ACM 978-1-4503-0062-9/10/06/...$10.00" BER Performance of OFDM Systems in Mobile Multi-Hop Relaying Channels Hassan A. Ahmed * , Ahmed Iyanda Sulyman • , Hossam Hassanein + Electrical and Computer Engineering Department *• , School of Computing + *+ Queen’s University, Canada, Kingston, K7L 3N6 • King Saud University, Saudi Arabia, Riyadh 11421 Emali:{ * 3haea@queensu.ca, • asulyman@ksu.edu.sa, + hossam@cs.queensu.ca} ABSTRACT Orthogonal frequency division multiplexing (OFDM) system has been proposed as a technique for broadcasting digital signals and for wireless communication. The bit-error-rate (BER) performance of the OFDM system is severely aﬀected by the nonlinearity of the high power ampliﬁer and by the Doppler eﬀect impairments. In this paper, we analyze the eﬀects induced on the OFDM signal by the ampliﬁer non- linearity and by the Doppler eﬀect over multi-hop relaying channels. Moreover, simulation results are presented to vali- date the analysis. It is shown that the resulting inter-carrier interference (ICI) due to the cumulative eﬀects of the phase noise generated by these impairments per hop becomes very signiﬁcant in a multi-hop relaying communication system, and severely degrades the BER performance of the system. Theoretical results show perfect agreement with those ob- tained by simulation. Categories and Subject Descriptors C.2.1 [Network Architecture and Design]: wireless com- munication; C.2.3 [Computer-Communication Networks]: Network Operations—Network management General Terms Theory, Design, Performance. Keywords Mobile Multi-hop relaying, WiMAX networks, mesh mode, OFDM systems, Amplify-and-forward relaying, Broadband wireless access. 1. INTRODUCTION Multi-hop relaying has been adopted in several wireless networks such as 3G cellular, WLANs, and WiMAX systems Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proﬁt or commercial advantage and that copies bear this notice and the full citation on the ﬁrst page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior speciﬁc permission and/or a fee. "IWCMC’10, June 28- July 2, 2010, Caen, France. Copyright c 2010 as a cost-efective means of extending the reach or capacity of the wireless system. The multi-hop relay extension for IEEE 802.16e (mobile WiMAX) system, for instance, is the sub- ject of ongoing standardization activities within the IEEE 802.16j Task Group. The emerging IEEE 802.16j standard enhances the IEEE 802.16e PHY and MAC to enable sup- port of multi-hop communication between a mobile station (MS) and a base station (BS) through intermediate relay stations (RSs), which can be mobile or ﬁxed. In such a sys- tem the communication between MS and BS is done through two hops: ﬁrst hop between MS and RS and second hop be- tween RS and BS. Each RS amplify or decode users data and forward it to the BS. Deploying RSs in the coverage area of BS as deﬁned in IEEE 802.16j has been considered a promising solution that can replace the 802.16e mesh mode for coverage extension, throughput enhancement, and over- coming coverage holes. Broadband wireless access networks (BWANs) based on or- thogonal frequency division multiplexing (OFDM) have also gained tremendous attentions lately. OFDM provides an ef- ﬁcient broadband data transmission by sending parallel data over a number of closely-spaced subcarriers. For high data rate, it is desirable to increase the number of subcarriers per OFDM symbol. As the number of subcarriers increases for a ﬁxed channel size however, the frequency spacing between the subcarriers in the OFDM symbol is reduced. This makes the OFDM system more sensitive to phase noise which de- stroys the orthogonality of the subcarriers, causing inter- carrier interference (ICI). The main issues causing phase noise in OFDM systems can be enumerated as follows. (i) High PAPR (peak-to-average power ratio) with nonlinear ampliﬁer: In OFDM systems, because of high ﬂuctuations in the level of the instantaneous signals transmitted, PAPR is typically large, making opera- tion over linear region of high power ampliﬁer (HPA) diﬃ- cult. When some portions of the OFDM signals transmitted operate in the nonlinear region of the HPA, nonlinear dis- tortions are introduced in the OFDM signals, resulting in phase noise which contributes to the ICI [1]. (ii) Doppler Eﬀect: Relative speed between the transmitter and receiver in a wireless channel, introduces Doppler shifts in the re- ceived frequencies, contributing also to the ICI. However, these items still pose signiﬁcant problems in most practi- cal systems and should be given due considerations in the design of broadband multi-hop relaying communication sys- tems, where the OFDM symbols typically traverse several 905