Abstract In this paper, we investigate multi‐group linear turbo equalization using single antenna interference cancellation (SAIC) techniques to mitigate the intercell interference for multi‐carrier code division multiple access (MC‐CDMA) cellular systems. It is important for the mobile station to mitigate the intercell interference as the performance of the users close to cell edge is mainly degraded by the intercell interference. The complexity of the proposed iterative detector and receiver is low as the one‐tap minimum mean square error (MMSE) equalizer is employed for mitigating the intracell interference, while a simple group interference canceller is used for suppressing the intercell interference. Simulation results show that the proposed iterative detector and receiver can mitigate the intercell interference effectively through iterations for both uncoded and coded signals. Copyright © 2009 John Wiley & Sons, Ltd.

In this paper, we propose a feedback based cooperative transmission (FBCT) scheme for multiuser multichannel communication systems. We focus on uplink channels and consider a user, say user 1, who has a weak uplink channel. Another user (who is possibly located somewhere between the base station and user 1), say user 2, can be a relay for cooperative transmission. Using the feedback information of a positive acknowledgment (ACK) or negative acknowledgment (NACK) signaling from the base station, the signal from user 1 can be re-relayed by the relay (i.e., user 2) or re-transmitted by the source (i.e., user 1) in the proposed FBCT scheme. The resulting FBCT scheme can be seen as a hybrid of cooperative transmission and automatic repeat request (ARQ) protocol. We show that a mixed strategy of re-relay and re-transmission of the proposed FBCT scheme can perform better than a pure strategy of re-relay or re-transmission.

This paper presents a modified turbo principle (MTP) for iterative detection and decoding in multiple-antenna systems whose detector uses the max-log-MAP algorithm. The modified turbo principle intensionally introduces dependency between the messages of the max-log-MAP detector and the decoder. Our simulation results show that the proposed MTP significantly reduces the performance loss due to the suboptimal detection algorithm, max-log-MAP. We also substantiate our claims by analyzing the proposed MTP from an information theoretic perspective.

In this paper, we consider iterative detector/receiver based on single antenna interference cancellation (SAIC) techniques for multicarrier code division multiple access (MC-CDMA) cellular systems. Since the performance of the users close to cell edge is mainly degraded by the intercell interference, it is important to suppress the intercell interference. Using spreading in MC-CDMA, the intercell interfering signals can be detected and cancelled. A better performance can also be achieved if iterative detectors/receivers are used. We propose low complexity iterative detector and receiver using the one-tap minimum mean square error (MMSE) equalizer and a simple group canceller as they are employed at the mobile station.

This paper aims to improve the design of iterative detection and decoding (IDD) based on the soft interference cancellation with minimum mean squared error (SC-MMSE) detector, which shows low performance compared to the maximum a posteriori (MAP) detector. Such low performance is attributed to that the "pure" (original) turbo principle is not always best for IDD with SC-MMSE. Thus, we propose a new IDD architecture based on the SC-MMSE detector which uses new a priori information. Extrinsic information transfer (EXIT) chart analysis and simulation results show that the performance of the proposed IDD is very close to that of IDD based on the MAP detector.

The minimum mean square error with successive interference cancellation (MMSE-SIC) receiver is known to achieve the capacity of multiple-input multiple-output (MIMO) fast fading channels in the presence of knowledge of the channel at the receiver. This paper presents efficient and numerically stable Cholesky decomposition based detection algorithms for MMSE-SIC, exploiting a property of ordering of MMSE-SIC. The proposed algorithms are shown to significantly reduce the computational complexity of existing efficient algorithms for SIC in MIMO flat fading channels.

This paper presents a new ordering criterion for successive interference cancelations (SIC) in MIMO flat fading channels and provides a performance comparison with the original ordering of V-BLAST. Furthermore, an efficient and stable Cholesky decomposition based detection algorithm is proposed to further reduce the computational complexity of existing fast algorithms for SIC in MIMO flat fading channels.

For the code division multiple access (CDMA) downlink channel, we investigate the decision feedback equalizer (DFE) at chip-level to effectively suppress multiple-access interference (MAI) when the spreading sequences are orthogonal. The structure of the receiver with the chip-level DFE has been investigated and the minimum mean-square error solution has been derived. Due to the inherent structure of the chip-level DFE, some iterative techniques with hard and soft decisions have been proposed. It is shown that the proposed receivers with the chip-level DFE can provide satisfactory performance. In comparison with the adaptive chip-level linear equalizer, the number of users can be doubled by using adaptive chip-level DFE at a bit-error rate of 10/sup -3/. Throughout the paper, we assume that scrambled orthogonal codes are used for spreading sequences.

A constrained minimum mean square error (CMMSE)-RAKE receiver for multipath fading channels is developed by extending the CMMSE receiver for flat fading channels. Based on the observation that interpath interference causes a bias of the channel estimator in , a receiver that can remove such a bias is proposed, plus a closed-form expression of the bit-error rate of the receiver is derived. Computer simulation is used to demonstrate that the proposed receiver can outperform existing RAKE receivers.

The constrained minimum mean square error (CMMSE)-RAKE receivers for multipath fading channels are analyzed. In particular, it is shown that for known channels the output signal-to-interference and noise ratio (SINR) of the CMMSE-RAKE receiver with a single constraint (Type 1) is greater than or equal to that of the CMMSE-RAKE with multiple constraints (Type 2). For Type 1 CMMSE-RAKE receiver, a maximum likelihood (ML) channel estimate is derived. Computer simulation results indicate that CMMSE receivers improve the bit error rate (BER) as compared with the conventional RAKE and currently available adaptive MMSE receivers, and that Type 1 performs the best.

