Laboratory experiments of TDD/SDMA OFDM wireless backhaul in a downlink for hierarchical broadband wireless access systems

In next generation broadband wireless access (BWA) systems, cell shrinkage and blind zones might occur at cell edges due to constraints on transmission power and the increase in propagation loss with broadband and higher frequency bands, respectively. Thus, hierarchical BWA systems consisting of micro and macro base transceiver stations (BSs) installed around the macro BS are considered as one of the candidate radio access network architectures for the next generation BWA systems. In the hierarchical BWA system, a wireless backhaul between micro and macro BSs needs high capacity transmission to serve all subscriber stations (SSs) in each micro BSs. In this paper, we developed a prototype of a time division duplex (TDD) / orthogonal frequency division multiplexing (OFDM) wireless broadband backhaul, with a space division multiple access (SDMA) technique applied, for the next generation hierarchical BWA systems and carried out laboratory experiments in an anechoic chamber room to evaluate the transmit beamforming performances. The laboratory experiments showed that the TDD/SDMA OFDM wireless backhaul was capable of realizing accurate transmit beamforming with a calibration function in each sub-carriers, and the throughput performance with a 4-element array antenna was achieved more three times more than that without SDMA.