IB‐DFE receivers with space diversity for CP‐assisted DS‐CDMA and MC‐CDMA systems

Abstract

SUMMARYMulti-Carrier Code Division Multiple Access (MC-CDMA), currently regarded as a promising multiple access scheme for broadband communications, is known to combine the advantages of an Orthogonal Frequency Division Multiplexing (OFDM)-based, Cyclic Prefix (CP)-assisted block transmission with those of CDMA systems. Recently, it was recognised that DS-CDMA (Direct Sequence) implementations can also take advantage of the benefits of the CP-assisted block transmission approach, therefore enabling an efficient use of Fast Fourier Transform (FFT)-based, chip level Frequency-Domain Equalisation (FDE) techniques. When employing a linear FDE with both MC-CDMA and DS-CDMA, the FDE coefficients can be optimised under the Minimum Mean Squared Error (MMSE) criterion, so as to avoid significant noise enhancement. The residual interference levels can be very high, especially for fully loaded scenarios, since the FDE/MMSE does not perform a perfect channel inversion. This paper deals with CP-assisted DS-CDMA systems and MC-CDMA systems with frequency-domain spreading. We consider the use of Iterative Block Decision Feedback Equalisation (IB-DFE) FDE techniques as an alternative to conventional, linear FDE techniques, and derive the appropriate IB-DFE parameters in a receiver diversity context. Our performance results show that IB-DFE techniques with moderate complexity allow significant performance gains in both systems, with bit error rate (BER) that can be close to the single-code matched filter bound (MFB) (especially for the CP-assisted DS-CDMA alternative), even with full code usage.

Analytical BER Performance Evaluation in SISO and MIMO Environments with SC-FDE Modulations and IB-DFE Receivers

“…These performance are usually obtained by means of lengthy Monte Carlo simulations and it would be desirable to analytically obtain the BER performance of IB-DFE receivers. Since the overall noise plus residual Inter-Symbol Interference (ISI) is approximately Gaussian at the FDE output, it can be obtained an estimate of the BER from the MSE at the FDE output for each iteration [8]. These BER estimates are very accurate for the linear FDE, however, they present a non-negligible error in the iterative FDE case and in this paper it is shown that the error is a function of the number of multipath propagation components, as well as the MSE.…”

Performance evaluation of IB-DFE-based strategies for SC-FDMA systems

“…For this reason, there has been significant interest in the design of nonlinear FDE in general and decision feedback FDE in particular, with the IB-DFE being the most promising nonlinear FDE [7,8]. IB-DFE was originally proposed in [9] and was extended for a wide range of scenarios in the last 10 years, ranging from diversity scenarios [10,11], MIMO systems [12], CDMA systems [13,14], and multi-access scenarios [15,16], among many other. Essentially, the IB-DFE can be regarded as a low complexity turbo equalizer [17][18][19][20] implemented in the frequency domain that do not require the channel decoder output in the feedback loop, although true turbo equalizers based on the IB-DFE concept can also be designed [21][22][23].…”

Multi-User Joint Tx/Iterative Rx MMSE-FDE and Successive MUI Cancellation for Uplink DS-CDMA

“…However, the residual ICI after MMSE-FDE limits the achievable BER performance improvement. In order to further reduce the residual ICI after MMSE-FDE, iterative MMSE-FDE and ICI cancellation (ICIC) was proposed [4,5]. In this scheme, at first, a series of one-tap Rx FDE and despreading is carried out.…”