Multiuser receivers for randomly spread signals: fundamental limits with and without decision-feedback

Abstract

Synchronous code-division multiple-access (CDMA) communication systems with randomly chosen spreading sequences and capacity-achieving forward error correction coding are analyzed in terms of spectral efficiency. Emphasis is on the penalties paid by applying single-user coding in conjunction with suboptimal multiuser receivers as opposed to optimal joint decoding which involves complexity that is exponential in the number of users times the codeword length.The conventional, the decorrelating, and the (re-encoded) decorrelating decision-feedback detectors are analyzed in the nonasymptotic case for spherical random sequences. The re-encoded minimum mean-squared error (MMSE) decision-feedback receiver achieving the same performance as joint multiuser decoding for equal power users is shown to be suboptimal in the case of equal rates.

Spectral efficiency of CDMA with random spreading

“…We see from (56) that a CDMA system with random sequences, chips per symbol, a single-user matched-filter front-end whose output signal-to-noise ratio in the absence of interfering users is SNR, and a target output signal-to-noise ratio of sumf can accommodate up to sumf SNR (66) users-a result obtained independently in [51] without analyzing capacity. Equation (56) gives the capacity per user and per symbol ( chips).…”

Wardrop Equilibrium for CDMA Systems

“…Assuming perfect cancellation of decoded users, successive interference cancellation with MMSE filter achieves the optimum capacity [20]. The following proposition ensues from this fact.…”

“…α log 2 1 + β + − α log 2 (e) β In the left-hand side of (38), P (y) is given by a MMSE power allocation similar to the one given by (20 (20), we obtain the power allocation (21). We want to show that the expectation of the random variable …”

Fading Channels: InformationTheoretic and Communications Aspects

“…Random signature sequences modeling the practically appealing cases of long-signature sequences spreading many coded symbols are stressed. Other nonlinear front-end detectors, as decision-feedback decorrelator and MMSE processors, were also addressed (see [196] and [254, references]) and shown to be very efficient. The literature on the information-theoretic aspects of this issue is so vast that the references here, along with the reference lists therein, provide hardly more than a glimpse on this issue (see [308] and references therein for more details).…”

“…See for example [139], [123], [32], [36], [40], [311], [324], [277], [70], [317], [196], [291], [145], and [169]. Some of the most interesting results concern the -out-of-model [53], which captures the fact that although there are many potential users per cell, only a relatively small part of them are actually active.…”