MIMO Beamforming, Spatial Multiplexing and Diversity in Wireless InSite
Wireless InSite provides features to model the following common MIMO techniques:
At a receiver, antenna diversity methods provide a more robust signal with less fading by taking advantage of differences in the signal received by antennas spaced half a wavelength or more apart from each other. Wireless InSite supports three approaches for receiver diversity:
Selection Combining: choose signal from receive antenna with strongest SNR
Equal Gain Combining: sum magnitudes of voltage from each receive antenna
Maximum Ratio Combining: apply weights to each channel to align the phase for voltage from each antenna, while also adjusting magnitude (normalizing by SNR to increase weaker channels)
At a transmitter, diversity techniques use precoding of the transmit antenna elements to increase the chances that received signals will be uncorrelated, improving the success of diversity techniques at the receiver. Wireless InSite supports transmit diversity by allowing the user to define custom precoding tables in which any set of codes can be defined. This allows the user to specify any of a number of common precoding methods, such as the use of Alamouti codes or pseudo codes.
When both transmit and receiver diversity techniques are enabled, Wireless InSite will use these together to combine the signals, resulting in overall improved reception for a single stream of data between the transmitter and each receiver point.
Wireless InSite models spatial multiplexing using Singular Value Decomposition (SVD). Spatial multiplexing takes advantage of the differences in the channels between transmitting and receiving antenna pairs to provide multiple independent streams between the transmitting and receiving antennas, increasing throughput by sending data over parallel streams. SVD decomposes the channel into multiple streams, where the number is the smaller of the number of transmitting or receiving antennas. For scenarios such as 2x2 or 4x2 MIMO, for example, SVD can generate two independent streams, while for 4x4 (four transmit antennas and four receive antennas), it can generate four streams, potentially quadrupling the throughput.
In practice, the actual throughput will depend on how high of an SINR can be achieved in each of the streams. Using the SVD technique, Wireless InSite splits the transmit power over the streams and computes the SINR for each, allowing it to predict the throughput for each parallel stream, which it then sums to determine the total throughput from all streams.
The objective of beamforming is to use multiple antennas to form beams, increasing the SINR, and thereby the throughput, to a receiver. Wireless InSite currently supports two methods for beamforming:
Maximum Ratio Transmission (MRT): maximizes the beam (adaptively) between Tx and Rx points
Precoding Tables: allows a user to define tabulated beams, supporting a number of approaches (codebooks, etc.) that allow for selection from predefined beams
MRT uses information about the channel between the transmitter and receiver antennas to form an optimum beam to the receiver. In practice, this technique would typically be used for a time-division-duplexing (TDD) system, in which the uplink and downlink share the same band, allowing the receiver to send a pilot signal that can be used by the base stations to adaptively form this optimum beam.
Wireless InSite’s precoding tables are more general purpose in nature. A user can define multiple sets of predefined beamforming weights, and Wireless InSite will evaluate the different weightings and choose the strongest beam to each receiver point. This simulates a MIMO base station that has predefined beams (e.g., codebooks), and uses one of a variety of methods to determine the best to use for a given channel.
Combining Beamforming with Receiver Diversity
The main objective of beamforming is to maximize the power at a receiver point, which is used to represent the potential location of user equipment (UE). When the UE has multiple receiver antennas, Wireless InSite allows it to combine transmit beamforming with receiver diversity techniques, so that SINR can be further improved to provide the strongest possible data stream for later calculation of performance metrics, such as throughput and bit error rate (see Communications Analysis).