A robust error control scheme for data transmission in CDMA-based cellular systems is proposed which employs outer Reed-Solomon codes concatenated with inner convolutional codes. The performance of this scheme is analyzed assuming nonperiodic random spreading sequences and a Rake receiver with perfect knowledge of the channel. In particular, a simple model for the memoryless inner coding channel that encompasses the effects of multiple access interference, self-noise and thermal noise is first derived. Using new tight upper bounds on bit- and symbol-error probabilities of convolutional codes over Nakagami, Rayleigh, and Rician fading multipath channels, the performance of the concatenated coding scheme is then evaluated. The Reed-Solomon/convolutional coding scheme has been adopted by the European RACE Project Code Division Testbed (CODIT) and implemented in an experimental testbed. The code design methodology, which has been used to specify the 9.6-, 64-, and 128-kbit/s data traffic channels of the CODIT testbed, is presented and the single-cell CDMA capacity is computed.

For an aperiodic invertible sequence of length L, the optimality criterion is chosen to be the minimum noise enhancement factor of the corresponding aperiodic inverse filter. The noise enhancement factor is defined as the ratio of the noise energies at the outputs of the aperiodic inverse and the normalized matched filters of the sequence due to white noise at the filter inputs. Optimal binary sequences of length L/spl les/32 and optimal binary skew symmetric sequences of length 33/spl les/L/spl les/59 were found by exhaustive search. The search for near-optimal sequences is shown to have strong connections to the search for sequences with large products of Golay (1977) merit factor times the minimum of the energy density spectrum. Based on Legendre sequences and on Kronecker product sequences, long near-optimal binary sequences are found that have associated noise enhancement factors close to 1 dB.

A user-separating (US) demodulator for a multiple-access system with digital transmission is defined to be a demodulator that, without knowledge of the channel codes of the various users, provides the decoder for each user with a scalar-valued output for each symbol period that permits maximum-likelihood decoding of that user's data. It is shown that such a US demodulator exists in general only for approximations to the true statistics of the modulator-input sequences of the interfering users. It is argued that approximating the interfering modulator-input sequences as independent, white Gaussian processes is the practical compromise between accuracy and simplicity in code-division multiple-access (CDMA) systems. The US demodulator for this approximation is shown to consist of a kind of matrix whitening fitter followed by a kind of matched-filter for the user in question. It is further shown that this US demodulator can often be well approximated as the symbol-by-symbol demodulator that makes the linear minimum mean-squared error estimate of each modulator-input symbol for the user in question. Simulation results are presented to confirm the theory of US demodulation and to illustrate its practical utility.>

It is shown that the sum capacity of the symbol-synchronous code-division multiple-access channel with equal average-input-energy constraints is maximized precisely by those spreading sequence multisets that meet Welch's lower bound on total squared correlation. It is further shown that the symmetric capacity of the channel determined by these same sequence multisets is equal to the sum capacity.>