A communication channel in a wireless communication network can be shared by different wireless stations in the network. One example of channel sharing is that wireless stations, such as customer premises equipment (CPEs), transmit signals on the same frequency at different times or on different frequencies at the same time.
A wireless communication network that employs spatial division multiple access (SDMA) utilizes spatial diversity to increase the capacity of a network. In such a system, the CPEs sharing the same communication channel transmit signals on the same frequency at the same time.
In order to prevent the signals transmitted by the CPEs on the same frequency at the same time from interfering with one another, a base transceiver station (BTS) needs to isolate the signals in such a way that they will not reach unintended wireless stations. In other words, these CPEs must be able to reliably detect and retrieve the signals that are sent to them.
There are two common methods to provide isolation among the CPEs sharing the same communication channel in a wireless communication network that employs SDMA. They are polarization isolation and spatial isolation. Polarization isolation is a more technically challenging method, and yet, it only provides a limited degree of isolation among the CPEs. In an environment with severe multi-path, polarization isolation only provides a difference of 5 to 10 dB in gain between the signals and interference.
An antenna array system on a BTS in a wireless communication network provides a practical solution for spatial isolation. The BTS selects a set of CPEs to participate in SDMA such that the degree of isolation among them is greater than a predetermined threshold. Spatial isolation among CPEs is achieved by using beamforming and interference nulling for antenna arrays.
For example, in a system employing SDMA, the BTS determines the spatial signatures of CPEs A and B, which are identified as candidates for sharing a communication channel, and generates a different beamforming weighting vector for CPEs A and B by using their spatial signatures jointly.
When the BTS transmits a signal to CPE A, the beamforming weighting vector of CPE A is applied to the antenna array. The antenna beam pattern created with the beamforming weighting vector has a nulling angle positioned toward the direction of arrival (DOA) of the antennae beam pattern of CPE B, i.e., CPE A will not receive signals intended for CPE B. The same mechanism is also applied to CPE B. The method described above is called SDMA via nulling.
One issue related to an SDMA via nulling method is that the effectiveness of antenna nulling is very sensitive to the accuracy of the beamforming weighting vector generated from the spatial signatures. If the beamforming weighting vector is not accurate enough, employing an SDMA operation might not lead to an improvement in system capacity. It might even make the channel unusable for the CPEs sharing the same channel, which subsequently reduces the overall capacity of the wireless communication network.
For example, in order to support 16 QAM modulation in a wireless network employing SDMA, each CPE must have an SINR greater than 20 dB. Assume that CPEs A and B both have an SINR greater than 20 dB and both support 16 QAM modulation before sharing a communication channel. If the wireless communication network employing SDMA via nulling cannot provide an SINR greater than 20 dB for both CPEs A and B, employing SDMA will bring down the communication channel for both of them.
SDMA via nulling eliminates co-channel interference (CCI) by applying beamforming weighting vectors of the CPEs that are almost orthogonal to each other. The effectiveness of the elimination of the CCI by employing SDMA via nulling depends on the accuracy of the spatial signatures of a CPE.
However, in reality, the spatial signatures calculated from receiving signals are never ideal; therefore, it is not uncommon for a CCI leakage to occur in the wireless communication network employing SDMA via nulling. A CCI leakage produces a fixed noise level and puts a hard limit on the bit error rate (BER) of the wireless communication network. As such, what is desired is a system and method for providing a robust SDMA via nulling.