PERFORMANCE ANALYSIS OF CHANNEL ESTIMATION FOR LDPC-CODED OFDM SYSTEM IN MULTIPATH FADING CHANNEL

In this paper, the channel estimation techniques for Orthogonal Frequency Division Multiplexing （OFDM） systems based on pilot arrangement are studied and we apply Low Density Parity Check （LDPC） codes to the system of IEEE 802.16a with OFDM modulation. First investigated is the influence of channel cstimation schemes on LDPC-code based OFDM system in static and multipath fading channels. According to the different propagation environments in 802.16a system, a dynamic channel estimation scheme is proposed. A good irregular LDPC code is designed with code rate of 1/2 and code length of 1200. Simulation results show that the performance of LDPC coded OFDM system proposed in this paper is better than that of the convolution Turbo coded OFDM system proposed in IEEE standard 802.16a.     

OPTIMAL ANTENNA SUBSET SELECTION AND BLIND DETECTION APPROACH APPLIED TO ORTHOGONAL SPACE-TIME BLOCK CODING

An approach combining optimal antenna subset selection with blind detection scheme for Orthogonal Space-Time Block Coding （OSTBC） is proposed in this paper. The optimal antenna subset selection is taken into account at transmitter and/or receiver sides, which chooses the optimal antennas to increase the diversity order of OSTBC and improve further its performance. In order to enhance the robustness of the detection used in the conventional OSTBC scheme, a blind detection scheme based on Independent Component Analysis （ICA） is exploited which can directly extract transmitted signals without channel estimation. Performance analysis shows that the proposed approach can achieve the full diversity and the flexibility of system design by using the antenna selection and the ICA based blind detection schemes.     

A Novel Event Model for Performance Analysis of IEEE 802.11 DCF

After considering the memory effect among series events occurring on the channel, we propose a novel event model to analyze the channel status more precisely. The memory effect is caused by the backoff freezing regulation of IEEE 802.11 Distributed coordination function （DCF）, which has been ignored before and thus resulted in the inaccurate evaluation of the network performance. Based on our new event model, the network performance of IEEE 802.11 DCF, including throughput, packet delay distribution and energy efficiency is analyzed. Simulation results show that our model is highly accurate.     

Joint Maximum-Likelihood Estimation of Frequency Offset and Channel in MIMO Systems

This paper addresses the problem of frequency offsets and channel gains estimation for a Multiinput multi-output （MIMO） system in fiat-fading channels. The general case where the frequency offsets are possibly different for each transmit antenna is considered, The Maximum-likelihood （ML） estimation of the carrier frequency offset for each transmit antenna in a MIMO system is investigated in this paper. The exact solution to this estimation problem turns out to be too complex as it involves a search over a multi-dimensional domain. However, by making use of the Particle swarm optimization （PSO） method, we efficiently solve the above complex problem. So based on the PSO theory, a novel joint estimation algorithm of frequency offsets and channel gains is proposed. Simulation results show that the proposed algorithm has better performance as compared with the correlation-based estimation algorithm and asymptotically achieves the Cramer-Rao lower bound （CRLB）, which provides a new idea to solve the problem of joint frequency offsets and channel gains estimation for MIMO systems.     

Channel de-allocation schemes for GSM/GPRS networks

Several channel de-allocation schemes for GSM/GPRS（General Packet Radio Service） networks are proposed in this paper. For DRA （Dynamical Resource Allocation） with de-allocation mechanism, if a new voice call arrives and finds that all the channels are busy, then one of the GPRS packets which occupy more than one channel for data transmission may release a channel for the new voice call. This paper presents 5 de-allocation mechanisms, i.e. DA-RANDOM, DA-RICHEST, DA-POOREST, DA-OLDEST and DA-YOUNGEST, to select the GPRS packet for releasing the appropriate channel. Simulation results show that DA-OLDEST achieves the best performance, especially in packets blocking probability, among all the de-allocation schemes. Although the performance of the proposed de-allocation schemes is not significantly different, they are all much better than that of the scheme without de-allocation.     

A NOVEL LINK ADAPTATION SCHEME TO ENHANCE PERFORMANCE OF IEEE 802.11G WIRELESS LAN

A novel link adaptation scheme using linear Auto Regressive （AR） model channel estimation algorithm to enhance the performance of auto rate selection mechanism in IEEE 802.11g is proposed. This scheme can overcome the low efficiency caused by time interval between the time when Received Signal Strength （RSS） is measured and the time when rate is selected. The best rate is selected based on data payload length, frame retry count and the estimated RSS, which is estimated from recorded RSSs. Simulation results show that the proposed scheme enhances mean throughput performance up to 7%, in saturation state, and up to 24% in finite load state compared with those non-estimation schemes, performance enhancements in average drop rate and average number of transmission attempts per data frame delivery also validate the effectiveness of the proposed schelne.     

