Dual-hop wireless communications with combined gain relays T.A. Tsiftsis, G.K. Karagiannidis and S.A. Kotsopoulos Abstract: A dual-hop relayed wireless communication system is presented where the gain of the relay, called combined gain relay (CGR), is produced after combining the channel state information from both hops, depending on the mean hop’s signal-to-noise ratio (SNR). The proposed scheme can be efficiently applied in dual-hop transmissions with unbalanced mean SNRs due to the long-term fading effects produced by the movement of the user in the area served by the wireless network. The overall system performance is studied in Rayleigh fading channels. Closed- form expressions are derived for important system performance metrics, such as average end-to- end SNR, average error probability and outage probability. The CGR’s average power consumption is investigated which in certain cases is lower compared with existed relays. Numerical results and simulations show an improvement in the end-to-end system performance. 1 Introduction Multihop relaying technology is a promising solution for throughput and high data-rate coverage requirements in future cellular and ad-hoc wireless communication systems without the need to use large power at the transmitter, and to combat fading and shadowing through spatial/multiuser [1–4] . Nowadays, there is great interest in the research community on the potential of multihop and especially on dual-hop transmissions [4–8]. Dual-hop transmission can be classified into two main categories, regenerative or non- regenerative, depending on the relay type used. Nonregen- erative relays just amplify and retransmit the information signal, as opposed to regenerative relaying nodes which decode the signal and then retransmit the detected version to the next node [5, 6] . Additionally, relays of nonregen- erative systems are classified in two main subcategories, as channel state information (CSI)-assisted relays, where they use the CSI from the previous hop to produce their gain leading to a power control of the retaransmitted signal, and fixed-gain relays with lower complexity compared with CSI- assisted ones, and which introduce a fixed gain and thus a variable signal power at the output. Looking through the recent open technical literature, the performance of dual-hop wireless communication systems is studied in [1, 5–8] . Hasna and Alouini have presented a useful and semianalytical framework for the evaluation of the end-to-end outage probability of multihop wirelsess systems with nonregenerative CSI-assisted relays over Nakagami-m fading channels [1] . Moreover, the same authors have studied the outage and the error performance of dual-hop systems with regenerative and nonregenerative CSI-assisted relays over Rayleigh [5] and Nakagami-m [6] fading channels. The analysis in [1, 5, 6] is based on an upper bound for the end-to-end signal-to-noise ratio (SNR) which leads to lower bounds for the system’s outage and average error probability. This bound corresponds to an ideal relay capable of inverting the channel in the previous hop (regardless of the fading state of that hop) without limiting the output power. Furthermore, in [7] the end-to- end performance of dual-hop systems equipped with nonregenerative fixed-gain relays is investigated and a specific relay is proposed, called semiblind, that benefits from the knowledge of the first hop’s average fading power. Tsiftsis et al. presented a new upper bound for the end-to- end SNR and efficiently evaluated the average error probability in dual-hop collaborative diversity systems, especially at low SNRs [8] . Anghel and Kaveh in [3] have studied the error performance of a co-operative network of dual-hop transmissions with parallel CSI-assisted relays in Rayleigh fading, where multiuser spatial diversity is used to combat the signal’s impairments. Karagiannidis has studied the performance bounds for multihop relayed transmissions with blind (fixed-gain) relays over Nakaga- mi-n (Rice), Nakagami-q (Hoyt) and Nakagami- m fading channels [4]. In this paper we present a dual-hop transmission system, where the gain of the relay, called combined gain relay (CGR), is produced using CSI from both hops, depending on the mean hop’s SNR. The proposed scheme can be efficiently used in dual-hop wireless transmissions with unbalanced mean SNRs between the hops due to the long- term fading effects produced by the movement of the user in the area served by the wireless network. The overall system performance is studied in Rayleigh fading channels as follows: Closed-form expressions for the moments, the average bit error probability (ABEP) and the outage probability of the end-to-end SNR are derived. We study the average power consumed by the CGR and it is shown that in certain cases it is less compared with existing relays. Moreover, numerical examples and Monte Carlo simula- tions show that CGR results in a significant improvement in the end-to-end system performance, compared with existing gain relay schemes. T.A. Tsiftsis and S.A. Kotsopoulos are with Department of Electrical & Computer Engineering, University of Patras, Rion, GR-26500, Patras, Greece G.K. Karagiannidis is with Department of Electrical & Computer Engineering, Aristotle Unilversity of Thessaloniki, GR-54124, Thessaloniki, Greece E-mail: tsiftsis@ee.upatras.gr r IEE, 2005 IEE Proceedings online no. 20045268 doi:10.1049/ip-com:20045268 Paper first received 27th October 2004 and in final revised form 4th May 2005 528 IEE Proc.-Commun., Vol. 152, No. 5, October 2005