Meaning
Spatial multi-user interference in multi-antenna systems can be cancelled at the transmitter before the signals are radiated. Employing zero forcing precoding generates signal weights that project each user’s data stream into the null space of the other users’ channel vectors. This spatial isolation technique enables concurrent downlink transmissions over the same frequency channel.
Interference Cancellation
Transmitting multiple independent data streams concurrently to different users causes severe inter-user interference. Precoding algorithms solve this by mathematically inverting the composite channel matrix. This inversion allows the transmitter to steer nulls toward non-intended receivers while delivering the desired signal power to the targeted receiver.
Power Allocation
Power penalties occur during channel inversion when some channel paths are deeply faded. The inversion of very small channel gains requires substantial transmitter power, which reduces the power available for other subcarriers. Transmitters must apply power scaling limits or regularized algorithms to prevent these power spikes from saturating the power amplifiers.
Channel Inversion
Reconstructing the user signals requires the transmitter to calculate the pseudo-inverse of the estimated channel matrix. While computationally simpler than non-linear precoding strategies, this method’s performance depends heavily on the accuracy of the feedback matrix. If the channel state estimates are outdated, the nulls will be misaligned, leading to substantial leakage of interfering energy between user streams and causing high packet loss rates across the system.
