We consider the resource allocation and beamforming design for system utility maximization in a cloud radio access network (C-RAN) under given fronthaul rate constraints and quality of service (QoS) requirements at users. The considered problem is known to be NP-hard and thus it is difficult to obtain global optimum solutions. Under practical assumptions, we approximate the considered problem using a quasi-geometric-programming problem. By analyzing its optimality conditions, we propose a fast iterative algorithm with low complexity to solve it. We show that the proposed algorithm is locally convergent. Simulations are carried out to verify the proposed algorithm. Compared to using standard convex optimization software, the proposed algorithm can reduce the processing time to as low as 1/700 while achieving the same final results.1

We consider a heterogeneous cellular network with densely underlaid small cell access points (SAPs). Wireless backhaul provides the data connection from the core network to SAPs. To serve as many SAPs and their corresponding users as possible with guaranteed data rates, admission control of SAPs needs to be performed in wireless backhaul. Such a problem involves joint design of transmit beamformers, power control, and selection of SAPs. In order to tackle such a difficult problem, we apply$\ell_1$-relaxation and propose an iterative algorithm for the$\ell_1$-relaxed problem. The selection of SAPs is made based on the outputs of the iterative algorithm, and we prove such an algorithm converges locally. Furthermore, this algorithm is fast and enjoys low complexity for small-to-medium sized systems. However, its solution depends on the actual channel state information, and resuming the algorithm for each new channel realization may be unrealistic for large systems. Therefore, we make use of the random matrix theory and also propose an iterative algorithm for large systems. Such a large-system iterative algorithm can produce the asymptotically optimum solution for the$\ell_1$-relaxed problem, which only requires large-scale channel coefficients irrespective of the actual channel realization. Near optimum results are achieved by our proposed algorithms in simulations.

In the new IEEE 802.11ah standard, a CRC code of 4 parity bits provides error detection for the SIG field of length 36 or 48 bits, giving rise to a significant performance penalty, which may be acceptable if there is an additional 32-bit Frame Check Sequence check on the payload in layer 2. However, in 802.11ah, there exist new Medium Access Control (MAC) control frames which are Null Data Packets (NDPs), which squeeze all critical information into the SIG field, and do not contain any payload at all. Hence, error checking is solely on CRC-4. These NDPs signal critical control functions in 802.11ah, and will affect MAC performance and energy consumption greatly if false positives occur. To achieve better performance, we propose that the number of parity bits be increased, and give the generator polynomials that achieve the improved performance. For the best performance-compatibility trade-off, an existing IEEE 802.11 CRC-8 generator can be used; the number of parity bits increased can be kept to 2, with a moderate performance gain, which can be achieved by a CRC-6 generator. Further, if the current generator and NDP control frame formats are fixed, but have some essential information bits to be received with high probability that they are correct, we may nest an inner CRC code to increase the error detection performance of the essential bits. For the inner code, an existing IEEE 802.11 CRC-8 generator can be used with a loss of 8 non-essential bits. We also propose an inner CRC-4 generator for a moderate performance gain and minimal loss of non-essential bits, and inner CRC-6 generators for enhanced performance.

Heterogeneous network (HetNet) is expected to be widely deployed in 4G cellular systems such as LTE-Advanced. One important requirement for LTE/LTE-Advanced networks is to support seamless and robust mobility so that a user can be handed over from one cell to another smoothly without service disruption. To ensure satisfactory user experience, 3GPP has started a study to evaluate the performance of mobility in HetNet and possibly make enhancement to improve mobility in HetNet. The main objective of this paper is to evaluate the mobility performance in HetNet based on the 3GPP study item simulation framework. The impact of HetNet on mobility is characterized for various user speeds and small cell densities. Our simulation result shows that compared to conventional macro-only layout, deployment of small cells poses challenges for mobility management. HetNet increases the probability of handover failure, especially if the user speed is high. Existing methods such as MSE based handover does not work well in HetNet either. Therefore more intelligent mobility management method is needed to enhance mobility management in HetNet.

