Subcarrier Grouping Research Articles

Antenna Selection for Noncoherent Space–Time–Frequency-Coded OFDM Systems

This paper studies receive antenna selection (AS) for multiantenna systems that operate over frequency-selective channels, where the channel state information (CSI) is known neither at the transmitter nor at the receiver. We consider AS for noncoherent space-time-frequency (STF)-coded orthogonal frequency-division multiplexing (OFDM) systems that employ unitary signals. An OFDM receiver with a single radio-frequency chain is employed, and the selection is based on the received signal power, where the receive antenna with the largest received signal power averaged over all subcarriers is chosen. Theoretical analysis and simulations show that by using AS, the maximum spatial and multipath diversity can still be obtained without CSI at the receiver, which is the same as the full-complexity multiantenna OFDM systems. We simplify code design and decoding using subcarrier grouping, which enables us to apply existing full-diversity codes for noncoherent and differential STF-OFDM systems to each group, to obtain maximum spatial and multipath diversity when AS is used. We also present a low-complexity suboptimal decoder that can easily be implemented in practice.

Multidimensional Subcarrier Mapping for Bit-Interleaved Coded OFDM With Iterative Decoding

A power and bandwidth-efficient bit-interleaved coded modulation (BICM) with orthogonal frequency-division multiplexing (OFDM) and iterative decoding (BI-COFDM-ID) using combined multidimensional mapping and subcarrier grouping is proposed for broadband transmission in a frequency-selective fading environment. A tight bound on the asymptotic error performance is developed, which shows that subcarrier mapping and grouping have independent impacts on the overall error performance, and hence, they can be independently optimized. Specifically, it is demonstrated that the optimal subcarrier mapping is similar to the optimal multidimensional mapping for bit-interleaved coded modulation with iterative decoding (BICM-ID) in frequency-flat Rayleigh fading environment, whereas the optimal subcarrier grouping is the same with that of OFDM with linear constellation preceding (LCP). Furthermore, analytical and simulation results show that the proposed system with the combined optimal subcarrier mapping and grouping can achieve the full channel diversity without using LCP and provide significant coding gains as compared to the previously studied BI-COFDM-ID with the same power, bandwidth, and receiver complexity.

Subcarrier Group Assignment for MC-CDMA Wireless Networks

Two interference-based subcarrier group assignment strategies in dynamic resource allocation are proposed for MC-CDMA wireless systems to achieve high throughput in a multicell environment. Least interfered group assignment (LIGA) selects for each session the subcarrier group on which the user receives the minimum interference, while best channel ratio group assignment (BCRGA) chooses the subcarrier group with the largest channel response-to-interference ratio. Both analytical framework and simulation model are developed for evaluation of throughput distribution of the proposed schemes. An iterative approach is devised to handle the complex interdependency between multicell interference profiles in the throughput analysis. Illustrative results show significant throughput improvement offered by the interference-based assignment schemes for MC-CDMA multicell wireless systems. In particular, under low loading conditions, LIGA renders the best performance. However, as the load increases BCRGA tends to offer superior performance.

Subcarrier Grouping for OFDM With Linear Constellation Precoding Over Multipath Fading Channels

This paper presents a general analysis of the achievable diversity and coding gains of orthogonal frequency-division multiplexing systems using linear constellation precoding and subcarrier grouping over multipath fading channels. It shows that the maximum diversity gain can be achieved with any subcarrier grouping scheme, as long as its group size F is not less than the number of effective resolvable paths L. For F ges L, the maximum coding gain can be achieved if F is an Euler number or an integer power of 2. Otherwise, the achievable coding gain approaches at least 70% of the maximum one. The analysis also proves the existence of a unique solution for optimum subcarrier grouping to achieve the maximum coding gain when F = L. Simulation results support the analytical prediction on the diversity and coding gains.

Bit-Interleaved Coded OFDM With Signal Space Diversity: Subcarrier Grouping and Rotation Matrix Design

This paper investigates the application of bit-interleaved coded modulation and iterative decoding (BICM-ID) in orthogonal-frequency-division-multiplexing (OFDM) systems with signal space diversity (SSD) over frequency-selective Rayleigh-fading channels. Correlated fading over subcarriers is assumed. At first, a tight bound on the asymptotic error performance for the general case of precoding over all N subcarriers is derived and used to establish the best achievable asymptotic performance by SSD. It is then shown that precoding over subgroups of at least L subcarriers per group, where L is the number of channel taps, is sufficient to obtain this best asymptotic error performance, while significantly reducing the receiver complexity. The optimal joint subcarrier grouping and rotation matrix design is subsequently determined by solving the Vandermonde linear system. Illustrative examples show a good agreement between various analytical and simulation results

