining k/2 symbol substreams are
provided to a Q-channel coset-encoding network 190. As is described
hereinafter, in exemplary implementations of the coset-encoding networks
180 and 190 the symbol substreams are encoded using orthogonal sets of
coset codes of length p, where p=k/2. The coset-encoded symbol substreams
within the networks 180 and 190 are then summed into I-channel and
Q-channel composite symbol streams I.sub.c and Q.sub.c, respectively.
Although for completeness both an I-channel and a Q-channel coset-encoding
network are depicted in FIG. 2, in particular implementations it may be
desired to partition the symbol stream S.sub.INT into only k/2 symbol
substreams for transmission over either the I-channel or the Q-channel.
Referring again to FIG. 2, a pair of identical Walsh waveforms are provided
to I-channel and Q-channel modulation and spreading networks 200 and 205
by a Walsh waveform generator 210. The Walsh waveforms are used within the
networks 200 and 205 to modulate the I-channel and Q-channel composite
symbol streams I.sub.c and Q.sub.c. In addition, PN spreading signals are
also respectively provided to the modulation and spreading networks 200
and 205 by PN.sub.I and PN.sub.Q sequence generators 215 and 220. The
PN.sub.I sequence is used to spread the composite symbol stream I.sub.c
into an I-channel code spread sequence S.sub.I. Similarly, the PN.sub.Q
sequence is utilized by the network 205 to spread the composite symbol
stream Q.sub.c into a Q-channel code spread sequence S.sub.Q. The
resultant I-channel and Q-channel code spread sequences S.sub.I and
S.sub.Q are used to bi-phase modulate a quadrature pair of sinusoids
generated within an RF transmitter 225. The modulated sinusoids will
generally be summed, bandpass filtered, shifted to an RF frequency, and
amplified prior to being radiated via an antenna over I and Q
communication channels.
FIG. 3 shows a block diagrammatic representation of the I-channel coset
encoding network 180, it being understood that the Q-channel coset
encoding network may be realized in a substantially identical manner. The
encoding network 180 includes a plurality of coset encoders 250 to which
are supplied the k/2 symbol substreams from the demultiplexer 170. The
encoders 250 are operative to generate k/2 sequences {a(1), a(2), . . .
a(k/2)} in which
a(1)=A(1)[.sym.]S.sub.1
a(2)=A(2)[.sym.]S.sub.2,
a(k/2)=A(k/2)[.sym.]S.sub.k/2
where S.sub.1, S.sub.2, . . . S.sub.k/2 form a set of k/2 orthogonal coset
codes of length p, and where the operation [.sym.] is defined as follows.
Let A=(a.sub.1, . . . , a.sub.r) be a sequence of length "r" and let
B=(b.sub.1, b.sub.2, . . . , b.sub.k), be a sequence of length "k", then A
[.sym.] B denotes the sequence (a.sub.1 .sym.b.sub.1, . . . , a.sub.1
.sym.b.sub.k, a.sub.2 .sym.b.sub.1. . . , a.sub.2 .sym.b.sub.k, . . . ),
where .sym. denotes the exclusive-OR operation. In generating the
sequences {a(1), a(2), . . . a(k/2)} each symbol within the symbol
substreams {A(1), A(2), . . . A(k)} is repeated "p" times, with the
"p.sup.th " repeated symbol being exclusive-OR'ed with the p.sup.th
coefficient of the corresponding coset code. This operation has been
characterized by those skilled in the art as encoding using a "rate 1/p
repetition coset code".
FIG. 4 is a block diagrammatic representation of a rate 1/p coset encoder
300 disposed to use a coset code C to encode an input symbol stream
R.sub.s into an output coset-encoded symbol stream R.sub.s,enc, where C
{c.sub.1, c.sub.2, . . . , c.sub.p }. The coset encoder includes a
demultiplexer 305 for providing each symbol r.sub.i included within the
symbol stream R.sub.s to a set of p exclusive-OR gates 310. Each of the
symbols r.sub.i is exclusive-OR'ed with one of the coset code coefficients
c.sub.p, with the result being supplied to a p:1 multiplexer 315. The
multiplexer 315 then produces the coset-encoded symbol stream R.sub.s,enc,
where R.sub.s,enc {r.sub.1 .sym.c.sub.1, r.sub.1 .sym.C.sub.2, . . .
r.sub.1 .sym.c.sub.p, r.sub.2 .sym.c.sub.1, r.sub.2 .sym.c.sub.2, . . .
r.sub.2 .sym.c.sub.p, . . . , r.sub.i .sym.c.sub.p, . . . }. More
generally, for each symbol r.sub.i the rate 1/p coset encoder produces a
sequence,
(r.sub.i .sym.c.sub.1,r.sub.i .sym.c.sub.2, . . . , r.sub.i
.sym.c.sub.p)=r.sub.i [.sym.]C.
Referring again to FIG. 3, in the preferred embodiment the substreams
{A(1), A(2), . . . A(k)} and the coset codes S.sub.1, S.sub.2, . . . ,
S.sub.k/2 are composed of the logical values 0 and 1, as are the sequences
{a(1), a(2), . . . a(k/2)} generated by the coset encoders 250. The
sequences {a(1), a(2), . . . a(k/2)} are converted to an integer, i.e.,
.+-.1, representation by a set of binary-to-integer conversion circuits
260 as follows:
0.fwdarw.+1
1.fwdarw.-1
As shown in FIG. 3, the sequence I.sub.c is then created by combining the
outputs from the conversion circuits 260 within a digital adder 270.
