Fast simulation of OFDM systems over frequency selective fading channels

The authors extend the formulation for both classical Monte Carlo (MC) and importance sampling (IS) techniques to include the bit error rate (BER) and symbol error rate (SER) of multilevel systems based on symbol by symbol observation. A new IS technique and analysis of BER and SER based on the extended formulation are presented. The authors briefly summarize the previous work on BER of binary systems. The extended formulation is then presented. The conventional IS technique for BER of binary systems is shown to be applicable in simulating BER and SER of M-ary systems, and a new improved IS technique is presented. Numerical results for all the methods are provided. >

In this paper, we analyze the Bit Error Rate (BER) performance of Golden coded Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system over Rician multipath fading channel. We also compare the performance of the MIMO-OFDM system using Golden code in Rayleigh and Rician multipath fading channels. We discuss the effects of the presence of line-of-sight (LoS) component in the multipath fading environment which renders the improvement in the overall performance of the Golden coded MIMO-OFDM. This paper discusses the performance of Golden codes in a frequency selective Rician fading channel. To deal with the frequency selective fading channel, we use the OFDM (Orthogonal Frequency Division Multiplexing) modulation. The BER performance of the Golden coded MIMO-OFDM system under several propagation conditions has been evaluated. Keywords—MIMO, OFDM, Golden STBC, Rayleigh fading, Rician fading.

This work explores the performances of Space-Time and Space Frequency Coded Orthogonal Frequency Division Multiplexing (OFDM) with simple two branches transmit diversity scheme. The combination of multiple-antenna and orthogonal frequency division multiplexing (OFDM) provides reliable communications over frequency selective fading channels. We focus on the application of space-time block codes (STBC) and space-frequency block codes (SFBC) in OFDM systems over time-varying and frequency-selective channels. SFBC transmitter shows superior performance in fast varying channels while STBC shows better performance in frequency selective channels. A switching technique is presented that selects an appropriate transmission scheme between the STBC and SFBC assuming Rayleigh fading model.

In the new generation wireless communication systems where high data rates are desired, orthogonal frequency division multiplexing (OFDM) has become the standard method because of its advantages over single carrier modulation schemes on multi-path, frequency selective fading channels. However, inter-carrier interference due to Doppler frequency shifts, and multi-path fading severely degrades the performance of OFDM systems. Estimation of channel parameters is required at the receiver. In this paper, we present a time-varying channel modeling and estimation method based on the discrete evolutionary transform that provides a time-frequency procedure to obtain a complete characterization of a multi-path, fading and frequency selective channel. Performance of the proposed method is tested on different levels of channel noise, and Doppler frequency shifts.

Modified efficiency importance sampling (MEIS), an importance sampling technique which can be used to yield a significant saving of run time for the assessment of bit error rate (BER) in digital communication systems using computer simulation, is described. The concept of equivalent noise source is introduced to avoid the effect of system memory on the efficiency of importance sampling, and a nonGaussian probability density function (p.d.f.) is selected as the biased equivalent noise to further improve run time. MEIS has been used successfully in the simulation of M-ary quadrature amplitude modulation (MQAM) systems. The MEIS is shown to be effective in the simulation of systems with large memory length, and in mildly nonlinear systems. The experimental results show that MEIS is a great improvement over Monte Carlo (MC) methods and that is even more efficient than the efficient importance sampling (EIS) technique. >

Adaptive coded modulation (ACM) is a key technique to increase spectral as well as power efficiency for orthogonal frequency division multiplexing (OFDM) systems. In this paper, an efficient rate adaptive low density parity check (LDPC) coded modulation (RALCM) algorithm is proposed, which maximizes the payload rate for OFDM systems under the constraint of the given transmit power and the target bit error rate (BER). Taking advantage of the two dimensional look up table for the signal to noise ratio (SNR) thresholds, the RALCM algorithm assigns different combinations of the LDPC code rates and quadrature amplitude modulation (QAM) modulations for subcarriers according to the channel frequency response (CFR). Both theoretical analysis and computer simulation show that the proposed algorithm not only efficiently improves the payload rate of the LDPC coded OFDM system over frequency selective fading channels, but also inherently reduces the computational complexity of the system optimization.

We propose a sub-band spreading technique for adaptive modulation (AM) in orthogonal frequency division multiplexing (OFDM) systems in order to reduce signaling overheads and to average frequency selective fading channels causing different signal-to-noise ratio (SNR) values for subcarriers in each sub-band. The conventional sub-band based AM schemes can also reduce signaling overheads and complexity for allocating a resource per sub-band at a time. However, they may suffer from the channel variation in a sub-band when the sub-band size is larger than the channel coherence bandwidth (BW). The sub-band spreading at the transmitter enables the received symbols in each sub-band to have an identical reliability even in a frequency selective fading channel. We rigorously analyze the averaged SNR value at the receiver of the sub-band spreading system and the analyzed average SNR in a sub-band is used for an adaptation criterion. The proposed AM scheme outperforms the conventional sub-band based OFDM scheme without spreading, and it can yield better throughput performance than the conventional subcarrier based AM schemes when we consider the signaling overheads.

