Multi-channel multi-carrier testbed

Multi-carrier modulation (MCM) is a technique that uses multiple orthogonal subcarriers to transform a wideband channel into multiple narrowband sub-channels whose magnitude responses become more flat as the number of subcarriers increase. High transmission rates are possible by breaking the data stream into parallel lower-rate streams transmitted over flat-fading sub-channels. This not only reduces implementation complexity, but also uses spectrum more efficiently thanks to the fine granularity of each subchannel and the use of overlapping spectra among the orthogonal subchannels.

MCM is prevalent in many recent and emerging broadband wireline and wireless communication systems such as digital subscriber line (DSL), digital video broadcasting (DVB), and wireless local area networks such as IEEE 802.11a/g/n. While essentially the same, the technique is referred to as discrete multi-tone modulation (DMT) or orthogonal frequency division multiplexing (OFDM) in wireline and wireless communication systems, respectively.

For numerous reasons, using more than one physical channel between the transmitter and receiver might be desirable, e.g. to increase data rate, increase reliability, or both. Most generally, this is implemented using multiple transmitters and multiple receivers, which is a configuration known as multi-input multi-output (MIMO). Other configurations are single-input multi-output (SIMO) and multi-input single-output (MISO). All of these modes have application in wireless communications where different paths of wave propagation through the wireless medium constitute different channels between the source and the destination in the presence of multiple antennas located sufficiently apart. In wireline communications, it is a matter of increasing the number of wires (and corresponding transmitters and receivers) connecting the source and the destination.

We have developed a hardware testbed which implements a bidirectional dualchannel wired MIMO DMT system. In each direction of communication, there exist two transmitters and two receivers. The two channels are two wire-pairs bundled together in a cable. As a result of close proximity of the wires, there is considerable crosstalk between the two channels. Some of the main impairments associated with dual-channel operation are echo, near-end and far-end crosstalk (NEXT and FEXT). With proper mitigation of these impairments using low-complexity algorithms that run in realtime, we have doubled the data rate of the single-channel case, in practice. Some algorithms/subsystems implemented in the tested are listed as follows:

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