The performance of multi-user cooperative diversity in an asynchronous CDMA uplink

Abstract

This paper investigates the impact of interuser non-orthogonality and asynchronous communication on the information-outage probability performance of multiuser decode-and-forward (DF) cooperative diversity in a code-division multiple-access (CDMA) uplink. Each user in the proposed system transmits its own data towards the base station and also serves as a relay for other users. We assume full-duplex communication so that each user can transmit and receive simultaneously at the same frequency. Each user attempts to decode the messages of a plurality of other users and forwards the superposition of multiple re-encoded and re-spread messages. Our cooperative scheme employs a sub-optimum decorrelating receiver to suppress the multiuser interference at both the base station and the relay-side. We evaluate the informationoutage probability performance of the proposed scheme in an underloaded, fully-loaded and overloaded CDMA uplink. We consider combining schemes at the base station where the source information is code combined with the relayed information, while the information from multiple relays is either code combined or diversity combined. Under the system parameters contemplated in this paper, diversity combining of the relayed information is nearly as good as code combining because of the associated probabilities of decoding at the relays. We then examine the effect of using practical modulation formats on the information-outage probability performance of the proposed DF multiuser sharing scheme under diversity combining. We see that the performance loss due to modulation constraints and the use of diversity combining instead of code combining is relatively small.

Decode-and-forward relaying based on orthogonalcomplementary codes

“…As seen in Eqs. (8) and (9), the signals from the source and relay terminals are transmitted in the same bands and the destination terminal needs to separate them successfully. Figure 3c draws the block diagram of the destination terminal.…”

“…The destination correlates the received signals in Eqs. (8) and (9) with c 11 (t) and c 12 (t), respectively, and obtains the signal from the source terminal r s (t) as…”

Distributed space-time block codes

“…Note that, although CDMA is a spread-spectrum signaling format, it does not need to operate in a spectrally inefficient mode. In fact, it was shown in [19] that the information outage probability of an asynchronous cooperative CDMA uplink under decorrelating multiuser detection is minimized when the system is slightly overloaded, i.e., when the number of relays is slightly larger than the processing gain. This is not surprising because an overloaded CDMA system is operating at high spectral efficiency.…”

Uplink Performance Evaluation of CDMA Communication System with RAKE Receiver and Multiple Access Interference Cancellation

“…The difference between multistage receivers and successsive interference canceller (SIC) receivers is that stead of using previous bit decisions to cancel interfereence from desired user's signal as in SIC, tentative decisions on each user are used to improve signal quality [10]. Receiver structure is called multistage, since when decisions are made, they can be used to either make a final decision on data or to enhance the signal through cancellation.…”