We address uplink user admission control problems in multiuser wireless systems with multiple single-antenna users and a multi-antenna receiver. Our aim is to select the maximum number of users that can simultaneously satisfy the given quality of service (QoS) and power constraints from a large number of candidates in the system. Both sum and per-user transmit power constraints are considered. By utilizing uplink-downlink duality, we formulate the uplink user admission problems into sparsity-maximization problems, which are NP-hard. Inspired by compressive sensing techniques, we propose novel methods to tackle them by first using convex relaxation and then applying fast iterative methods on the convex-relaxed solutions. Simulations show that the proposed algorithms achieve excellent performance. The numbers of admitted users are close to the optimum ones obtained by exhaustive search when the QoS requirement is medium or low. Compared to a greedy algorithm, the proposed methods can admit up to 24% more users in the simulations.

Joint transmission (JT) is an attractive coordinated multipoint (CoMP) downlink transmission technique in multi-cell networks. It offers high system throughput but needs to distribute each user data to multiple base stations (BSs), which can lead to huge backhaul signaling overhead when the number of users is large. In this paper, we address the problem of jointly optimizing the BS assignment for each user data and the BS transmit beamformer design subject to per-BS power constraints and given quality-of-service (QoS) requirements. Our aim is to minimize the backhaul user data transfer from the data center to the BSs for the JT approach. We formulate such a problem into a sparsity-maximization problem. However, this problem is NP-hard. We propose to tackle it by first applying convex relaxation to an equivalent form of the original problem and improving the obtained results by solving a series of convex ℓ1-norm minimization problems. Compared to full BS cooperation, the proposed method saves about 13%–35% backhaul user data transfer in our simulations for moderate number of users and QoS requirement.

Coordinated multipoint (CoMP) transmission has been proposed as an important strategy to improve the signal quality for cell-edge users in future mobile cellular standards. However, when the joint transmission technique is applied in the CoMP downlink, the user data transferred from the data center to the base stations (BSs) can lead to a huge backhaul signaling overhead. Subject to quality-of-service (QoS) and per-BS power constraints, minimizing the user data transfer in the backhaul needs to jointly optimize the beamformer design and the BS assignment for each user data. We show that this problem can be cast into a sparsity-maximization problem, which is NP-hard. A semi-distributed algorithm is proposed for that problem to obtain suboptimal solutions. Such an algorithm is based on the uplink-downlink duality in multicellular networks. It utilizes the projected subgradient method and iterative function evaluation. Simulations show that the proposed method can reduce the backhaul user data transfer by 10%–30% compared to full cooperation for moderate number of users and QoS requirement.

When the joint processing technique is applied in the coordinated multipoint (CoMP) downlink transmission, the user data for each mobile station needs to be shared among multiple base stations (BSs) via backhaul. If the number of users is large, this data exchange can lead to a huge backhaul signaling overhead. In this paper, we consider a multi-cell CoMP network with multi-antenna BSs and single antenna users. The problem that involves the joint design of transmit beamformers and user data allocation at BSs to minimize the backhaul user data transfer is addressed, which is subject to given quality-of-service and per-BS power constraints. We show that this problem can be cast into an \ell_0-norm minimization problem, which is NP-hard. \changedv{Inspired by recent results in compressive sensing, we propose two algorithms to tackle it. The first algorithm is based on reweighted \ell_1-norm minimization, which solves a series of convex \ell_1-norm minimization problems. In the second algorithm, we first solve the \ell_2-norm relaxation of the joint clustering and beamforming problem and then iteratively remove the links that correspond to the smallest transmit power. The second algorithm enjoys a faster solution speed and can also be implemented in a semi-distributed manner under certain assumptions.} Simulations show that both algorithms can significantly reduce the user data transfer in the backhaul.

In this paper, precoding and receive filter matrices are designed such that minimum mean square error (MMSE) is achieved for interference alignment using 3-symbol extension in the multiple-input multiple-output (MIMO) X channels which comprise two transmitter-receiver pairs with M antennas at each transmitter/receiver. The performance of the proposed algorithm is demonstrated through numerical simulation and compared with the conventional scheme where the precoding matrices are randomly generated and the respective linear equalizers are employed at the receivers. The bit-error-rate (BER) performance of the proposed algorithm is superior compared with that of the conventional scheme across all signal-to-noise ratio (SNRs). Its BER improves with the iterative procedure to refine the precoding and receive filter matrices. Moreover, only a small number of iterations is required to achieve significant improvement in the BER performance.

