Systems and methods for adaptive bit loading in a multiple antenna orthogonal frequency division multiplexed communication system
In an orthogonal frequency division multiplexed (OFDM) system, a transmitter and/or receiver communicate separate data streams on non-orthogonal spatial channels. Each spatial channel may use the same set of OFDM subcarriers and may take advantage of the multipath characteristics of the spatial channel allowing the communication of additional data without an increase in frequency bandwidth. Space-frequency subcarrier modulation assignments may be dynamically assigned on a per subcarrier basis as well as a per spatial channel basis to help maximize the data-carrying capacity of the channel. In some embodiments, each of the spatial channels may be associated with one of a plurality of spatially diverse antennas. In other embodiments, beamforming may be performed to allow the transmission and/or reception of signals within the spatial channels.
1. A multiple-input-multiple output (MIMO) transmitter for transmitting a group of sequential orthogonal frequency division multiplexed (OFDM) symbols on a time-division duplexed (TDD) channel using a plurality of antennas comprising:
subcarrier modulators to individually modulate groups of OFDM subcarriers in accordance with spatial-frequency subcarrier modulation assignments to generate groups of symbol-modulated subcarriers;
circuitry to combine corresponding symbol-modulated subcarriers for transmission on each of a plurality of spatial channels, the spatial channels being non-orthogonal and having different multipath characteristics; and
a beamformer to perform beamforming on the combined symbol-modulated subcarriers for transmission over the spatial channels,
wherein the TDD channel comprises a plurality of the groups of the OFDM subcarriers,
wherein the OFDM subcarriers within each group of subcarriers and within each group of sequential OFDM symbols have the same spatial-frequency subcarrier modulation assignments, and
wherein the spatial-frequency subcarrier modulation assignments for each group are provided by a receiver and determined from spatial channel characteristics of the spatial channels prior to transmission.
2. The MIMO transmitter of claim 1 further comprising a spatial-frequency parser to parse a block of bits into spatial-frequency groups of a variable number of coded bits, each spatial frequency group being associated with a spatial component and a frequency component of the OFDM symbols, the spatial component being associated with one of the spatial channels, the frequency component being associated with one group of the OFDM subcarriers.
3. A multiple-input-multiple output (MIMO) receiver for receiving a group of sequential orthogonal frequency division multiplexed (OFDM) symbols, the receiver comprising:
a beamformer to perform beamforming on signals received through a single antenna that were transmitted by a MIMO transmitter over a plurality of spatial channels, the beamformer to help separate signals of the spatial channels, the spatial channels being non-orthogonal and having different multipath characteristics;
circuitry to determine spatial-frequency subcarrier modulation assignments for groups of OFDM subcarriers from channel characteristics of the spatial channels for transmission to the MIMO transmitter; and
subcarrier demodulators to demodulate received OFDM subcarriers that comprise the OFDM symbol based on the spatial-frequency subcarrier modulation assignments for the groups of OFDM subcarriers,
wherein the same spatial-frequency subcarrier modulation assignments are selected for each subcarrier within the groups of OFDM subcarriers, and
wherein the MIMO transmitter is configured to transmit a group of sequential OFDM symbols on a time-division duplexed (TDD) channel with each subcarrier having the same spatial-frequency subcarrier modulation assignment.
4. The MIMO receiver of claim 3 wherein the beamformer performs beamforming on signals that are received through each of a plurality of single antennas, the signals having been transmitted by the MIMO transmitter over the plurality of spatial channels.