An efficient multiuser detector with low decision delay for multiple chip rate DS/CDMA mobile radio systems

Abstract

In this paper, an efficient multiuser detection scheme with low decision delay is presented for asynchronous multiple chip rate (MCR) DS/CDMA systems in mobile radio channels without any restrictions on processing gains and chip rates. An equivalent synchronous single bit rate DS/CDMA system is first formulated to break up the detection problem into the blocks of finite length called processing windows. Then, a low delay multipath-combining decision-feedback multiuser detector (LDMCDF) is proposed based on the equivalent system model. Since the LDMCDF makes the decisions for data bits in every processing window, the decision delay is less than the interval of one processing window. The effect of the processing window length on the performance of the LDMCDF is evaluated, and the simulation results show that the LDMCDF provides good performance with the negligible decision delay.

Introduction

The multiple access interference (MAI) and near–far problem limit the capacity and performance of DS/CDMA systems significantly [20]. There has been a great deal of interest in improving DS/CDMA detection through the use of multiuser detectors since the optimum multiuser detector [31] was proposed. The computational complexity of the optimum detector has prompted the development of suboptimal multiuser detectors with less complexity. Among the suboptimal detectors, the most well-known detector is the linear decorrelator [16], [17]. The linear decorrelator eliminates the MAI at the cost of increased noise power and does not require the knowledge of the received signal strengths. In [32], [35], multipath-decorrelating strategies were studied in the frequency-selective fading channel [24] which results in multipath propagation. Since these strategies apply the decorrelating technique to multipath signals, the complexity of the decorrelating operation increases as the number of paths being combined does. The partial decorrelating decision-feedback detector [4], [5] is somewhat more complex than the linear decorrelator. By utilizing the bit decisions of the stronger users as feedback terms to reduce the MAI for the weaker users, this detector outperforms the linear decorrelator at the cost of user power estimation.

Future DS/CDMA systems must be able to support multiple data rate (henceforth, multirate) services such as speech, image, data, and even moving picture. Therefore, receiver design for multirate DS/CDMA systems is a research area of great interest. There are several alternatives to implement the multirate services. Among them, multimodulation, variable processing gain (VPG), and multichannel systems alter the modulation level, processing gain, and number of the signature sequences used, respectively, according to the data rate [21]. Considering the issue of radio frequency (RF) resource management, since these methods use a single RF bandwidth, they can hardly achieve the wide range of service bit rates which range from a few kbits/s to as high as $\text{2}\text{Mbits/s}$ [1]. For the efficient RF resource management, multiple chip rate (MCR) systems using multiple RF bandwidths corresponding to different chip rates were proposed [1], [18]. In the MCR-DS/CDMA system, signals are transmitted at different chip rates, processing gains, and carrier frequencies according to their own service requirements; e.g., low-rate services can be implemented on narrowband channels and high-rate services on wideband channels. Note that the VPG-DS/CDMA system is the special case of the MCR-DS/CDMA system with the same carrier frequencies and the same chip rates. Services of different RF bandwidths can use the different frequency bands or the identical frequency band [1], and the latter is more efficient in the RF resource management than the former. The latter spectrum allocation is assumed in this paper.

There have been several studies on receiver design for multirate DS/CDMA systems. For a dual rate synchronous VPG-DS/CDMA system, the linear decorrelator was investigated in [26], [27] and the partial decorrelating decision-feedback detector in [28]. The papers [9], [10], [11] considered the multirate DS/CDMA systems of which data rates are the integer multiples of the lowest data rate. Multistage multiuser detectors were investigated in [9] for the VPG and multichannel schemes. In [10], [11], successive interference cancellation detectors were studied in the context of the multimodulation and multichannel schemes. However, since the aforementioned studies have been performed in the restricted range of multirate DS/CDMA systems, particularly in the single chip rate systems, more studies need to be done for the MCR-DS/CDMA system to mitigate those restrictions.

The linear decorrelator for asynchronous DS/CDMA systems becomes a non-causal linear time invariant filter with infinite impulse response when an infinite length transmission is assumed [17]. Hence, for practical asynchronous DS/CDMA systems, the memory length and decision delay approach infinity. This fact makes it impractical to implement the linear decorrelator. There have been numerous approaches to implement the linear decorrelator for the asynchronous single rate DS/CDMA system. The sliding window decorrelating algorithm (SLWA) [32] breaks up the detection problem into finite length blocks by utilizing the structure of coded DS/CDMA systems. However, the SLWA works only with coded DS/CDMA systems. In [34], Zheng proposed the insertion of isolation bits into the information bit sequence before modulation to realize a finite sequence length decorrelator. This technique causes a reduction in the effective throughput of the system. Kajiwara and Nakagawa [14] treat the asynchronous DS/CDMA system as a synchronous DS/CDMA system with the reduced processing gain. The reduced correlation time of the matched filters used in [14] may cause performance loss. The one-shot linear decorrelator realizes the bit-by-bit detection by treating every user as two independent users and using maximal-ratio combination [23]. The FIR decorrelator [12] is the truncated linear decorrelator with a finite memory length. In [13], the FIR decorrelator has been applied to the asynchronous VPG-DS/CDMA system whose bit rates are the integer multiples of the lowest bit rate by formulating an equivalent single rate DS/CDMA system with the lowest bit rate. However, the decision delay may become large for the high bit rate user when this scheme is applied to the system of the wide range of service bit rates.

In this paper, we consider the multiuser detection scheme for the asynchronous MCR-DS/CDMA system in the frequency-selective slowly varying Rayleigh fading channel without any restrictions on the processing gain and chip rate. Motivated by the studies on implementing the linear decorrelator and the good performance of the partial decorrelating decision-feedback detector, we develop a multiuser detector which achieves good performance with negligible decision delay. In order to reduce the decision delay, we break up the detection problem into the finite length blocks called processing windows by deriving an equivalent synchronous single rate DS/CDMA system model from the asynchronous MCR-DS/CDMA system. Based on this equivalent system model, a low delay multipath-combining decision-feedback multiuser detector (LDMCDF) is proposed. The multipath signals are coherently combined to alleviate the fading effects of the channel. The MAI cancellation is performed on the combined multipath signals so that the complexity of the MAI cancellation may no more depend on the number of paths being combined. The decisions of the transmitted data bits are obtained by using the decision-feedback and maximal-ratio combining detection schemes in every processing window. Hence, the decision delay becomes less than the interval of one processing window.

The rest of this paper is organized as follows. In Section 2, the received signal model of the MCR-DS/CDMA system is described. The proposed equivalent system model is formulated in Section 3. In Section 4, the proposed LDMCDF is described. Section 5 presents numerical results. Finally, we draw conclusions in Section 6.