Bounds on the Maximum Number of Concurrent Links in MIMO Ad Hoc Networks with QoS Constraints

arXiv:1004.4297v1 [cs.NI] 24 Apr 2010 1 Bounds on the Maximum Number of Concurrent Links in MIMO Ad Hoc Networks with QoS Constraints Pengkai Zhao, and Babak Daneshrad Wireless Integrated Systems Research (WISR) Group, Electrical Engineering Department, University of California, Los Angeles, CA 90095 USA (e-mail: pengkai@ee.ucla.edu; babak@ee.ucla.edu) Abstract Multiple-Input Multiple-Output (MIMO) based Medium Access Control (MAC) protocols have received a good deal of attention as researchers look to enhance overall performance of Ad Hoc networks by leveraging multi antenna enabled nodes [1]–[5]. To date such MAC protocols have been evaluated through comparative simulation based studies that report on the number of concurrent links the protocol can support. However, a bound on the maximum number of concurrent links (MNCL) that a MIMO based MAC protocol should strive to achieve has hitherto been unavailable. In this paper we present a theoretical formulation for calculating the bound on the MNCL in a Mobile Ad Hoc Network (MANET) where the nodes have multiple antenna capability, while guaranteeing a minimum Quality of Service (QoS). In an attempt to make our findings as practical and realistic as possible, the study incorporates models for the following PHY layer and channel dependent elements: (a) path loss and fast fading effects, in order to accurately model adjacent link interference; (b) a Minimum Mean Squared Error (MMSE) based detector in the receiver which provides a balance between completely nulling of neighboring interference and hardware complexity. In calculating the bound on the MNCL our work also delivers the optimal power control solution for the network as well as the optimal link selection. The results are readily applicable to MIMO systems using Receive Diversity, Space Time Block Coding (STBC), and Transmit Beamforming and show that with a 4 element antenna system, as much as 3× improvement in the total number of concurrent links can be achieved relative to a SISO based network. The results also show diminishing improvement as the number of antennas is increased beyond 4, and the maximum allowable transmit power is increased beyond 10 dBm (for the simulated parameters). June 10, 2018 DRAFT 2 Index Terms MIMO, MANET, MAC, MMSE, Concurrent Links, Receive Diversity, STBC, Beamforming. I. INTRODUCTION Wireless Ad Hoc networking has emerged as an important aspect of next generation communi- cation systems. For conventional Single-Input Single-Output (SISO) system, interference among nodes drastically limits the number of concurrent (simultaneous) links in Ad Hoc networks. Multi antenna, multi-input-multi-output (MIMO), based wireless communications has the ability to spatially null interference and in so doing increase the number of concurrent links within a Mobile Ad Hoc Networks (MANET), thus increasing overall network throughput. In fact some work found in the literature [1]–[6] look to MIMO capable MAC protocols as a means of increasing the network efficiency and its sum-throughput. The maximum number of concurrent links is a metric used in the literature [1], [7], [8] to evaluate the capacity of a network. Examples of MACs that support concurrent links in a network where all nodes have multiple antennas at their disposal can be found in [1]–[5], [7]. For convenience these MAC protocols will be referred to as concurrent-based MACs in this paper. The Null-Hoc [2] and SPACE-MAC [3] protocols look to enable concurrent links by using the Gram-Schmidt Orthonormalization, so as to create orthogonal channels among different links. The protocol in [4] uses adaptive interference cancellation both at the transmitter and at the receiver, as well as a simple power control protocol for each link. Multiple links are assumed to access the channel sequentially and work simultaneously. The MIMA-MAC protocol [5] uses space division multiple access techniques to guarantee the concurrency of different communicating links in the network. Although these concurrent-based MAC protocols have proved to outperform the conventional SISO based MACs such as the IEEE 802.11 DCF [9], a natural question to ask is that how close they actually come to the theoretical bound (limit) of concurrency in Ad Hoc networks. Furthermore, since MIMO systems enable a variety of approaches in utilizing multiple antennas in the physical layer [10], it is also concerned that how this bound of concurrency is affected by the choice of MIMO algorithms and associated physical layer techniques. In this work we identify the theoretical Maximum Number of Concurrent Links (MNCL) in the network by considering June 10, 2018 DRAFT 3 the following PHY layer and channel dependent elements: (a) path loss and fast fading effects; (b) different MIMO transmit/receive algorithms; (c) a Minimum Mean Squared Error (MMSE) based detector in the receiver; (d) optimal power control and optimal link selection. The derived MNCL acts as a performance benchmark fo

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