When the joint processing technique is applied in the coordinated multipoint (CoMP) transmission downlink, the user data for each mobile station (MS) needs to be distributed among multiple base stations (BSs), which incurs the exchange of user data via the backhaul. In order to alleviate the requirement on the backhaul capacity, we propose to minimize the exchange of user data among the BSs, subject to certain QoS requirements and power constraints. We formulate this problem into an l0-“norm” minimization problem, and propose two heuristic methods to solve it. Simulations show that both methods can significantly reduce the data exchange among BSs in the backhaul, only at the cost of marginal increase in the sum transmit power at the BSs.

In this paper, a class of quasi-orthogonal space-time block codes (Q-STBC) is proposed for systems with two transmit antennas and three time slots, where the Alamouti code is not applicable due to the odd time slots. The proposed Q-STBC codes achieve rate one and full diversity with low complexity maximum likelihood decoding. The Q-STBC design also shows excellent properties in other practical aspects, such as the compatibility with the single antenna transmission mode, low power fluctuation, and low receiver decoding and transmitter encoding complexity.

In this paper, a class of quasi-orthogonal space- time block codes (Q-STBC) is proposed for systems with 2 antennas and 3 time slots, where the orthogonal Alamouti code can not be applied due to the odd, also known as Orphan, symbol problem inherited in the 3GPP frame structure. The proposed codes can achieve rate one and full diversity with lower complexity maximum likelihood detection. The design also takes into consideration other issues, such as the power fluctuation, the compatibility with single antenna transmission mode, the receiver and transmitter complexity etc.

In this paper, a class of quasi-orthogonal space-time block codes (Q-STBC) is proposed for systems with 2 antennas and 3 time slots, where the orthogonal Alamouti code can not be applied due to the “odd symbol” problem inherited in the 3GPP frame structure. The proposed codes can achieve rate one and full diversity with lower complexity maximum likelihood detection. The design takes into consideration the signal power fluctuation and minimizes the potential peak-to-average ratio (PAPR) in the transmitted signals. The application to Single Carrier Frequency Division Multiple Access (SC-FDMA) has been investigated further. A transmission scheme is proposed to preserve the good PAPR property of SC-FDMA. In addition, linear and iterative decoding algorithms are proposed to simplify the receiver complexity. Simulations have been conducted to demonstrate the superior performance.

Future cellular systems such as LTE-Advanced systems offer improved coverage, system throughput, and cell-edge throughput through the effective use of coordinated multi-point processing (CoMP). In this paper, we propose cooperative multicell automatic repeat request (ARQ), which exploits both the cooperation among base stations via backhaul links and ARQ. First, it involves minimal change in the physical-layer and MAC specifications of the mobile to BS connection. Second, spatial diversity is captured with efficient use of bandwidth resource as long as the backhaul links are available. We study three protocols, where BSs cooperate with different multicell processing: opportunistic decode-and-forward, amplify-and-forward, and compress-and-forward. We derive their respective average packet error rates and throughput. The obtained results show that cooperative multicell ARQ protocols are promising in terms of average packet error rate and throughput compared to the conventional ARQ. The parameters that effect the performance of each protocol include the operating SNRs of the main link and backhaul link, the number of neighboring BSs, and the network topology.

Zero-IF architecture is a competitive alternative to the conventional super-heterodyne architecture for RF-to-baseband conversion in OFDM systems. Despite its advantages of low cost and high integratability, it suffers from the I/Q imbalance that contributes amplitude and phase mismatches between in-phase and quadrature branches, resulting in signal distortion and performance degradation. In this paper, we revisit the problem in a theoretical viewpoint through a minimum distance analysis. In particular, we aim at proving that when the subcarriers are paired symmetrically to a center frequency and the maximum likelihood decoding is employed, the transmitter I/Q imbalance can provide an additional degree of diversity in frequency selective fading channels.