It is well known that conventional detectors of direct sequence-code division multiple access systems are interference-limited. Multiuser detection has been considered as a promising approach to combat this problem. Since an optimal multiuser detector is too complex to implement, several suboptimal multiuser detector techniques have been developed. The one-shot linear decorrelating detector (LDD) was proposed to reduce the computational complexity and delay required in the conventional LDD, but it still require high complexity. Another one-shot LDD with low complexity was proposed. To improve its performance, this detector uses a maximal ratio combining (MRC) at the two outputs of the left piece of the present bit and the right piece of the next bit in an one-shot window; this detector is called as a one-shot LDD with post-MRC. In spite of the aid of MRC, the performance of the one-shot LDD with post-MRC is worse than the one-shot LDD. To overcome this problem, we propose the one-shot LDD with pre-SDC/post-MRC that employs selection diversity combining (SDC) with multiple antenna elements in front of the one-shot LDD with post-MRC. In asynchronous random code division multiple access systems over fading channels, the largest signal-to-noise ratio (SNR) of SDC, near-far resistance (NFR) of the one-shot LDD, and the output SNR of MRC are random variables. We derive the probability density functions (PDFs) for these random variables and calculate the bit error rate) of the proposed detector using the obtained PDFs. In a comparison of mathematical analysis and computer simulation results, we observe good agreement.

Adaptive linear minimum mean square error (LMMSE) receivers for frequency selective fading channels are developed by modifying the constrained LMMSE receiver of Kim, Jeong and Choi (see IEEE Trans. Commun., vol.48, p.1793-96, 2000). In particular, additional constraints regarding interpath interference are imposed on the constrained optimization proposed by Kim et al., and simple adaptation rules are derived by converting the constrained MMSE problem into an unconstrained optimization. It is shown that the instantaneous SINR of the proposed receiver is always higher than that of the RAKE receiver. Computer simulation results indicate that the proposed receivers can perform better than the existing RAKE and adaptive LMMSE receivers in multipath fading environments.

The one-shot linear decorrelating detector (LDD) was proposed to reduce the computational complexity and delay required in the conventional LDD, but it still requires high complexity. Another one-shot LDD with low complexity was proposed. To improve its performance, this detector uses maximal ratio combining (MRC) at the two outputs of the left piece of the present bit and the right piece of the next bit in a one-shot window. In spite of the aid from MRC, the performance of the one-shot LDD with post-MRC is worse than the one-shot LDD. To overcome this problem, we propose a one-shot LDD with pre-SDC/post-MRC that employs selection diversity combining (SDC) with multiple antenna elements in front of the one-shot LDD with post-MRC. In asynchronous random CDMA systems over fading channels, the largest SNR of SDC, near-far resistance (NFR) of the one-shot LDD, and the output SNR of MRC are random variables. We derive the probability density functions (PDFs) for these random variables and calculate the bit error rate (BER) of the proposed detector using the obtained PDFs. In comparison of mathematical analysis and computer simulation results, we observe good agreement.

The convergence problem of minimum mean square-error (MMSE) receivers is discussed, and to overcome the problem, a constrained MMSE receiver is proposed. In addition, we propose the orthogonal decomposition-based least mean square algorithm to implement the constrained MMSE receiver adaptively. Through computer simulations, it is shown that the proposed receiver provides significant performance improvement in the bit-error rate over the conventional matched filter receiver and currently available MMSE receivers.

Realistic direct sequence code division multiple access (DS/CDMA) channels are severely time-varying. Standard minimum mean squared error (MMSE) receivers have various problems in tracking the rapidly changing channel characteristics in fading environments. In addition, they require a training sequence for their adaptation. This paper presents a constrained MMSE receiver with a modified MMSE criterion. The constrained MMSE receiver is adaptively implemented using the orthogonal decomposition-based least mean square (LMS) algorithm. In order to improve the accuracy of the reference signal and channel estimates, a pilot symbol-aided scheme is employed. It also makes the proposed MMSE receiver not require any separate training sequence. The simulation results show that the proposed MMSE receiver provides significant performance improvements in bit error rate (BER) over the conventional matched filter (MF) receiver and the currently available MMSE receivers, and is insensitive to the speed of Rayleigh fading.

a new direction-based handoff call control scheme (DBHCCS) for CDMA cellular systems is proposed. In the proposed scheme, a neighbor base station ignores unnecessary soft handoff calls requested by mobile stations which are not actually approaching the neighbor base station, while protecting real soft handoff calls by employing a queue. We proposed a new method of determining the priorities of queued handoff calls. In this method, a mobile station predicts the time when its handoff call is forcedly terminated, by using the changing rate of the received pilot strength from the serving BS and using the current time broadcast by the sync channel of the serving BS as a reference time. The service priority of queued handoff calls is determined based on the predicted forced termination time reported by mobile stations. The performance of the DBHCCS is compared with that of the first-in/first-out (FIFO) scheme and the measurement-based prioritization scheme (MBPS). The simulation results show that the probability of new call blocking for the DBHCCS is lower than that of the FIFO scheme or the MBPS queuing discipline. It is also shown that the probability of forced termination for the DBHCCS is less than or equal to that of the conventional CDMA cellular system with the MBPS queuing discipline.