I. Introduction
The successful deployment of wireless local area networks, such as IEEE 802.11a/b/g/n, makes wireless communications prevailing and indispensable in our daily life. The advent of bandwidth-hungry applications and products, such as video streaming, cellular backhaul, and cellular data offloading, drives the demand for high-throughput wireless communication systems to deliver digital contents in the big-data era [1]. As a result, multi-gigabit wireless communications, standardized in IEEE 802.11ac and 802.11ad, are under development. Some of the key features toward the multi-gigabit wireless include wide signal bandwidth and MIMO techniques. In 802.11ac, channel bandwidths of 40 MHz, 80 MHz, and 160 MHz are supported. With carrier aggregation, the 160 MHz channel bandwidth can be constituted by two contiguous or non-contiguous 80 MHz channels. Besides, with MIMO techniques, the system capacity can be boosted by the spatially-multiplexed streams. A total of eight streams can be transmitted and each user can share up to four data streams.