Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-08-17
2003-08-26
Trost, William (Department: 2683)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S320000, C370S335000, C370S441000, C370S535000, C370S537000
Reexamination Certificate
active
06611513
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority of European Patent Application No. 98306720.8, which was filed on Aug. 21, 1998.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a CDMA system, including a transmitter and receiver, for use in e.g. a digital wireless communications system. In particular, the invention relates to a method of and apparatus for mapping and demapping CDMA signals.
2. Description of Related Art
Recently iterative decoding algorithms have become a vital field of research in digital communications. The first discovered and still most popular encoding scheme suited for iterative decoding is the parallel concatenation of two recursive systematic convolutional codes, also referred to as ‘Turbo Codes’, as described in J. Hagenauer, “The Turbo Principle: Tutorial Introduction and State of the Art”, Symposium on Turbo Codes, Brest, France, September 1997. In the past few years other applications of the ‘Turbo Principle’ have been found e.g. G. Bauch, H. Khorram, J. Hagenauer, “Iterative Equalization and Decoding in Mobile Communications Systems”, in Proc. EPMCC'97, Bonn, pp. 307-312, Germany, October 1997.
Channel coding is used to make the transmitted digital information signal more robust against noise. For this the information bit sequence is encoded at the transmitter by a channel encoder and decoded at the receiver by a channel decoder. In the encoder, redundant information is added to the information bit sequence in order to facilitate error correction in the decoder. For example, in a systematic channel encoding scheme the redundant information is added to the information bit sequence just as additionally inserted, ‘coded’ bits. In a non-systematic encoding scheme the outgoing bits are all coded bits, and no ‘naked’ information bits remain. The number of incoming bits (information bits) at the encoder is smaller than the number of outgoing bits (information bits plus inserted coded bits, or all coded bits). The ratio of incoming/outgoing bits is called the ‘code rate R’ (typically R=½).
Concatenated coding schemes apply to at least two parallels or serially concatenated encoders. There are iterative decoding algorithms for either parallel or serially concatenated coding systems and the classical ‘Turbo’ Codes are parallel concatenated codes.
FIG. 1
shows a genuine serially concatenated coding scheme with the transmission performed on a block-by-block basis. The signal sequence is encoded twice at the transmitter in a serial manner. The binary signal from the digital source (e.g. an analogue to digital converter with analogue input signal from a microphone) is first encoded by an outer encoder. The output of the outer encoder gets passed through an interleaver which changes the order of the incoming bit symbols to make the signal appear more random to the following processing stages. After the interleaver the signal is encoded a second time by an ‘inner encoder’. Correspondingly, at the receiver the signal is first decoded by the inner decoder, deinterleaved, and decoded by the outer decoder. From the outer decoder soft values are fed back as additional ‘a priori’ input to the inner decoder. The soft values are reliability values of the quality of the decoded signal. The feedback of these values helps to reduce the bit error rate of the hard decision values 0,1 at the output of the outer decoder in further, iterative decoding steps. The iterative decoding of a particular transmitted sequence is stopped with an arbitrary termination criterion, e.g. after a fixed number of iterations, or until a certain bit error rate is reached (the termination criterion is not important at all for the invention). It should be noted that the ‘a priori’ soft value input to the inner decoder is set to zero for the very first decoding of the transmitted bit sequence (‘0th iteration’).
The inner and outer binary codes can be of any type: Systematic, or non-systematic, block or convolutional codes.
At the receiver the two decoders are soft-in/soft-out decoders (SISO decoder). A soft value represents the reliability of the bit decision of the respective bit symbol (whether 0 or 1). A soft-in decoder accepts soft reliability values for the incoming bit symbols. A soft-out decoder provides soft reliability output values on the outgoing bit symbols. The soft-out reliability values are usually more accurate than the soft-in reliability values since they can be improved during the decoding process based on the redundant information that is added with each encoding step at the transmitter.
Method and apparatus for iteratively demapping a signal is described in European Patent Application number 98302653.5 filed on Apr. 3, 1998, the contents of which are incorporated herein by reference.
CDMA (Code Division Multiple Access) systems have a plurality of users separated by different codes. Each transmitted bit of a user k is substituted by a number N
c
of ‘shorter’ bits, called ‘chips’, which are chosen according to the ‘channelization code’ of the particular user k. Since the occupied bandwidth expands by a factor of N
c
after the ‘substitution’, this process is called ‘spreading’, and a CDMA system is often referred to as ‘spread spectrum system’. After spreading, the signal for each user occupies the total available bandwidth B. At the receiver the desired user is detected by means of correlation with the appropriate channelization code.
Multicode CDMA is a method that provides higher data rates to a single user in a CDMA system. The particular user is assigned N channelization codes, instead of having only one in a conventional CDMA system. Hence the ‘multicode’ user can transmit at an N times higher data rate than a ‘single-code’ user. The N binary antipodal codes are added up at the multicode transmitter to form a N+1-level amplitude modulated signal, instead of a binary antipodal signal for the single code case.
FIG. 2
shows a conventional multicode CDMA (M-CDMA) single user transmitter/receiver. The binary signal from the digital source (e.g. an analogue to digital converter with analogue input signal from a microphone) is first encoded by the channel encoder. The output of the channel encoder gets passed through an interleaver which changes the order of the incoming bit symbols to make the signal appear more random to the following processing stages. After the interleaver the coded bit stream is split into N parallel bit streams by a demux (serial to parallel conversion). Each bit stream
1
, . . . ,N gets spread (i.e. multiplied) by a binary antipodal channelization code (codeword
1
, . . . ,N) of length N
c
chips. Typically, the N channelization codes are orthogonal. After spreading the N binary antipodal chip streams are added up on the chip rate to form the amplitude modulated (N+1 levels) chip symbols. Hence N coded bit symbols result in N
c
chip symbols. Each block of N
c
chip symbols is referred to as a composite multicode CDMA symbol. Optionally, scrambling on the chip rate can be applied to even further randomize the signal. Typically the scrambling sequence is a binary antipodal pseudo-random sequence. The composite signal is then put to the transmission channel.
FIG. 2
shows base band processing only and up-conversion to radio frequency etc. is omitted for the sake of clarity. For simplicity of the description we assume a real signal processing. However, the real channelization codes could be complex channelization codes as well, or there could be a complex scrambling sequence.
On the channel the signal is distorted by additive noise, or any other noise form.
Correspondingly, at the receiver the signal is descrambled (optionally) and correlated with the N channelization codes (codewords
1
, . . . ,N). The correlation consists of a multiplication with the respective channelization code and an accumulation over N
c
chips. After multiplexing (parallel to serial conversion) and deinterleaving the N correlation results are put to the channel decoder. Finally, the information bi
Agere Systems Inc.
Torres Marcos
Trost William
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