Dedicated LSAS for Metro-Cell Wireless Backhaul - Part I: Downlink
02 January 2013
We propose a dedicated Large-Scale Antenna System (LSAS) to provide wireless backhaul service for metro-cells. The LSAS operates in a different band of frequencies than the metro-cells - the unlicensed 750 MHz band - and it uses special equipment that is designed specifically for the backhaul task. Hence there are no back-compatibility issues, and standards-committee conflicts, which are typically associated with efforts to introduce radical new wireless technologies, are circumvented. Our scenario begins with a network of hexagonal macro-cells, where each macro-cell is augmented with eighteen (18) randomly-located metro-cells. In turn, each macro-cell is provided with an LSAS array that is required to provide backhaul service to the eighteen metro-cells. The required net down-link backhaul rate is thirty (30) megabits/second per metro-cell, or equivalently 540 megabits/second total throughput per LSAS array. The LSAS arrays utilize a spectral bandwidth of 20 MHz for which the maximum permitted radiated power per array is a total of 40/3 ~ 13 Watts. Our objective is to determine the number of antennas in each LSAS array that are typically needed to deliver the stipulated backhaul throughput. The LSAS arrays operate independently of each other with no coordinated transmission of backhaul data, and they only cooperate with respect to power-control, which is globally optimal according to the max-min criterion. The result of our analysis is the cumulative distribution of the number of antennas per LSAS array that is needed to deliver the stipulated backhaul to the metro-cells. For conjugate beam-forming, 190 antennas are sufficient 90% of the time, while for zero-forcing 170 antennas are sufficient 90% of the time. If the 5% most-troublesome metro-cells are dropped from service, 78 antennas are sufficient for conjugate beam-forming and 56 antennas for zero-forcing. The total area occupied by each LSAS array ranges from 1.7 square-meters for 56 antennas arranged in a 8 by 7 grid to 6.6 square-meters for 190 antennas arranged in an 16 by 12 grid where the antennas are spaced ½ wavelength center-to-center.
