Performance of higher order modulation schemes in relay networks

Abstract

Due to ever increasing demands of wireless applications and devices, spectrally e - cient high data-rate transmission is the prime requirement of practical communication systems. To meet such demands, cooperative relaying is considered as a core technique due to its ability to provide spectral e ciency with improved capacity and coverage over severely degraded channel conditions, without considering multiple antennas at di erent nodes. Additionally, orthogonal frequency division multiplexing (OFDM) is one of the promising multi-carrier modulation schemes which provides spectrally e cient high data-rate transmission with improved robustness even in severely degraded channel conditions. Thus, cooperative communication in conjunction with OFDM is considered in long-term evolution (LTE)-Advanced, and provides a solid platform for future 5G and beyond wireless communication systems. For further data-rate enhancement with optimum spectral e ciency, higher order modulation schemes such as the family of quadrature amplitude modulations (QAM) (i.e. squared QAM (SQAM), rectangular QAM (RQAM), cross QAM (XQAM) and hexagonal QAM (HQAM)) have gained increased attention due to their high power and bandwidth e ciency. In practice, perfect knowledge of channel state information (CSI) at all the communication nodes is not feasible, which introduces channel estimation error (CEE). CEE has signi cant detrimental impact on the system performance which cannot be ignored for realistic system design. While moving towards 5G and beyond systems, with the increased multimedia applications through wireless channels, the bandwidth requirement has increased signi cantly. This makes the design of a linear power ampli er (PA) very di cult. In cooperative relaying, high peak-to-average-power ratio (PAPR) occurs not only in uplink but also in downlink due to the presence of non-linear PA (NLPA) at relays which introduces signi cant non-linear distortion (NLD) in the received signal. Therefore, studying the impact of NLD on the system performance is equally important. In this thesis, initially a 3-hop orthogonal frequency division multiple access (OFDMA) based amplify-and-forward (AF) relay network is considered, which is based on shared user equipment side distributed antenna system (SUDAS). A mixed Rayleigh/Rician fading environment is considered and closed-form expressions of lower-bound (LB) and upper-bound (UB) of outage probability are derived. Further, limiting case of SUDAS network is assumed and performance of 3-hop AF OFDM relay network over mixed Rayleigh/Rician fading is analyzed. For the performance analysis, analytical expressions of outage probability, asymptotic outage probability, and ergodic capacity are derived. Also, average symbol error rate (ASER) expressions of RQAM, XQAM, and HQAM schemes are derived. Finally, a comparative study of various QAM schemes is presented. Further, a dual-hop AF single relay network with maximum ratio combining (MRC) receiver is considered over the generalized independent and non-identically distributed (i.n.i.d.) Nakagami-m fading channels with integer valued fading parameter. Practical constraint of imperfect CSI is considered and closed-form expressions of outage probability, asymptotic outage probability, and ASER of RQAM, XQAM, and HQAM schemes are derived. The impact of Nakagami-m fading parameter and imperfect CSI are observed on the system performance and important insights are highlighted. Furthermore, a dual-hop AF multi-relay network with MRC receiver is considered over i.n.i.d. Nakagami-m fading channels with both the integer and non-integerfading parameters. Practical constraints of imperfect CSI and NLPA at the relay are considered. Analytical expressions of outage probability, asymptotic outage probability, and ASER of RQAM, XQAM, and HQAM schemes are derived for both the integer and non-integer fading parameters. A comparative study of various QAM schemes is presented and the impact of fading parameter, number of relays, imperfect CSI, and NLPA are highlighted on the system performance, and important insights are driven. In parallel, over the last few years, free space optics (FSO) has tremendously been studied as a substitute to the radio frequency (RF) systems due to its low cost and easy to install system which operates in unlicensed spectrum. It is a line-of-sight technique which provides improved transmission security and immunity from the electromagnetic interference in a given unlicensed spectrum. However, atmospheric turbulence and pointing error result in severe performance degradation. Out of the several statistical channel models proposed for the turbulence induced faded FSO channels, Gamma-Gamma fading model is most commonly preferred which consists of both the small and large scale atmospheric uctuations. Cooperative relaying has emerged as one of the solutions to counter the turbulence induced fading, and increasing the range of FSO systems. Relaying takes the advantage of shorter hops to improve performance of the FSO link as well as coverage of the system. Mixed RF/FSO can ll up the connectivity gap between the RF network and a backbone network. Thus, a dual-hop variable gain AF mixed RF/FSO system is considered.Nakagami-m and generalized Gamma-Gamma distributions are considered for the RF and FSO links, respectively. Outdated CSI at relay, and atmospheric turbulence with pointing error for FSO link are considered. For performance analysis, analytical expressions of outage probability and ergodic capacity are derived in terms of Meijer-G function and extended generalized bivariate Meijer-G function (EGBMGF). Generalized ASER expressions for RQAM, XQAM, and HQAM schemes are derived in terms of Meijer-G function. Further, a detailed comparative study of various modulation schemes is presented and the impact of pointing error, atmospheric turbulence, outdated CSI, and Nakagami-m parameter are highlighted on the system performance. Lastly, numerical results are compared with Monte-Carlo simulations to verify the correctness of derived expressions. ii