Network MIMO is a technology being developed towards lifting the interference limits on wireless network through collaborative transmissions from multiple base stations. However its great potential is shadowed by the stringent requirement of high speed/capacity backbones facilitating information exchange, including data, control signals, and channel state information amongst the base stations. In this paper, we propose a user-selective collaboration scheme to mitigate the backbone requirement by reducing the information exchange required. We define a certain threshold for users in each base station and only those users exceeding the threshold will be served by multiple base stations collaboratively. The other users will be served by a single base station as in a conventional non-collaborative transmission. The threshold is determined in such a way that the users with collaborative transmissions enjoy maximized sum rates whereas the others without collaborative transmissions sacrifice only marginal potential collaboration gain in terms of achievable maximized sum rates. As the amount of information to be exchanged will be a square function of the number of users served by the collaborative base stations, the user-selective scheme can reduce the backhaul overhead significantly.

In this paper, we propose a space-time block code (STBC) coding scheme with three decoding algorithms for the frequency-domain generation of single-carrier frequency division multiple access (SC-FDMA) which addresses the issue of orphan symbols. The proposed algorithms only require one time-slot to perform Alamouti coding as compared to the space-time coding and mapping (STCM) Scheme I which uses two time-slots. The performance of the third proposed algorithm approaches that of STCM Scheme I in coded systems.

In multi-cell OFDMA systems, inter-cell interference can severely degrade the system throughput, particularly for cell-edge users. To mitigate the inter-cell interference problem, interference coordination is a promising approach that effectively restricts and allocates certain resources among users in different cells to minimize the effect of inter-cell interference. However, most approaches are studied for static user distribution/traffic load and usually require a centralizer controller to solve a complicated optimization problem. In this paper, we propose an adaptive and distributed interference coordination algorithm that can achieve an efficient frequency reuse for a given user distribution and traffic load. The proposed algorithm only requires minimal coordination between base stations and does not need any a priori frequency planning. To enable reduction in system optimization, we classify the users into two groups, namely cell-interior and cell-edge user groups. With such user group differentiation, we can efficiently decompose a multi-cell optimization problem into distributed optimization problems, which is composed of solving single-cell resource allocation problem.

In this paper, an energy efficient structure for beacon signal transmission is proposed for incumbent wireless microphone protection in a cognitive radio system. The proposed structure consists of two consecutive parts: synchronization with index and synchronization with payload, transmitted as in-phase and quadrature components respectively. As opposed to the current beacon design, it can achieve more reliable and balanced error protection for the beacon components, while keeping the system complexity low.

In this paper, we consider the detection of OFDM signals used for cognitive radios. Based on the special feature - cyclic prefix embedded in OFDM signals, we propose an optimal Neyman-Pearson likelihood test for the signal detection. We have derived rigorously the probability distribution function of the corresponding decision statistics under frequency selective fading channels and obtained the closed forms of the probability of signal detection and the false alarm rate. The optimal threshold is also derived with respect to a given false alarm rate. Finally, simulations have been conducted to illustrate the promising performance of the proposed detection for OFDM signals under multipath fading environments.

Cognitive radio (CR) is seen as one of the revolutionary techniques for future spectrum utilization. The first and probably the most challenging task for CR is to detect/sense the white space of the spectrum for its usage. In this paper, detection of DVB-T signals is studied since TV band has been considered by Federal Communications Commission (FCC) to be pioneered for CR usage. Cyclostationary feature of DVB signals is explored so that the robust single-cycle detector (SCD) can be employed to handle the problem of noise uncertainty, to which the widely used energy detector (ED) is susceptible. Due to complexity concerns for real-time implementation, a blind twin peak (BTP) detector is proposed to approximate the performance of SCD while keep the complexity as low as ED.

We detail a code mapping scheme designed especially for devices in the enhanced wireless personal area networks operating in both the 868 MHz and 915 MHz bands for the emerging IEEE 802.15.4b standard. This carefully designed coding scheme, called cyclic odd bit inversion length 16 (COBI-16), meets all the following three essential criteria: (a) no spikes in the frequency spectrum, (b) low complexity correlation operations and (c) simple frequency offset and DC compensation operations. The scheme is tested in both AWGN and Rayleigh fading (flat and frequency selective) channels. Both coherent and non-coherent demodulations are investigated. Also investigated are the synchronous false alarm rate and synchronous miss rate in the AWGN channel. The results show that the scheme conforms to all the stringent requirements defined by the IEEE 802.15.4b protocol. The scheme has been accepted by the IEEE 802.15.4b task group as the standard for data transmission.