Reducing Detection Complexity of MIMO-OFDM by Using New Nulling Vectors

We propose new nulling vectors for the subcarrier grouping based low-complexity detection scheme of V-BLAST (vertical Bell Laboratories layered space-time) coded MIMO-OFDM. In each subcarrier group, the center subcarrier uses the conventional ZF-DFE (zero-forcing decision-feedback- equalization) algorithm and the non-center subcarriers use the reduced-complexity ZF-DFE with the new nulling vectors. The subcarrier grouping based detection scheme, ZF-DFE-SG, can significantly reduce the detection complexity compared with ZF-DFE-EX which exhaustively applies the conventional ZF-DFE at each subcarrier independently. The performance loss is very small. We conduct the simulations for channel coded V-BLAST OFDM system under estimated channel frequency responses. It is shown the complexity can be reduced by 80.6% with only 1.0 dB performance loss for a 4x4 system.

Fair Allocation of Subcarrier and Power in an OFDMA Wireless Mesh Network

This paper presents a new fair scheduling scheme for orthogonal frequency-division multiple-access-based wireless mesh networks (WMNs), which fairly allocates subcarriers and power to mesh routers (MRs) and mesh clients to maximize the Nash bargaining solution fairness criterion. In WMNs, since not all the information necessary for scheduling is available at a central scheduler (e.g., MR), it is advantageous to involve the MR and as many mesh clients as possible in distributed scheduling based on the limited information that is available locally at each node. Instead of solving a single global control problem, we hierarchically decouple the subcarrier and power allocation problem into two subproblems, where the MR allocates groups of subcarriers to the mesh clients, and each mesh client allocates transmit power among its subcarriers to each of its outgoing links. We formulate the two subproblems by nonlinear integer programming and nonlinear mixed integer programming, respectively. A simple and efficient solution algorithm is developed for the MR's problem. Also, a closed-form solution is obtained by transforming the mesh client's problem into a time-division scheduling problem. Extensive simulation results demonstrate that the proposed scheme provides fair opportunities to the respective users (mesh clients) and a comparable overall end-to-end rate when the number of mesh clients increases

Downlink beamforming subject to the equivalent isotropic radiated power constraint in WLAN OFDM systems

Range extension by means of downlink beamforming in a wireless local area network (WLAN) orthogonal frequency-division multiplexing (OFDM) system is addressed. A maximum ratio (MR) combining beamformer scaled according to the equivalent isotropic radiated power (EIRP) constraint adopted in the European regulations, is compared to the conventional total power (TP) restricted solution in typical propagation conditions. It is shown that the EIRP-scaled MR beamformer demonstrates significant performance degradation compared to the TP case. A joint optimization problem for the beamforming weights over all the sub-carriers subject to the EIRP constraint is formulated based on maximization of the minimum signal-to-noise ratio (SNR) over all the working sub-carriers at the receiver. A two-stage sub-optimal solution is proposed that exploits optimization over groups of non-adjacent sub-carriers and normalization to maintain the overall EIRP constraint. Its efficiency is illustrated by means of comparison with the EIRP-scaled MR beamformer and another sub-optimal algorithm based on the conventional grouping of highly correlated sub-carriers in the IEEE 802.11 environment.

Two-Dimensional Mapping for Wireless OFDMA Systems

The recent Orthogonal Frequency Division Multiple Access (OFDMA) transmission technique is gaining popularity as a preferred technology in the Broadband Wireless Access (BWA) emerging standards. In standards 802.16-2004 and 802.16e, the basic allocation units are comprised of sub-channels and OFDMA time symbols; each sub-channel is a group of sub-carriers, so that all the sub-channels are considered equally adequate to all users. We study the naturally arising new approach of two-dimensional mapping of incoming requests into the matrix that represents the system resources, where each allocation is of an arbitrary multi-rectangular shape (to the best of our knowledge, this approach has not been discussed elsewhere). We define a cost model and constraints related to practical OFDMA systems, which depend on the spatial shape of the two-dimensional allocation; the main objective function is the spatial efficiency. We show that the arising problem, even in its simplest form, is NP-hard . We present run-time efficient heuristic solutions for various mapping problems, taking into account the above QoS and OFDMA related constraints. In particular, a novel solution for two-dimensional mapping under priority constraints is suggested. Extensive simulations with parameters of real systems were used to investigate the performance of the proposed solutions in terms of throughput, delay and system load. The results show that high throughput can be achieved with relatively simple mapping algorithms. We believe that the proposed two-dimensional mapping approach is prospective, due to its fitness to modern standards