Improved Low-Density Parity-Check Codes Based on Single Parity-Check Block and Their Performance Analysis on the Binary Erasure Channel

Based on some characteristics of the decoding and the error floors of Low-density parity-check （LDPC） codes, an approach of adding single parity-check blocks to original LDPC blocks over GF（2） is proposed, which can correct multiple block errors in a block group. When this approach is set to have correcting capacity of no more than two block errors in a code group, the performance of ensemble C（n, x^l-1,x^r-1） on the Binary erasure channel （BEC） with this approach is analyzed. The simulation results indicate that when the Block error probability （BEP） drops, the performance of the given LDPC codes can be improved exponentially. Also, the error floors of LDPC codes can be suppressed to much lower levels. Therefore, the short-length LDPC codes have a wide range of applications.     

Performance Investigation of IEEE 802.11e EDCA Under Non-Saturation Condition Based on the M/G/1/K Model

The IEEE 802.11e network provides different with QoS （Quality of service） guarantee for different traffic streams. The Enhanced distributed channel access （EDCA） in IEEE 802.11e MAC protocol is proposed to support prioritized QoS on the basis of the Distributed coordination function （DCF） in IEEE 802.11. We first build a new Markov chain model for the EDCA. The model takes into account both the idle state that represents that there are no packets to be transmitted and differentiation based on different AIFS-value （Arbitration inter-frame space） for different traffic. We can use the signal transfer function of the generalized Z-transform domain state transition diagram to derive a probability distribution of the MAC layer service time and analyze the performance of EDCA from a non-saturated channel to a saturated medium by using M/G/1/K queuing model. The proposed model is calculated numerically and validated against simulation results, we observed a good match between the analytical model and simulation. Simulation and theoretical results show that despite providing prloritized QoS, the EDCA still can not support strict QoS for real-time application. On the other hand, it is inevitable that there is unfairness of channel access to prloritized traffic due to EDCA.     

A TSK Fuzzy Approach to Channel Estimation for OFDM Systems

This paper proposes a TSK fuzzy approach to channel estimation for Orthogonal Frequency Division Multiplexing （OFDM） systems. The information of dispersive fading channel is described by using TSK fuzzy model, which is updated by the pilot symbols. The proposed approach can trace the variation of channel and it is computationally simple. Its performance is tested via simulations. Results show that it is comparable to that of ideal Minimum Mean-Square-Error （MMSE） method, especially at the low Signal to Noise Ratio （SNR）.     

Parametric channel modeling based OFDM channel estimation

A simplified parametric channel estimation approach was proposed for orthogonal frequency division multiplexing （OFDM） systems. Based on parametric channel model, this algorithm is composed of two parts： the estimation of channel parameters and channel interpolation. The exponentially embedded family （EEF） criterion is exploited to determine the number of channel paths as well as the multipath time delays. Consequently, the channel frequency responses is acquired via the estimated parameters. Additionally, the authors＇ scheme is computationally efficient owing to the needless of the eigenvalue decomposition or the estimation of signal parameters by the rotational invariance technique （ESPRIT）. Simulations are provided to validate the performance of this algorithm from perspectives of the probability of correct estimation and the mean square error （MSE）. It is demonstrated that this approach exhibits a superior performance over the existing algorithms.     

Rate-adaptive transmission over correlated fading channels

In this paper, we investigate link adaptation and incremental redundancy (IR) retransmission schemes over correlated wireless channels. While computer simulations have been used to study the performance of these techniques, a numerically tractable analytical approach is more desirable to analyze generic protocols, and to reveal insights into the performance tradeoffs. An error-recursion approach is developed in this paper to mathematically analyze the throughput, delay, and energy efficiency of rate-adaptation techniques over fading channels with arbitrary correlations between retransmissions. Using Reed-Solomon codes as an example, we quantitatively predict the performance tradeoff of throughput and latency for IR schemes and the performance dependency on the channel correlation. Numerical results also show that reactive rate-adaptation schemes with IR retransmission outperform proactive rate-adaptive schemes, even with perfect channel side information, in terms of throughput and energy efficiency.     

Adaptive Modulation over Nakagami Fading Channels

We first study the capacity of Nakagami multipath fading (NMF) channels with an average power constraint for three power and rate adaptation policies. We obtain closed-form solutions for NMF channel capacity for each power and rate adaptation strategy. Results show that rate adaptation is the key to increasing link spectral efficiency. We then analyze the performance of practical constant-power variable-rate M-QAM schemes over NMF channels. We obtain closed-form expressions for the outage probability, spectral efficiency and average bit-error-rate (BER) assuming perfect channel estimation and negligible time delay between channel estimation and signal set adaptation. We also analyze the impact of time delay on the BER of adaptive M-QAM.     