Embodiments Supporting High Data Rates
I. 4.times.Nominal Rate
FIG. 5 shows a block diagrammatic representation of a pair of I-channel and
Q-channel coset encoding networks 350 and 360 utilized in a preferred
embodiment of the invention to transmit data at four times the nominal
rate. In particular, a rate 1/2 encoded and interleaved symbol stream at a
rate of eight times (e.g., 76.8 ksps) the nominal rate (e.g., 9.6 ksps) is
demultiplexed by sequentially assigning symbols to one of four substreams
{A(1), A(2), A(3), A(4)}, where A(1)={A.sub.11, A.sub.12, . . . }, A(2)=
{A.sub.21, A.sub.22, . . . }, A(3)={A.sub.31, A.sub.32, . . . }, and
A(4)={A.sub.41, A.sub.42, . . . }. In the implementation of FIG. 5 the
rate 1/2 encoded and interleaved symbol stream is derived from an input
data bit sequence (not shown) of a rate equivalent to four times the
nominal rate. As is indicated by FIG. 5, the substreams A(1) and A(2) are
respectively provided to rate 1/2 coset encoders 370 and 372 within the
I-channel coset encoding network 350, while substreams A(3) and A(4) are
respectively routed to rate 1/2 coset encoders 375 and 377 within the
Q-channel coset encoding network 360. For rate 1/2 repetition the coset
code (0,0) is used by the encoders 370 and 375 to encode the symbol
substreams A(1) and A(3), while the coset code (0,1) is supplied to the
coset encoders 372 and 377 for encoding the symbol substreams A(2) and
A(4). The encoded substreams from the I-channel coset encoders 370 and 372
are transformed into an integer format (.+-.1) by a pair of
binary-to-integer conversion networks 380, and combined within digital
adder 385 into the real sequence I.sub.c,4. In like manner the substreams
from the Q-channel coset encoders 375 and 377 are put into an integer
format by the binary-to-integer conversion networks 390, and are then
added within digital adder 395 to form the real sequence Q.sub.c,4.
FIG. 5 also shows preferred implementations of the I-channel and Q-channel
modulation and spreading networks 200 and 205. The I-channel network 200
includes a multiplier 400 for multiplying the sequences I.sub.c,4 and
Q.sub.c,4 by a Walsh function W provided by the Walsh generator 210 in an
integer (i.e., .+-.1 format), where in an exemplary implementation
W=(W.sub.1, W.sub.2, . . . , W.sub.32, W.sub.33, . . . , W.sub.64). In
this way the coset encoding networks 350 and 360 operate in conjunction
with the spreading networks 200 and 205 to effectively assign the Walsh
function W to the substreams A(1) and A(3), and to assign a Walsh function
W* to the substreams A(2) and A(4), where W*=(W.sub.1, W.sub.2, . . . ,
W.sub.32, -W.sub.33, . . . , -W.sub.64).
A PN.sub.I sequence is provided to a multiplier 402 operative to spread the
sequence I.sub.c,4 into the I-channel code spread sequence S.sub.I,4
produced by the I-channel network 200. Similarly, a PN.sub.Q sequence is
used by multiplier 404 in spreading the sequence Q.sub.c,4 into a
Q-channel code spread sequence S.sub.Q,4 produced by the network 205. The
resultant I-channel and Q-channel code spread sequences S.sub.I,4 and
S.sub.Q,4 are used to bi-phase modulate a quadrature pair of sinusoids
generated within an RF transmitter (not shown).
II. 8.times.Nominal Rate
FIG. 6 shows a block diagrammatic representation of I-channel and Q-channel
1/4 rate coset encoding networks 450 and 460 utilized in a preferred
embodiment of the invention to transmit data at eight times the nominal
rate. An input bit sequence at eight times the nominal rate is rate 1/2
encoded and interleaved into a symbol stream at sixteen times (e.g., 153.6
ksps) the nominal rate (e.g., 9.6 kbps) is demultiplexed by sequentially
assigning symbols to one of eight substreams A(i),i=1, . . . , 8, where
A(i)={A.sub.i1, A.sub.i2, . . . }, i=1, . . . , 8.
As is indicated by FIG. 5, the substreams A(1)-A(4) are respectively
provided to I-channel rate 1/4 coset encoders 470, 472, 474 and 478 within
the I-channel coset encoding network 450, while substreams A(5)-A(8) are
respectively routed to Q-channel rate 1/4 coset encoders 480, 482, 484 and
488 within the Q-channel coset encoding network 460. A rate 1/4 coset code
S.sub.1 is used by the encoders 470 and 480 to encode the symbol
substreams A(1) and A(5), a coset code S.sub.2 is used by the encoders 472
and 482 to encode the symbol substreams A(2) and A(6), a coset code
S.sub.3 is used by the encoders 474 and 484 to encode the symbol
substreams A(3) and A(6), while the coset code S.sub.4 is used by the
encoders 478 and 488 to encode the symbol substreams A(4) and A(8). The
coset codes S.sub.1 through S.sub.4 are defined as follow | | |