A novel MC DS-CDMA system scheme with high spectral efficiency

This article presents a novel multicarrier direct sequence code-division multiple-access (MC DS-CDMA) scheme based on the loosely synchronous (LS) codes, which can achieve very high spectral efficiency. Instead of the single subcarrier modulation in a conventional MC DS-CDMA scheme, this novel scheme introduces the nonorthogonal subcarrier-group modulation, whose interferences caused by the non-orthogonal spacing in a subcarrier group can be reduced by a multicode detector. A detailed analysis for this system architecture and also a multiple-input-multiple output (MIMO) MC DS-CDMA solution to satisfy the requirement of beyond 3G mobile communication system are proposed. Simulated results demonstrate that this scheme can obtain excellent bit error rate (BER) performance, as well as achieve more than twice the spectral efficiency as that of conventional MC DS-CDMA systems.

Local ML detection for multicarrier DS-CDMA downlink systems with grouped linear precoding

Abstract-A multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) downlink system with linear precoding over a group of subcarriers is considered. This scheme preserves user orthogonality independently of the underlying frequency-selective channel, collects the channel diversity and enables low-complexity decoding. In this context, we examine a local maximum-likelihood (LML) detection technique that searches for the maximum-likelihood (ML) solution in the neighborhood of the output provided by the minimum mean-squared error (MMSE) detector. By exploiting the soft information of the MMSE detector output and the precoder structure, we introduce useful criteria to reduce the computational complexity of the LML search. Simulations illustrate that the LML-MMSE detector with minimum neighborhood size yields considerable BER improvement with respect to MMSE, and outperforms a block decision-feedback equalization (DFE) approach at comparable complexity.

Joint Interference Suppression and Frequency Offset Compensation Techniques for a Reverse Link MC-CDMA System

This paper addresses several relevant issues in detection process of a reverse link multicarrier code division multiple access system with frequency offset. Due to the presence of frequency offsets, power of a group of subcarriers is spread to all the available subcarriers. In order to collect all the scattered signal power, we propose a two stages receiver. First stage of the proposed receiver performs maximum ratio combining. Following from the first stage, multiple access interference is suppressed in the second stage. We propose various multiple access interference suppression techniques, having different complexity and performance. Simulation results demonstrate the advantage of the proposed receiver. We also investigate the performance of 2 major subcarrier assignment schemes, namely continuous subcarrier assignment scheme and interleaved subcarrier assignment scheme, in both ideal independent fading channel and practical correlated fading channel.

Differential space-time-frequency coded OFDM with maximum multipath diversity

Addressing orthogonal frequency division multiplexing (OFDM) transmissions over frequency-selective Rayleigh fading channels, we propose a differential space-time-frequency (DSTF) coding scheme with maximum multipath diversity. Resorting to subcarrier grouping, we convert the system into a set of parallel DSTF systems, within which DSTF coding is considered. Through pairwise error probability (PEP) analysis, we prove that full diversity gain, which is the product of the number of transmit antennas, receive antennas, and the multipath channel length, can be achieved without channel state information (CSI) at the receiver. We analyze the performance of group DSTF coding over correlated fading channels and establish the design criteria for subcarrier selection and code selection. In addition to applying existing differential unitary diagonal codes, we propose a systematic approach to designing nondiagonal codes for DSTF systems, which are block diagonal (BD) and have the necessary unitary and full diversity properties. Based on permutation techniques, we have devised new excellent-performing BD DSTF codes, which significantly improve the coding gain as compared to their diagonal counterparts. Corroborating simulations confirm the merits of our approach.

Effects of phase noise on performance of ofdm systems using an ici cancellation scheme

This paper investigates the effects of phase noise on the performance of orthogonal frequency division multiplexing (OFDM) systems using an intercarrier interference (ICI) cancellation scheme. In this case, the common phase error (CPE) and ICI caused by phase noise depend on the overall spectrum of each weighted group of subcarriers rather than on the spectrum of each individual subcarrier. This means that the system performance can be improved by filtering the phase noise to fit a particular spectrum. It is shown that the ICI cancellation scheme can significantly improve the bit error rate (BER) performance in the presence of phase noise.

