Applying Spatial Diversity to a Fixed Point-to-Point Microwave Radio System Chadi Khirallah, Paul Coulton, Saad Qazi, and Nader Zein Department of Communication Systems, Lancaster University, Lancaster, LA1 4YR Email: c.khirallah@lancaster.ac.uk Abstract-In this paper we present the performance of a practical enhancement of the current Point-To-Point radio system, which increases the channel capacity from 11 bps/Hz to 22 bps/Hz. The system proposed uses a Co-Channel Dual Spatial Dual Polarisation mode allowing the transmission of 622 Mbps STM4 signals on four sub-channels. A Least-Mean-Square nterference canceller is used at the receiver to negate the various interference effects. 1. INTRODUCTION Multiple-Input-Multiple-Output (MIMO) refers to radio links with multiple antennas at the transmitter and the receiver thus allowing the spatial dimension to be exploited to improve the performance and capacity of the wireless link. This promised capacity enhancement has motivated research interest in MIMO and space time coding (STC) for both fixed and mobile radio access including: 3rd Generation (3G) mobile; Point-to- Multipoint (P2MP); and Point-to-Point (P2P) systems, which are becoming an essential component in communications networks. In this paper we consider the Synchronous Digital Hierarchy (SDH) standards, which has different transmission rates ranging from 155 Mbps (normally referred to as Synchronous Transport Model level 1(STM1)) to higher rates developed from integer multiples of STM1 rate. These systems operate in frequency bands 4- 38 GHz with channel spacing ranging 28- 60 MHz and carry multiple STM signals in various diversity combinations [1], [2], [3]. The current standard for P2P fixed radio systems [3] will be used in local and regional networks at data rates between 2 x STM-1 and 4 x STM-1 or STM-4 . The carriers in these three system configurations are arranged in the following (see Fig. 1): 1. Adjacent Channel Alternate Polarization (ACAP) – 2 basic capacities on 2 x 56 MHz channels using alternate (dual) polarization; with bandwidth efficiency 5.5bps/Hz; 2. Adjacent Channel Co-Polarization (ACCP) – 2 basic capacities on 2 x 56 MHz channels using the same polarization; with bandwidth efficiency 5.5bps/Hz; 3. Co-Channel Dual Polarized (CCDP) – 2 basic capacities operating in the same 56 MHz channel using dual polarization. For this configuration a Cross Polar Interference Canceller (XPIC) can be used; with bandwidth efficiency 11 bps/Hz. This paper introduces a new spatial-cross polarization mode, which increase the bandwidth efficiency to 22 bps/Hz for the same 28 MHz bandwidth using four carriers, 128 QAM, and dual polarized antennas. This new system is defined as Co-Channel Dual Spatial Dual Polarisation mode (CCDSDP) and is shown in Fig. 1. Fig. 1: System Carrier Configurations 2. SYSTEM MODEL Fig. 2 shows the block diagram of the transmitter where data is generated at the rate of 4 STM-1, which is equivalent to 622 Mbps, and then encoded using a (208,188) Reed Solomon (RS) Code, which is a shortened version of (255,235) RS codes . This encoded data source is then demultiplexed into four independent data streams that are modulated using 128 QAM. Fig. 2: CCDSDP Transmitter ISBN: 1-9025-6009-4 © 2003 PGNet