CHANNEL MODELS FOR COOPERATIVE MIMO SYSTEMS

Abstract

Cooperative MIMO is a promising technology which can exploit the spatial diversity gains inherent in multiuser wireless systems without the need of multiple antennas at each node. Cooperative MIMO technology finds its application in cellular systems as well as resource constrained wireless sensor network in which the nodes can cooperate to optimally allocate resources, so as to maximize the performance and network lifetime. A deep understanding of the channel is necessary before setting up any wireless network. The various cooperative relaying protocols like Amplify and Forward, Detect and Forward and Coded Cooperation are briefly explained in this thesis. The statistical properties like time domain correlation, doppler spectrum, level crossing rate and signal to noise ratio of the fixed gain relay channel are elaborated which provides understanding about the characteristics of the relay fading channel. The tapped delay model forms a basis for simulation of such fading channels. A tapped delay model for a MIMO channel is presented before giving an overview of standardized point to point MIMO channel models like WINNER II model, 3GPP SCM, SUI and 802.16j channel model. While many issues in cooperative MIMO channel, modeling have been covered by the existing standardized point-to-point MIMO channel models, there are still some.challenges that remain to be addressed, which are concisely brought out. The mobile to mobile (M2M) channel is an integral part of Cooperative MIMO systems. The "double-ring" scattering model proposed by Patel et. al. is used to simulate the M2M channels. This double ring model is verified in terms of statistical properties like pdf of complex envelope, autocorrelation, variance of the autocorrelation and level crossing rate (LCR). The performance of DPSK in terms of BER and outage probability over such M2M channel is evaluated and compared with the available analytical results. The single relay acting as Amplify and Forward (A&F) node forms the simplest scenario for cooperative communication. The concept of double ring model is extended to simulate this relay channel as three ring model, in which the ring of scatterers is also considered around the relay node. Under flat fading conditions, the overall channel from the source to the destination via the relay in A&F systems is "double Rayleigh" with properties quite different from a typical cellular channel. The three ring model is verified in terms of statistical properties such as the envelope pdf, autocorrelation, level crossing rate and doppler spectrum. The performance of DPSK over such a relay channel is evaluated. The plots of BER versus SNR are obtained and compared with the analytic results. The Maximum Ratio Combining (MRC) is considered to study the multichannel cooperative diversity (CD) networks. The double ring and three ring model are used to simulate these CD networks. The performance of BPSK over these multichannel networks is obtained in terms of BER which validate the diversity gains associated with cooperation. The doppler is introduced in the channel model to see the effect of mobility of terminals on channel coefficients. iv