基于独立分量分析的正交空时分组编码盲检测方案

通过在不同天线发射的信号之间引入时域和空域信息,正交空时分组编码(OSTBC)可以获得发射分集增益,而且可以在不牺牲带宽的情况下获得更高的编码增益。但是其译码却需要精确的信道状态信息(CSI),因此,信道信息估计的准确性严重影响系统的性能。基于独立分量分析(ICA)的盲源分离(BSS)技术可以在不进行信道估计的情况下对发射信号实现有效检测。通过利用OSTBC的正交特性,提出了2种基于ICA的盲检测方案,同时,一些基于信道估计的检测算法也被用来进行性能比较。瑞利衰落信道下的仿真结果表明,2种新方案均具有较好的系统适应性和误码率性能。     

Binary lattice vector quantization with linear block codes andaffine index assignments

We determine analytic expressions for the performance of some low-complexity combined source-channel coding systems. The main tool used is the Hadamard transform. In particular, we obtain formulas for the average distortion of binary lattice vector quantization with affine index assignments, linear block channel coding, and a binary-symmetric channel. The distortion formulas are specialized to nonredundant channel codes for a binary-symmetric channel, and then extended to affine index assignments on a binary-asymmetric channel. Various structured index assignments are compared. Our analytic formulas provide a computationally efficient method for determining the performance of various coding schemes. One interesting result shown is that for a uniform source and uniform quantizer, the natural binary code is never optimal for a nonsymmetric channel, even though it is known to be optimal for a symmetric channel     

Simplified block adaptive diversity equalizer for cellular mobileradio

We propose a reduced complexity antenna diversity combiner-equalizer receiver structure to combat multipath fading in cellular mobile radio (CMR) communications. The technique utilizes block adaptation based on interpolated channel estimates and linear or decision feedback equalization. The receiver offers complexity reduction relative to previously proposed block adaptation methods without sacrificing performance     

Differentially encoded M-PSK block codes

Two schemes for differential encoding of blockcoded M-ary PSK signals are presented and compared. Both proposed schemes perform differential encoding after channel decoding for minimizing the performance penalty associated with differential schemes. The first scheme requires the block codes to be rotationally invariant and linear, whereas the second requires rotational invariance only. Examples of bit error rate performance are also presented     

Symbol-level adaptive modulation for coded OFDM on block fading channels

It has recently been recognized by many researchers that adaptive modulation is most effective when the channel diversity order is small. In this letter, we propose a simple adaptive modulation scheme for orthogonal frequency division multiplexing (OFDM) systems on channels that provide a small order of diversity. The proposed adaptation algorithm is based on a novel and very simple analytical formula we derive for the performance of BICM on block fading channels. Simulation results show that the derived analytical formula is very tight when the targeted bit error probability is small. OFDM systems on indoor channels provide small orders of diversity and form an ideal scenario for adaptation. In order to keep system complexity and feedback requirements at a minimum, we will consider symbol-level adaptive modulation wherein all the subcarriers in an OFDM symbol use the same modulation.     

Performance of truncated type-II hybrid ARQ schemes with noisyfeedback over block fading channels

This paper considers truncated type-II hybrid automatic repeat-request (ARQ) schemes with noisy feedback over block fading channels. With these ARQ techniques, the number of retransmissions is limited, and, similar to forward error correction (FEC), error-free delivery of data packets cannot be guaranteed. Bounds on the average number of transmissions, the average coding rate as well as the reliability of the schemes are derived using random coding techniques, and the performance is compared with FEC. The random coding bounds reveal the achievable performance with block codes and maximum-likelihood soft-decision decoding. Union upper bounds and simulation results show that over block fading channels, these bounds can be closely approached with simple terminated convolutional codes and soft-decision Viterbi decoding. Truncated type-II hybrid ARQ and the corresponding FEC schemes have the same probability of packet erasure; however, the truncated ARQ schemes offer a trade-off between the average coding rate and the probability of undetected error. Truncated ARQ schemes have significantly higher average coding rates than FEC at high and medium signal-to-noise ratio even with noisy feedback. Truncated ARQ can be viewed as adaptive FEC that adapts to the instantaneous channel conditions     

Transmit antenna shuffling for quasi-orthogonal space-time block codes with linear receivers

In this letter, we propose a transmit antenna shuffling scheme for quasi-orthogonal space-time block codes (QO-STBCs). We show that by adaptively mapping the space-time sequences of the QO-STBC to the appropriate transmit antennas depending on the channel condition, the proposed scheme can improve its transmit diversity with limited feedback information. The performance of the scheme with various numbers of shuffling patterns is analyzed. The bit error probability of the schemes is evaluated by simulations. It is demonstrated that with the linear zero-forcing (ZF) and the minimum mean squared error (MMSE) receivers, the closed-loop QO-STBC using two feedback bits can achieve almost the same performance as the ideal 4-path diversity and it is about 4-5 dB better than the open loop schemes.     

Double differential space-time block coding for time-selectivefading channels

Most existing space-time coding schemes assume time-invariant fading channels and offer antenna diversity gains relying on accurate channel estimates at the receiver. Other single differential space-time block coding schemes forego channel estimation but are less effective in rapidly fading environments. Based on a diagonal unitary matrix group, a novel double differential space-time block coding approach is derived in this paper for time-selective fading channels. Without estimating the channels at the receiver, information symbols are recovered with antenna diversity gains regardless of frequency offsets. The resulting transceiver has very low complexity and is applicable to an arbitrary number of transmit and receive antennas. Approximately optimal space-time codes are also designed to minimize bit error rate. System performance is evaluated both analytically and with simulations