Analysis of Doppler Spread Perturbations in OFDM(A) Systems

Abstract

Abstract. We analyze the effects of Doppler spread in mobile channels on Orthogonal Frequency Division Multiplex (OFDM) systems. This is important, since channel variations during one OFDM symbol cause Inter Sub-carrier Interference (ICI) in OFDM systems, which degrades the performance, since ICI can be seen as additional near-Gaussian noise. The analysis is presented and closed-form results are given for numerous important practical Doppler spread encountered in mobile channels and applications such as mobile reception of DVB-T. We also show that for the case of asymmetrical Doppler spreads, the frequency correction of the receiver can be adjusted so as to minimize the ICI. To extend our work to OFDM(A) mobile radio systems where many users' contributions to the ICI overlap, we have introduced the concept of an equivalent single-user which causes the total ICI, in order to use the same analytical framework, and compared the uplink and downlink cases. Simulation results confirm our analysis and show the importance of the analytical tool to perform realistic system analysis without having to resort to time consuming time-domain simulations.

Effect of power and rate adaptation on the spectral efficiency of MQAM/OFDM system under very fast fading channels

“…Solving (21) yields the optimal power adaptation (22) where the value of the Lagrange multiplier λ can be obtained from the average power constraint using numerical methods. The optimal rate adaptation is then given as…”

“…We can obtain an upper bound from (22) as (27) For the worst case, the ICI is determined by the largest power, when γ [k] = ∞ and a lower bound on the maximum spectral efficiency in adaptive OFDM/MQAM systems is obtained.…”

“…Much of the research on the performance analysis of OFDM in fast fading channels are based on an approximation of the ICI components as Gaussian random variables or by using the average ICI power. Such simple closed-form expressions and bounds for the total ICI power are presented in [11,[20][21][22]. These have been employed to obtain approximate expressions for the average signal-to-interference plus noise ratio (SINR) [23][24][25], and error rates [3][4][5].…”

Artificial Wideband Multi User Channels for Rural High Speed Vehicle to Vehicle Links

“…It should be noted that with the sampling used there could potentially have been higher Doppler frequency harmonics, though this is unlikely given that two vehicles were not moving in the direction of each other at 130km/h. However, what is of more interest here is that the duration of such instances of high Doppler shift will be short and thus can be considered as "chirps" in the Doppler spectrum, which potentially is disadvantageous to maintaining a carrier to interference ratio for OFDM with such high values of Doppler shift [15], which would have to be compromised by extended packet lengths at a cost of available bandwidth.…”

Hybrid link adaptation for inter-carrier interference adjustment in orthogonal frequency division multiple access cellular uplink systems

“…However, when applied to the uplink, the use of rate adaptation technique may cause a serious intercarrier interference (ICI) problem. The user is only affected by its own ICI in the downlink [16], but it can be affected by ICI from other users in the uplink [17,18]. The user who is near the BS and/or has high velocity can cause severe ICI to other users in the uplink.…”