achieve higher downlink data rates. However, CO-MIMO requires high-speed communication between the cooperating base stations. History Gregory Raleigh was first to advocate the use of MIMO in combination with OFDM. In a theoretical paper, he proved that with the proper type of MIMO system—multiple, co-located antennas transmitting and receiving multiple information streams using multidimensional coding and encoding—multipath propagation could be exploited to multiply the capacity of a wireless link. Up to that time, radio engineers tried to make real-world channels behave like ideal channels by mitigating the effects of multipath propagation. However, mitigation strategies have never been fully successful. In order to exploit multipath propagation, it was necessary to identify modulation and coding techniques that perform robustly over time-varying, dispersive, multipath channels. Raleigh published additional research on MIMO-OFDM under time-varying conditions, MIMO-OFDM channel estimation, MIMO-OFDM synchronization techniques, and the performance of the first experimental MIMO-OFDM system. Raleigh solidified the case for OFDM by analyzing the performance of MIMO with three leading modulation techniques in his PhD dissertation: quadrature amplitude modulation (QAM), direct sequence spread spectrum (DSSS), and discrete multi-tone (DMT). QAM is representative of narrowband schemes such as TDMA that use equalization to combat ISI. DSSS uses rake receivers to compensate for multipath and is used by CDMA systems. DMT uses interleaving and coding to eliminate ISI and is representative of OFDM systems. The analysis was performed by deriving the MIMO channel matrix models for the three modulation schemes, quantifying the computational complexity and assessing the channel estimation and synchronization challenges for each. The models showed that for a MIMO system using QAM with an equalizer or DSSS with a rake receiver, computational complexity grows quadratically as data rate is increased. In contrast, when MIMO is used with DMT, computational complexity grows log-linearly (i.e., n log n) as data rate is increased. Raleigh subsequently founded Clarity Wireless in 1996 and Airgo Networks in 2001 to commercialize the technology. Clarity developed specifications in the Broadband Wireless Internet Forum (BWIF) that led to the IEEE 802.16 (commercialized as WiMAX) and LTE standards, both of which support MIMO. Airgo designed and shipped the first MIMO-OFDM chipsets for what became the IEEE 802.11n standard. MIMO-OFDM is also used in the 802.11ac standard and is expected to play a major role in 802.11ax and fifth generation (5G) mobile phone systems. Several early papers on multi-user MIMO were authored by Ross Murch et al. at Hong