10.1109/PIMRC.2007.4394856

IEEE 802.15.4b is an enhanced standard to the IEEE 802.15.4, addressing the needs for high-data rate transmission using 868 or 915 MHz band in the wireless personal area networks. This paper details a modulation and code mapping scheme designed specifically for the standard's high rate transmission schemes operating in the 868 MHz and 915 MHz bands; which leads to efficient transmission and low implementation complexity. The code mapping scheme is called Quadrature Cyclic Odd Bit Inversion length 16 or Quad COBI-16. Among the scheme's desirable properties is that the total phase rotation in the I/Q plane accumulates to 0 degree over the complete symbol. The scheme is tested in both AWGN and Rayleigh frequency selective fading channels. The results show that it conforms to all the stringent requirements defined by the standard.

IEEE 802.15.4b is an enhanced standard to the IEEE 802.15.4, addressing the need for high-data rate transmissions in the 868 and 915 MHz band in the wireless personal area networks. This paper details a code mapping scheme designed specifically for the standard's 915 MHz band that leads to efficient transmission and low implementation complexity. The scheme is called quadrature cyclic odd bit inversion length 16 or Quad COBI-16 and one of its desirable properties is that the total phase rotation in the I/Q plane accumulates to 0 degree over the complete symbol. The scheme is tested in both AWGN and Rayleigh fading (flat and frequency selective) channels. Both coherent and non-coherent demodulations are investigated. Also investigated are the synchronous false alarm rate and synchronous miss rate in the AWGN channel. The results show that the scheme conforms to all the stringent requirements defined by the IEEE 802.15.4b protocol.

We detail a cyclic odd bit inversion length 16 (COBI-16) code mapping scheme designed especially for devices in the enhanced wireless personal area networks operating in the 915 MHz for the emerging IEEE 802.15.4b standard. This carefully designed coding scheme meets all the following three essential criteria: (a) no spikes in the frequency spectrum, (b) low complexity correlation operations and (c) simple frequency offset and DC compensation operations. The scheme is tested in both AWGN and Rayleigh flat and frequency selective fading channels. Both coherent and noncoherent demodulations are investigated. Also investigated are the synchronous false alarm rate and synchronous miss rate in the AWGN channel. The results show that the scheme conforms to all the stringent requirements defined by the IEEE 802.15.4b protocol. The scheme has been accepted by the IEEE 802.15.4b task group as the standard for data transmission

In this paper, a pulse UWB system based on 16-ary ternary orthogonal keying (TOK) is proposed for low power low data rate wireless personal area networks (WPAN). The novel ternary sequence based common signaling in the UWB system is proposed to support efficient implementation for both simple energy detectors and complex coherent detectors. Three variants on different pulse compression and clustering are also discussed and simulations have been conducted for both additive white Gaussian noise channels, UWB line-of-sight and non-line-of-sight fading channels. The results have demonstrated the superior performance of the proposed UWB system over other systems.

We propose a pseudo-inverse minimum mean squared error (MMSE) based QR decomposition (QRD) M-algorithm (QRD-M) for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The proposed algorithm can effectively mitigate the detrimental effect of MIMO channel ill-conditions on the bit error rate (BER) performance. As a result, the number of retained branches (M value) can be reduced to half of the original QRD-M algorithm in [1] in order to achieve the same performance for both the uncoded and coded systems. The detection complexity is therefore greatly reduced.

The maximum likelihood (ML) estimator and the Cramer Rao Bound (CRB) for frequency offset estimation are derived and compared against each other in a multiband orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband (UWB) system. A total of 100 UWB channel realizations, as proposed by IEEE 802.15 Channel Modeling sub-committee, for each channel model (CM) 1: line-of-sight for 0-4m and CM2: non-line-of-sight for 0-4m are simulated to demonstrate the convergence of the ML estimator variance with the CRB for high signal-to-noise ratio (SNR).