This paper presents the effects of carrier frequency offset (CFO) and symbol timing offset (STO) in filtered multitone (FMT) systems over time and frequency selective multipath fading channels. The analysis is based on an FMT system using a square-root raised cosine pulse. The performance of FMT systems in the presence of CFO and STO over different fading channels are derived and compared with that of orthogonal frequency division multiplexing systems. The offsets degrade the performance of the FMT systems because they cause filter mismatch. The CFO changes the amplitude frequency characteristic of the receiver filter, while the STO changes the phase frequency characteristic of the receiver filter. The simulations are conducted over different fading channels. To achieve signal-to-interference ratio above 20 dB, over a flat static channel, the frequency offset relative to symbol duration must be less than 5% in the orthogonal frequency division multiplexing system, while the upper limit is 8% in the FMT system. The simulation results also show that the FMT system is less sensitive to CFO in time frequency selective channels. The signal-to-interference ratio expressions help predict the bit-error-rate performance of FMT with single tap subchannel equalization. The analysis also helps synchronization algorithm design after analyzing the synchronization offset effect in FMT systems under certain channel conditions. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper we propose an interleaved spread spectrum orthogonal frequency division multiplexing (ISS-OFDM) system and investigate its performance over frequency selective multipath fading channels. The purpose is to exploit frequency diversity capability, develop an e±cient spectrum spreading algorithm and improve the system performance over frequency selective multipath fading channels. At transmitter, a spectrum spread ISS-OFDM signal is generated by employing OFDM modulation and interleaving techniques. At the receiver, two solutions including serial demodulation and parallel demodulation for diversity combing are proposed, in which the received signals are combined by using a maximum ratio combining (MRC) technique. The simulation indicates that higher frequency diversity is achieved, and the Bit Error Rate (BER) and Peak-to-Average Power Ratio (PAR) performances of the proposed ISS-OFDM system over frequency selective multipath fading channels are improved signi¯cantly as compared with the conventional OFDM systems. Another unique characteristic is that the spectrum spreading factor and diversity order provided by the system are reconfigurable to achieve cognitive communications.

This paper deals with low-complexity joint channel estimation and decoding for faster-than-Nyquist (FTN) signaling over frequency selective fading channels. The inter-symbol interference (ISI) imposed by FTN signaling and the frequency selective channel are intentionally separated to fully exploit the known structure of the FTN-induced ISI. Colored noise due to the faster sampling rate than that of the Nyquist signaling system is approximated by autoregressive process. A Forney style factor graph representation of the FTN system is developed and Gaussian message passing is performed on the graph. Expectation propagation (EP) is employed to approximate the message from channel decoder to Gaussian distribution. Since the inner product between FTN symbols and channel coefficients is infeasible by belief propagation (BP), we propose to perform variational message passing (VMP) on an equivalent soft node in factor graph to tackle this problem. Simulation results demonstrate that the proposed low-complexity hybrid BP-EP-VMP algorithm outperforms the existing methods in FTN system. Compared with the Nyquist counterpart, FTN signaling with the proposed algorithm is able to increase the transmission rate by over 40%, with only negligible BER performance loss.

This paper compares diversity combining schemes for the downlink of spread spectrum orthogonal frequency division multiplexing (SS-OFDM) systems in frequency selective fading channels. In particular, symbol-level combining after despreading is compared to chip-level combining under maximal ratio combining (MRC) of signals from different diversity branches and minimum mean-square error (MMSE) equalization of spreading sequences. Symbol-level combining takes place after the operations of MMSE equalization and despreading, whereas the operations of equalization and despreading occur after MRC if chip-level combining is used. Chip-level combining combines diversity samples in an efficient manner while reducing inter-code interference (self-interference) that results from the loss of orthogonality of spreading sequences due to a frequency selective channel. This method is shown to be superior to symbol-level combining when the diversity branches are uncorrelated, and when the branches differ only due to subcarrier interleaving. Novel expressions for the bit error rate (BER) of the two methods, as well as the extension of the analysis to partially loaded systems are presented. The results are relevant to antenna diversity as well as temporal diversity achieved though re-transmission within an ARQ scheme.

We consider a three-node wireless relay system with frequency selective quasi-static fading channels between the nodes. We assume practical constraints require the relay to operate in half-duplex mode and employ a low-complexity non-orthogonal amplify-forward protocol. Assuming full channel state information and optimal energy allocation between source and relay, we study upper bounds of the outage probability for such channels. Compared to relay systems in flat fading channels, our results reveal that the additional diversity offered by frequency selective fading channels significantly improves outage performance. The results also suggest, however, that relays may offer a limited improvement in diversity when multipath fading in the source-destination link is sufficiently rich.

Orthogonal frequency division multiplexing (OFDM) is one key technology in wireless multimedia communication. In this paper, the performance of channel estimation methods for OFDM systems based on block-type under frequency selective fading channels is discussed. Analysis of a 3G carrier model with 1 MHz bandwidth under frequency selective fading channels is made. The pilot signal estimation based on LS, MMSE, LMMSE (a simplified MMSE algorithm with low-rank approximation using singular value decomposition) criteria, and channel interpolation based on the simple linear interpolation method. Simulation indicates its good anti-multi-path performance and anti-doppler frequency shift ability.

In this paper, we analysis the receiver of orthogonal frequency division multiplexing (OFDM) systems over frequency selective fading channels where the multipath delay exceeds the guard interval duration and propose a novel receiver scheme. The proposed receiver consists of the Inter Symbol Interference (ISI) canceller and the turbo equalizer. At each iteration, the proposed turbo equalization algorithm performs soft Inter Carrier Interference (ICI) cancellation and frequency domain minimum mean-square error (MMSE) filtering to minimize the performance degradation caused by ICI distortion and frequency-selective channels. Simulation results of the system performance show that the proposed receiver effectively recovers the contaminated OFDM signals.

For the reception of multicode direct sequence (DS)-CDMA signals, the MMSE frequency domain equalisation is applied instead of RAKE combining. The achievable BER performance in a frequency selective Rayleigh fading channel is evaluated by computer simulation. It is shown by computer simulation that the DS-CDMA with MMSE frequency domain equalisation outperforms the DS-CDMA with RAKE combining and shows only slight performance degradation compared to the MC-CDMA with minimum mean square error combining (MMSEC).