Abstract
Recently, a block spreading code division multiple access (BS-CDMA) technique was presented, whereby user-specific precoding along with orthogonal spreading codes is used to achieve multiuser interference- (MUI-) free reception when all users arrive at the base station simultaneously. In practice, however, imperfect synchronization destroys the orthogonality among users, and MUI occurs. To mitigate the MUI in BS-CDMA due to quasisynchronous reception, this paper proposes an iterative successive interference cancellation (SIC) receiver, where cancellation of interfering signals is ordered according to the times of arrival (TOA) of the signals from different users. The ordering criterion is justified through analysis and simulation on the average signal-to interference-plus-noise ratio (SINR) of different users, where it is shown that in a quasisynchronous BS-CDMA system, ordering with regard to increasing TOA is equivalent to ordering with respect to decreasing average SINR, when practical channels such as the exponentially decaying channel is considered. The proposed SIC receiver is shown to achieve a performance close to a system with synchronous reception for only two iterations. In addition, an algorithm to determine the detection order of different blocks is proposed such that parallel detection of the signals from different users with reduced latency can be achieved.
Highlights
Code division multiple access (CDMA) is a popular multiple access technique that is used to support multiple users simultaneously in a network
A novel block spread CDMA (BS-CDMA) [1] framework was presented, whereby the use of user-specific, channel-independent precoding along with orthogonal spreading codes leads to variants of well-known practical multiple access techniques such as orthogonal frequency division multiple access (OFDMA), which has been considered as a mandatory technology in the Long Term Evolution (LTE) standard [2]
We present the uncoded bit error rate (BER) performance of each user for the aforementioned quasisynchronous BS-CDMA systems using one or two iterations in Figures 7, 8, and 9, considering the three asynchronous scenarios discussed in Section 3, which are referred to as case 1, case 2, and case 3, respectively
Summary
Code division multiple access (CDMA) is a popular multiple access technique that is used to support multiple users simultaneously in a network. We consider a quasi-synchronous BSCDMA system and show through analysis and simulation that for practical channels such as the exponentially decaying channel, detection with an order of increasing times of arrival (TOA) is essentially the same as an order of decreasing average SINR This property of ordering holds only for particular channel models with an exponentially decaying power delay profile, it does not diminish the practicality of the proposed SIC receiver, because these channel models are considered to be in good agreement with practical channel measurements [19] and have been adopted by the 3rd Generation Partnership Project (3GPP) to model channels for cellular networks [20].
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