Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-04-09
2002-03-26
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06363103
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates in general to code division multiple access (CDMA) communications systems and, more particularly, to an improved method of interference cancellation for CDMA communications systems using M-ary orthogonal modulation.
Multiple access communications techniques enable multiple users, such as mobile telephones, to share the same path, for example a radio channel, to communicate to one receiver or base station at the same time. Examples of multiple access techniques include frequency division multiple access (FDMA) wherein different users are assigned to different frequency bands of the channel, time division multiple access (TDMA) wherein different users are assigned to non-overlapping time slots of the channel, and code division multiple access (CDMA). In CDMA, different users are assigned unique spreading codes, commonly pseudorandom noise (PN) codes, which are high bandwidth bitstreams used to multiply a respective baseband signal before transmission. Multiplying a baseband signal by a spreading code increases the bandwidth of the signal by a factor known as the spreading gain to spread the baseband signal across the channel.
Upon receipt at the base station, each user's signal is separated and decoded by first multiplying the total received signal by the complex conjugate of the desired user's spreading code. This removes the desired user's spreading code from the received signal or despreads the desired signal back to its original bandwidth and makes other user' signals look like high bandwidth noise. The despread signal, together with interference due to other user' signals, i.e. multiple access interference, is used in a conventional CDMA receiver to decode the desired user's transmitted bits, treating the interference as additive noise. The quality of reception at the base station can be substantially improved if the multiple access interference, rather than being treating as noise, is canceled from the received signal before decoding the desired user's signal.
To this end, interference cancellation (IC) techniques are employed to try to reduce multi-access interference in a CDMA receiver by estimating the interference due to other users and then subtracting the estimated interference from the received signal before the desired user's signal is decoded. A multistage or parallel interference canceler (PIC) consists of a number of concatenated stages which are usually identical to one another. The total received signal is passed to the first stage which makes tentative decisions as to the transmitted signals of all the users. While making a tentative decision on a particular user's signal, all other user' signals are treated as noise. For each user, an estimate of interference is obtained by respreading and combining the tentative signal decisions of all other users. The interference estimate is then subtracted from the received signal to form a “cleaner” signal for that user, which is passed to the next stage of interference cancellation. The next stage uses the cleaner signals for each user to again estimate and subtract interference. This is repeated for any desired number of stages with two to four stages being typical. Output signals from the final stage are used by a conventional CDMA decoder to make symbol decisions, i.e., to determine what symbols were in the received signal.
At each mobile telephone, bits of the user's signal can be modulated for example as binary phase-shift keying (BPSK) signals or as M-ary orthogonal signals (as in IS-95 North America CDMA standard) prior to spreading. With BPSK modulation, the baseband signal of a user takes the values +1 or −1 depending on whether the bit is a 0 or a 1. With M-ary orthogonal modulation, a group of log
2
M bits are mapped onto one of M Walsh codes, each Walsh code having M bits taking values −1 or +1. For example, M=64 in the uplink of IS-95 CDMA standard, so that 6 bits are modulated to one of 64 Walsh codes with each Walsh code being 64 bits long. All M codes are orthogonal to each other. Decoding a BPSK modulated signal after despreading involves integrating over the bit interval and hardlimiting the result. For M-ary orthogonal modulation, decoding is done by computing the correlations of the despread signal with all the M possible Walsh codes and determining the strongest among them.
FIGS. 1 and 2
illustrate a prior art parallel interference cancellation (PIC) arrangement for an IS-95-like CDMA system using M-ary orthogonal modulation with Walsh-Hadamard functions as symbol waveforms.
FIG. 1
schematically shows a general architecture of an N-stage PIC
100
. Carrier is removed from the received signal to obtain the complex baseband received signal r, which is the sum of all signals received from the K simultaneous telephones or users plus noise.
The output of each stage
102
,
104
,
106
of the PIC
100
is a set of estimates of all the user' received signals: u
1,n
, u
2,n
, . . . , u
k,n
, where lower case n is used to indicate the number of any stage and lower case k is used to indicate the number of any user. As shown in
FIG. 2
, signal u
k,n−1
, which consists of user k's received signal plus an interference component, is used by the nth stage to reconstruct user k's received signal. One of K conventional decoders
108
,
110
,
112
, coherent or non-coherent, for M-ary orthogonal CDMA signals are used to decide which one of the M Walsh functions or symbols was transmitted by the kth user. The one of decoders
108
,
110
,
112
also performs the despreading operation by multiplying the input signal with the complex conjugate of the kth user's PN code.
The M-ary decoders
108
,
110
,
112
are followed by Walsh code generators
114
,
116
,
118
which produce the corresponding symbol waveforms so that a single symbol waveform is used for reconstruction of each symbol waveform. The reconstructed symbol waveform for the kth user is then respread by multiplying it with the kth user's PN code and scaled by the complex valued channel estimate â
k
to obtain the reconstructed user k's baseband received signal. For the kth user, the interfering signals from all other users thus reconstructed are subtracted from the total received signal r to produce u
k,n
. If the symbol decisions in the nth stage are sufficiently accurate, u
k,n
will have a lower interference component than u
k,n−1
. In general, the amount of interference reduced in the nth stage will depend on the correctness of symbol decisions in that stage.
The received baseband signal r is given to all the inputs for the first stage. The outputs of the Nth stage are used by conventional M-ary decoders
120
,
122
,
124
such as the ones described above to make final symbol decisions for each user. If the PIC
100
is followed by an error correcting decoder, such as a Viterbi decoder, for channel encoded data, the M-ary decoder may provide soft decisions instead of hard symbol decisions.
SUMMARY OF THE INVENTION
The present invention comprises a multistage or parallel interference cancellation (PIC) arrangement for use in a communications system using code division multiple access (CDMA) with M-ary orthogonal modulation, such as the uplink specified by the IS-95 North American CDMA standard. All the M symbol waveforms, weighted according to their correlations with a despread signal, are combined to form a reconstructed symbol waveform for each user in each stage. The reconstructed symbol waveforms for all users are summed to form an estimated composite received signal having reduced interference which is subtracted from the composite received signal to generate a residual interference signal (RIS). While the total received signal is despread in the first stage of the PIC, the RIS or cancellation residue signal from the immediately preceding stage is used in succeeding stages of the PIC. A final decision is made by despreading the RIS from the final stage of the P
Buehrer R. Michael
Gollamudi Sridhar
Nicoloso Steven P.
Burd Kevin M.
Stevens & Showalter LLP
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