Orthogonal Frequency Division Multiplexing Code Division Multiple Access Research Articles

Symbol Timing Sequence Structure for OFDM-CDMA Communication System under Low SNR

Symbol synchronization offset of orthogonal frequency division multiplexing (OFDM) communication system will cause the rotation of data constellation at receiver even if such offset falls in the range of the appended cyclic prefix. When OFDM modulation is combined with spread spectrum technique, e.g., OFDM code division multiple access (OFDM-CDMA), the problem becomes more serious since OFDM-CDMA communication system generally works under very low signal-to-noise ratio. This paper focuses on symbol timing synchronization of OFDM-CDMA communication system and proposes an effective sequence structure on the basis of circular conjugate-symmetric properties of discrete Fourier transform. The proposed frequency-domain sequence not only can generate real time-domain signal to reduce calculation complexity, but also can resist peak-to-average power ratio of OFDM modulation by exploiting suitable computer search algorithm. The simulation results show a sharp cross-correlation peak can be obtained, which is quite helpful for accurate symbol timing synchronization of OFDM-CDMA communication system.

Iterative Equalizer for OFDM-CDMA Multiple Access Communication Systems

In this paper, the receivers with adaptive equalizer and turbo decoder are investigated for orthogonal frequency division multiplexing-code division multiple access (OFDM-CDMA) systems in multipath channels. The de-spreading equalizer which combines de-spreading and equalization is used to mitigate the MAI and ISI. The stochastic gradient based LMS algorithm is used as the adaptive algorithm for the equalizer. Three receiver architectures are investigated: The first one is the traditional receiver called de-spreading equalizer disjointed with turbo decoder. In this scheme, the equalizer and turbo decoder operate independently and the equalizer relies on slicer output to adapt its filter coefficients. However, the extrinsic information from turbo decoder can provide more reliable information of the transmitted data. Hence the second receiver architecture is called as de-spreading equalizer jointed with turbo decoder, where turbo decoder gives the extrinsic information as feedback to the adaptive equalizer as the desired information. Finally, the equalizer jointed with turbo decoder is extended to differential coded signaling in OFDM-CDMA system and its performance is presented and discussed.

Foreword of Special Issue on “Information Education and Technological Application”

Foreword of Special Issue on “Information Education and Technological Application”

Performance Analyses of OFDM–CDMA Receivers in Multipath Fading Channels

Orthogonal frequency-division multiplexing (OFDM) is a promising scheme for multicarrier (MC) transmission. The combination of OFDM and code-division multiple access (CDMA) which is referred to as OFDM-CDMA, has recently attracted much research interest in wireless communications. In this paper, we evaluate the downlink performance for two forms of adaptive OFDM-CDMA receivers in multipath fading channels: 1) MC-CDMA and 2) MC-direct sequence (DS)-CDMA. We propose theoretical minimum mean square errors (MMSEs) for MC-CDMA and MC-DS-CDMA to compare the performances in different situations. Moreover, one drawback of OFDM schemes is the rate reduction due to the cyclic prefix (CP) overhead, which significantly affects the channel utilization in MC-DS-CDMA systems. We propose adaptive receiver structures with both subcarrier- and code-interval equalizations to improve the performance of MC-DS-CDMA systems with shorter or no CP. Simulation results demonstrate the effectiveness of the proposed receivers and the correctness of the theoretical derivations.

Variable power and rate allocation using simple CQI for multiuser OFDMA-CDM systems

This paper presents variable power and rate allocation (PRA) for a multiuser orthogonal frequency division multiple access code division multiplexing (OFDMA-CDM) system which exploits both orthogonality among users and symbol-level diversity. To predict the link performance, we analyze the postprocessing SNR as the transmit power (TP) varies when the minimum mean square error combiner (MMSEC) is employed. It is shown from analysis that the MMSEC SNR is nonlinear in TP, but is bounded by the harmonic and the arithmetic means of the subcarrier SNRs. From the observation, we propose two variable PRA methods using a simple channel quality indication and a required TP estimation method. Simulation results show that the proposed methods not only guarantee the target performance but also increase the throughput at the same or slightly increased feedback overhead compared with the fixed TP allocation case.

On the Transmitter-Based Preprocessing for 2-D OFDM-CDMA Forward-Link Systems Over Time-Varying Rayleigh Fading Channels

Transmitter-based preprocessing is investigated for 2-D orthogonal frequency-division multiplexing code-division multiple-access (OFDM-CDMA) forward-link systems for improving performance and shifting signal processing complexity from a mobile unit to a base station. Preprocessing schemes that are based on zero forcing (ZF) with power normalization, minimum mean square error (MMSE), and ZF with multiuser water filling (ZF-MWF) criteria are jointly investigated with 2-D spreading pattern optimization and multiuser scheduling from an information-theoretic viewpoint. Numerical results show that 1) the performance of preprocessing is quite sensitive to the 2-D spreading pattern for SNRs of interest, for example, 20% degradation on the sum data rate is observed for MMSE preprocessing if the spreading pattern is not properly selected; 2) ZF-MWF may substantially outperform the other two criteria depending on the SNRs; and 3) multiuser scheduling provides a significant performance improvement on the system sum data rate.

