1. Field of the Invention
The present invention applies to the field of wireless communications systems using antenna arrays and, in particular, to estimating the power of a signal received on a spatial channel.
2. Description of the Prior Art
Adaptive antenna arrays and SDMA (Spatial Division Multiple Access) enable a wireless system to use strategies to reduce interference and enhance system capacity. These strategies include 1) increasing the signal to interference ratio on the uplink (user terminal to base station) by adjusting received signal samples based on the location of a remote terminal and the RF environment, 2) concentrating signal power to the intended user terminal (beam-forming), and 3) placing nulls to user terminals using similar or the same frequency resources, such as terminals using the same channel on the downlink (base station to user terminal), among others. With these strategies, adaptive arrays can greatly enhance the capacity of a wireless system.
Using various SDMA strategies a base station may be able to communicate with more than one user terminal on the same conventional communications channel. These user terminals sharing a conventional channel are here referred to as “co-spatial” user terminals. The number of user terminals with which a base station can successfully communicate using a single communications channel may vary. Using the SDMA strategies, the conventional channel may be divided into multiple spatial channels to be used by as many remote terminals.
It may be desirable to determine the power of the signals on each spatial channel. That is, it may be desirable to determine how much each signal on each spatial channel is contributing to the received signal. It may also be desirable to estimate the power of signals received from co-channel interferers, that is, terminals communicating with other base stations, or in other cells, reusing the same communications channel. These may be done by estimating the spatial signature of a remote terminal whose power level is to be determined, and calculating the norm, i.e. magnitude, of the spatial signature. The problem with this method is that the spatial signature estimate may be very noisy. For example, if the signal on a spatial channel from a remote terminal is somewhat strong relative to the signal from the remote terminal whose power level is being estimated, the spatial signature will be of poor quality. Given a poor spatial signature, taking the norm of this signature will result in a poor estimate of the power contribution.
One way to improve the power estimation is by having better spatial signature estimates. This may include using a joint maximum likelihood signature estimation. This strategy involves finding spatial signatures by minimizing a function of the received signal and two reference signals. The problem with this method is that a minimization needs to be performed, which is a complex and resource-consuming task. Furthermore, the two reference signals need to be available for calculation simultaneously, further consuming memory resources.