In this paper, a new pulse-based UWB system is proposed with low power ranging. The ternary sequence based common signaling is proposed to support power efficient energy detectors, differential detectors, and coherent detectors simultaneously. The preamble for ranging is designed to have good autocorrelation properties. The on-off keying based power efficient modulation is combined with chip mapping/spreading to provide robust system performance even in a multipath fading channel. Furthermore, a low complexity energy detection receiver structure is elaborated for leading edge detection in a multipath rich channel. System simulations are performed to demonstrate the promising performance of the proposed system

We detail a cyclic odd bit inversion length 16 (COBI-16) code mapping and modulation scheme designed especially for devices in the enhanced wireless personal area networks (WPAN) operating in the 868 MHz for the emerging IEEE 802.15.4b standard. This COBI-16 scheme has been accepted and incorporated in the draft IEEE 802.15.4b standard. This carefully designed coding and modulation scheme meets all the following three essential criteria: (a) no spikes in the frequency spectrum, (b) low complexity correlation operations and (c) simple frequency offset and DC compensation operations. The transmitted signal meets all the IEEE standard's stringent PSD mask requirements. The system is tested in both AWGN and Rayleigh fading (flat and frequency selective) channels. Both coherent and non-coherent demodulations are investigated. Also investigated are the synchronous false alarm rate and synchronous miss rate in the AWGN channel

This paper proposes a space-time coding architecture for single-carrier block spread CDMA systems. The proposed scheme employs a novel block spreading and despreading operation, which lead to significant reduction in the receiver complexity. The performance of the system is enhanced through space-time coding transmission. Extensive simulation studies are carried out and the performance of the proposed system is compared against conventional CDMA systems.

In this paper, we present a pre-transformed (PT) Multiple Input Multiple Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) system, in short PT-MIMO-OFDM system. We propose two receiver structures that combine the iterative subcarrier reconstruction and iterative interference cancellation to fully exploit both the spatial and the frequency diversity of MIMO-OFDM systems. Signi cant performance gain can be achieved. Based on the performance and receiver complexity tradeoff, we also present a set of guidelines for choosing either one of the two receivers in practical systems.

We present a comparative study of two tree search based detection algorithms, namely, the M-algorithm combined with QR decomposition (QRD-M) and the sphere decoding (SD) algorithms, for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. First, we show that nodes ordering before and during the tree search is important for both algorithms. With appropriate ordering, QRDM can improve detection performance significantly and SD can reduce decoding complexity substantially. Then we compare the implementation complexity of the two algorithms, in terms of the number of nodes required to search or the required number of multiplications to achieve maximum likelihood detection performance. It is interesting to show that the average complexity of SD is lower than that of QRD-M, whereas the worst case complexity of SD is much higher than that of QRD-M.

A list MIMO detection scheme is proposed to achieve near optimal maximum likelihood detection performance. The scheme is especially efficient in high data rate packet transmission systems, such as wireless LAN based hotspots, fixed broadband wireless access systems, 4G cellular based hotspots etc. The complexity of the optimal performance achieving scheme is significantly lower than that of the ML detection and the state of art technologies.

An algorithm based on distinguishing the first significant multipath (ETA) is proposed to address the critical issue of symbol timing synchronization and carrier frequency offset estimation in Ultra wideband (UWB) Multi-band OFDM (MB-OFDM) systems. FTA pinpoints the frame synchronization sequence and the start of its Fast Fourier Transform (FFT) window by accumulating multipath energies and then discerning for first significant multipath component through threshold comparison between consecutive accumulated energy samples. Thereafter, the frequency offset (FO) is estimated from phase differences between training symbols. Two channel models (CM), as proposed by IEEE 802.15 Channel Modeling sub-committee, CM1: line-of-sight for 0-4m and CM2: non-line-of-sight for 0-4m are simulated with 100,000 noisy realizations per threshold. The timing offset mean-squared errors for CM1 is 0.371 OFDM samples and CM2 is 4.548 OFDM samples under a typical signal-to-noise ratio (SNR) of 17 dB for high data rate applications. These are orders of magnitude lower compared to conventional OFDM-extended correlation methods. A maximum likelihood FO estimation algorithm is derived and its performance is compared with FTA simulation results. Variance of the FTA FO estimate approaches that of maximum likelihood FO estimate with a SNR loss of less than 0.5 dB.