Linear constellation precoding for OFDM with maximum multipath diversity and coding gains

Orthogonal frequency-division multiplexing (OFDM) converts a frequency-selective fading channel into parallel flat-fading subchannels, thereby simplifying channel equalization and symbol decoding. However, OFDM's performance suffers from the loss of multipath diversity, and the inability to guarantee symbol detectability when channel nulls occur. We introduce a linear constellation precoded OFDM for wireless transmissions over frequency-selective fading channels. Exploiting the correlation structure of subchannels and choosing system parameters properly, we first perform an optimal subcarrier grouping to divide the set of subchannels into subsets. Within each subset, a linear constellation-specific precoder is then designed to maximize both diversity and coding gains. While greatly reducing the decoding complexity and simplifying the precoder design, subcarrier grouping enables the maximum possible diversity and coding gains. In addition to reduced complexity, the proposed system guarantees symbol detectability regardless of channel nulls, and does not reduce the transmission rate. Analytic evaluation and corroborating simulations reveal its performance merits.

Intercarrier interference self-cancellation scheme for OFDM mobile communication systems

For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offsets in mobile radio channels distort the orthogonality between subcarriers resulting in intercarrier interference (ICI). This paper studies an efficient ICI cancellation method termed ICI self-cancellation scheme. The scheme works in two very simple steps. At the transmitter side, one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. The weighting coefficients are designed so that the ICI caused by the channel frequency errors can be minimized. At the receiver side, by linearly combining the received signals on these subcarriers with proposed coefficients, the residual ICI contained in the received signals can then be further reduced. The carrier-to-interference power ratio (CIR) can be increased by 15 and 30 dB when the group size is two or three, respectively, for a channel with a constant frequency offset. Although the redundant modulation causes a reduction in bandwidth efficiency, it can be compensated, for example, by using larger signal alphabet sizes. Simulations show that OFDM systems using the proposed ICI self-cancellation scheme perform much better than standard systems while having the same bandwidth efficiency in multipath mobile radio channels with large Doppler frequencies.

Applications of Chaotic Digital Code-Division Multiple Access (CDMA) to Cable Communication Systems

In this paper the technical details of chaotic digital code-division multiple access ((CD)2MA) communication systems used in cable communication systems are presented. The cable communication system may be a pure coaxial RF cable network, a hybrid fiber-coax network, or a pure optical fibre network for high-capacity data link. As an example of its many potential applications in cable communication systems, (CD)2MA is used to support the upstream digital data communications in cable TV systems occupying the very noisy 5–40 MHz portion of spectrum. Although the (CD)2MA proposed in this paper is only used to support current Internet services via cable TV networks, it can also be used to support the high-speed data-link provided in the 550–750 MHz band in hybrid fiber-coax networks. (CD)2MA is a new communication framework which uses band-limited chaotic carriers insted of linear ones. For the purpose of generality, in this paper the band-limited chaotic carriers are approximated by groups of linear sub-carriers, which distribute within the same bandwidth with a fixed amplitude, random phases and uniformly distributed frequencies. The theoretical result of the performance of (CD)2MA is given. We also provide the simulation results of the bit-error-rate (BER) performances of a synchronous (CD)2MA used in cable communication systems. The results show that the (CD)2MA system has a better performance than the synchronous CDMA system proposed for the same cable communication system. Technical details of (CD)2MA are also presented for the future design of prototype systems. We present the framework of the whole (CD)2MA system including carrier syncronization, timing recovery and the details of nonlinear carrier generators.

Joint Optimal Resource Allocation and PAPR Reduction Algorithm for OFDMA Systems

One of the drawbacks of an orthogonal frequency division multiplexing (OFDM) system is the larger peak to averaged power ratio (PAPR). The PAPR reduction scheme is a complicated problem, especially when a resource allocation is considered jointly. In this paper, a joint algorithm for determining an optimal resource allocation with PAPR awareness is proposed. Firstly, an adaptive tone reservation assignment is performed by optimally choosing a group of subcarriers, whose channel gains are sufficiently high, to be the data subcarriers and the remaining subcarriers will be used as the reserved tones for reducing PAPR to the desired level. Secondly, an adaptive power adjustment for PAPR reduction technique is introduced for determining the allocated power of data subcarriers by using the water-filling approach with the joint PAPR and capacity constraints. By using the proposed joint algorithm, the PAPR threshold level is satisfied while the desired capacity could be achieved. From the results, the proposed algorithm achieves the better results comparing with a conventional partial transmit sequence (PTS) technique and the tone reservation (TR) technique in terms of PAPR, and probability of error performances, in which the side information is not required, in contrast with the other techniques.