A Tree-Structured Blind Algorithm for Joint Parametric Channel Estimation and Non-coherent Data Detection in an OFDM-CDMA Wireless System with Antenna Arrays

A blind joint parametric channel estimation and non-coherent data detection algorithm is proposed for the downlink of an orthogonal-frequency-division-multiplexing code-division-multiple-access (OFDM-CDMA) system with multiple-input-multiple-output (MIMO) antenna arrays. To reduce the computational complexity, we first develop a tree-structured algorithm to estimate high dimensional parameters predominantly describing the involved multipath channels by employing several stages of low dimensional parameter estimation algorithms. In the tree structure, to exploit the space-time distribution of the receive multipath signals, spatial beamformers and spectral filters are adopted for clustered-multipath grouping and path isolation. In conjunction with the multiple access interference (MAI) suppression techniques, the proposed tree architecture algorithm jointly estimates the direction of arrivals, propagation delays, carrier frequency offsets and fading amplitudes of the downlink wireless channels in a MIMO OFDM-CDMA system. With the outputs of the tree architecture, the signals of interest can then be naturally detected with a path-wise maximum ratio combining scheme.

OFDMA-CDM performance enhancement by combining H-ARQ and interference cancellation

In this paper, superimposed packet allocation for orthogonal frequency-division multiple-access code-division multiplexing (OFDMA-CDM) is presented, where each transmitted packet is associated with one spreading code. An iterative algorithm which is a combination of parallel interference cancellation and hybrid automatic repeat request (H-ARQ) based on soft value combining (SVC) is proposed, and its performance is studied and compared with other existing H-ARQ schemes. The proposed algorithm exploits the reliability information of erroneously received copies of the same data packet to improve the performance of interference cancellation. The interference of correctly received packets is ideally reconstructed and subtracted. Thus, the overall system performance improves iteratively. As a result, the proposed algorithm outperforms conventional H-ARQ based on SVC, as well as H-ARQ based on maximum ratio combining.

Pre-Equalization With Subband Channel Loading: A Technique to Maximize Throughput in Uplink OFDM-CDMA Systems

This letter focuses on maximization of achievable throughput for uplink orthogonal frequency-division multiplexing code-division multiple-access (OFDM-CDMA) systems. In particular, a new pre-equalization technique is presented where all users employ spreading both in frequency (on sub-bands of the available spectrum) and time; moreover, bit and power loading is used at the mobile terminals to accomplish throughput maximization. To avoid interference among users, the proposed loading algorithm inherently inverts the channel amplitude on the sub-carriers of each sub-band, while the phase is adjusted separately.

Low-rate channel coding with complex-valued block codes

This paper addresses aspects of channel coding in orthogonal frequency-division multiplexing-code-division multiple access (OFDM-CDMA) uplink systems where each user occupies a bandwidth much larger than the information bit rate. This inherent bandwidth expansion allows the application of powerful low-rate codes under the constraint of low decoding costs. Three different coding strategies are considered: the combination of convolutional and repetition codes, the code-spread system consisting of one single very low-rate convolutional code and a serial concatenation of convolutional, Walsh-Hadamard and repetition code. The latter scheme is improved by combining the Walsh-Hadamard codes with an additional M-phase-shift keying modulation resulting in complex-valued Walsh-Hadamard codes (CWCs). Analytical performance evaluations will be given for these codes for the first time. The application of CWCs as inner codes in a serial code concatenation is also addressed. We derive a symbol-by-symbol maximum a posteriori decoding algorithm in the complex signal space in order to enable iterative decoding for the entire code. A comprehensive performance analysis by simulation of all the proposed coding schemes shows that the Walsh-Hadamard-based schemes are the best choice for low-to-medium system load. Note that even for fully loaded OFDM-CDMA systems, the concatenation with an inner complex-valued Walsh-Hadamard code leads to a bit-error rate less than 10/sup -5/ for an E/sub b//N/sub 0/ of about 6 dB.

Dynamic spreading code selection method for PAPR reduction in OFDM-CDMA systems with 4-QAM modulation

We present a new dynamic spreading code selection technique to obtain low peak-to-average-power ratio (PAPR) of an orthogonal frequency-division multiplexing code-division multiple-access (OFDM-CDMA) system with 4-QAM modulation for the down-link in mobile communication. In this method, initially, each user is assigned a low and a high PAPR spreading code, which allows selecting one when the system is operating. The spreading code of each user is dynamically selected so that total PAPR level of the whole user group present in the system is minimized. With the proposed technique, the PAPR level of a system with 10 users and 64 sub-carriers using 64 chip Walsh-Hadamard (WH) codes, can be limited to 15 dB while the worst case theoretical maximum could go up to 28.1 dB. Although 64 chip WH codes are employed to evaluate the performance it can be generalized to all other sets of spreading codes.