We propose a low complexity VBLAST OFDM detection algorithm. In this method, we partition the subcarriers into a number of groups. The group size is determined by the frequency correlation between different subcarriers. Within each group, we use conventional VBLAST detection on the centre subcarrier and record the detection order. For the rest of the subcarriers in the group, we use a QR decomposition technique according to the recorded detection order. We show that this algorithm significantly reduces the complexity of VBLAST OFDM detection. The performance degradation of the proposed algorithm is small compared to conventional detection.

A performance comparison between convolutional coded vertical Bell-lab layered space time (V-BLAST) and groupwise space time trellis coded (GSTTC) multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems is presented in this paper. Both systems have the same data rate and employ receivers with comparable complexity, which are zero-forcing, minimum mean square error (MMSE) and list sphere decoder (LSD) based turbo receivers. Simulation results show that V-BLAST significantly outperform the GSTTC systems when MMSE or LSD based turbo receiver is applied, while GSTTC perform similarly to or better than the V-BLAST systems when zero-forcing detection algorithm is applied.

Recently, an iterative detection method is proposed for a pre-transformed OFDM system, where significant performance gain is shown to be achieved through linear processing (Z.Lei et al., Apr. 2003). An analytical treatment of the subject is given. The optimum transform design and reconstruction criteria are given to maximize the minimum SNR. In the case when no iteration is applied (i = 0), we compare the effect of the transform on the performance in a channel with and without diversity. Under the assumption that the previous detection is error free, it is shown analytically that the iterative method achieves a diversity advantage of i + 1 in the i/sup th/ iteration, thus providing an explanation on its superior performance. Simulations are conducted to corroborate the analysis. Due to the generality of the transform design, the analysis conducted is applicable for other common systems (such as MC-CDMA and SC-FDE systems) as well.

In this paper, an iterative interference cancellation (IIC) scheme is proposed for the ordered zero-forcing (ZF) algorithm to regain the signal energies lost during the interference suppression (IS) process in groupwise space lime trellis coded (GSTTC) systems. Both hard decision and soft decision based IIC are studied. With hard decision based IIC, significant improvement over the ordered ZF algorithm is achieved and a performance comparable to that of the complex ordered maximum SNR (MaxSNR) algorithm is reached. With soft decision IIC, the proposed scheme can even outperform the ordered MaxSNR algorithm and provides a better performance-complexity tradeoff in systems with large number of antennas and simple space time trellis codes.

In this letter, an improved square-root algorithm for Bell Labs Layered Space-Time (BLAST) system is proposed. It speeds up the original square-root algorithm by 36% in terms of the number of multiplications and additions. Compared with the recursive algorithm, it requires only a 13.6% higher number of multiplications and a 38.9% higher number of additions. Since it uses unitary transformations to avoid the computation of any matrix inversion or "squaring" operation, it maintains the advantages of the square-root based algorithms that are numerically stable, robust, and hardware friendly.

This paper proposes a multiband multicarrier CDMA system with a simplified receiver structure for downlink transmission. An adaptive algorithm is proposed for allocating multiple subbands for users based on their channel gain and usage. The band allocation algorithm together with adaptive modulation can considerably increase the system capacity. A simplified receiver structure intended to reduce the complexity of channel equalizer-despreader structure of conventional multicarrier CDMA system is also proposed. The combined system is extensively simulated for different channel parameters and the results are discussed.

A scheme for selecting two beams in a switched beam antenna system for providing downlink transmit diversity based on space-time block coding (STBC) technique is proposed. The scheme involves selecting a pair of beams based on the uplink reception of the switched beam antenna system, and determining from the pair of beams a corresponding pair of orthogonal beams. The scheme also involves providing the pair of orthogonal beams as pair of transmit beams, the switched beam antenna system achieves nearly the same system performance, in terms of bit error rate (BER), as much complex adaptive antenna system with STBC. Simulation results demonstrate the promising scheme.

An algorithm is proposed for estimating the directions of arrival (DOAs) of multipath signals received from multiple users on the uplink of a code-division multiple-access system. The correlation matrices of the received signal before and after code-matched filtering are used to provide unique estimates even when the number of required DOAs exceeds the number of antenna array elements. This scenario is well known to cause conventional direction-finding algorithms (such as MUSIC) to fail. Both intersymbol interference (ISI) and multiple-access interference (MAI) are modeled exactly, and so the algorithm performs much better than those which model ISI and MAI as Gaussian noise.