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This paper presents an OFDM-based relay network with relay selection employing both decode-and-forward (DF) and amplify-and-forward (AF) approaches. For the network, we derive a lower-bound on the outage probability and the diversity order achievable in frequency selective fading channels. Simulation results show that both DF and AF relaying provides the diversity order same to that obtained from...
We investigate performance limits of a coded asymmetric OFDM (A-OFDM) system in multipath fading channels when maximum-likelihood (ML) detection is deployed. Our A-OFDM formulation is general and the analysis extends to coded OFDM (COFDM), precoded COFDM and single carrier systems. At first, an exact pairwise error probability is evaluated based on Craig's alternative form of the Q-function. Then...
System-level simulations of packet-based orthogonal frequency-division multiple access (OFDMA) cellular networks require a proper consideration of the channel-dependent fast fading/scheduling in order to produce meaningful results. However, generating the time and frequency selective fast fading channels and performing detailed channel-adaptive scheduling for each user in a multi-cellular system is...
In this work, we examined the performance of conventional DFT based OFDM and wavelet based OFDM (WOFDM) with and without Alamouti coding over multipath Rayleigh fading channels with exponential power delay profile. Results show that WOFDM has slightly better bit error rate performance than the conventional OFDM with and without Alamouti code. Besides the performance improvement of WOFDM as compared...
In this paper, alpha-OFDM, a generalization of the OFDM modulation, is proposed to enhance the outage capacity of bursty transmissions. This new flexible modulation scheme is easily implemented and only requires a symbol rotation of angle alpha after the IDFT stage. The induced rotation slides the DFT window and provides frequency diversity in block fading channels. Interestingly, the results show...
In this paper, we show that the use of cyclic delay diversity (CDD) on the downlink (DL) can improve the performance of uplink (UL) diversity methods such as antenna selection in a time-division duplexing (TDD) system. An analytical framework for showing why CDD transmission on the DL improves UL diversity transmission in TDD system is provided. The analysis is used to quantify the gain in the UL...
In the IEEE802.11a WLAN standard, orthogonal frequency division multiplexing (OFDM) modulation is employed. Diversity techniques are usually utilized to overcome fading channels in OFDM systems. Furthermore, a diversity technique with one antenna is desirable because the form factor of the terminal is important. Fractional sampling (FS) scheme is one of the diversity techniques with a single antenna...
OFDM can provide frequency diversity gain for point-to-point communications over frequency-selective slow fading channel. Recent works show that OFDM may also form a flexible and efficient multiple access method, which is often referred to OFDMA. However, the user outage probability and the optimal frequency diversity gain in OFDMA systems are not known. In this paper, random bipartite graph is used...
In this paper, we investigate the benefits of two-hop opportunistic relay in time division duplex (TDD) OFDMA cellular network configurations. The paper starts with a short analytical model for the two-hop opportunistic relay. The model expresses the probability of finding a suitable relay node in the presence of lognormal fading and it allows the computation of the expected number of out-of-coverage...
In this paper, a joint carrier synchronization and equalization algorithm is proposed for OFDM systems in the tracking stage. Significantly, the carrier synchronization scheme is a dual-loop, which is composed of an outer and an inner loops, with the multirate processing to eliminate the carrier frequency offset and the channel phase variation. In addition, the gain equalization loop is employed to...
In this paper, we consider an automatic-repeat-request (ARQ) retransmission protocol signaling over a block-fading multiple-input-multiple-output (MIMO) channel. Unlike previous work, we allow for multiple fading blocks within each transmission (ARQ round), and we constrain the transmitter to fixed rate codes constructed over complex signal constellations. In particular, we examine the general case...
In this paper, we investigate the SNR and BER performance of an OFDM system under imperfect synchronization which is caused due to noise, Doppler shift and frequency selective fades in the channel. An analytical expression for BER is derived assuming BPSK modulation and numerical results are presented to show the effect and extent of the above channel impairments in OFDM downlinks.
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