The present invention generally applies to wireless communication systems, and particularly applies to using receiver loop-back signals to improve communication system performance.
Wireless communication involves, in the most general sense, transmitting information from one location to another. A transmitter generates one or more transmit signals that is somehow varied, i.e., modulated, in accordance with desired transmit information. A receiver extracts the transmitted information from a received signal by detecting and decoding its signal modulations. In an ideal system, the originally transmitted signal modulations and, hence, the transmitted data, are perfectly preserved in the received signal.
Received signals seldom have perfect correspondence to transmitted signals because of downlink channel distortions. Propagation path characteristics of the radio medium include time-varying attenuation, phase shift, fading, and multipath reflections. Further variance between the transmit information and recovered received information arises from the non-ideal performance of transmitters and receivers. Signal transmission and reception involves signal filtering, amplification, and modulation, all of which may impart signal distortion.
Much of the business of robust wireless communication design involves detecting and correcting signal errors in an attempt to cope with the non-ideal realities of wireless signal transmission and reception. In a common approach, wireless receivers “learn” downlink channel characteristics and use the “learned” channel characteristics to compensate received signal distortions based on the estimated downlink channel. A common approach to downlink channel estimation involves the transmitter sending known information as part of the transmit signal. By looking at distortions in the known portion(s) of the received signal, the receiver develops estimates of the downlink channel, which it then uses to compensate the received signal. Because distortion changes rather rapidly in mobile environments, the typical mobile station receiver frequently updates its downlink channel estimates.
In a typical wireless communication network system, one or more network transmitters transmit signals to one or more mobile stations, which, in turn, transmit signals back to one or more network receivers. In such scenarios, the mobile stations commonly perform downlink channel estimation to compensate their respective received signals and, similarly, the network receiver(s) perform uplink channel estimation to compensate the signals received from the mobile stations. Thus, it is relatively common for a wireless receiver, whether at the mobile station or the network, to improve its reception performance by compensating its received signal using estimates of the channel through which the signal was received.
In a less common approach, a wireless transmitter pre-compensates its transmit signal for expected downlink channel distortions. That is, if the wireless transmitter has access to reasonably good estimates of the channel through which its transmitted signal will travel, it can alter the transmit signal in a manner that lessens the effects of transmit channel distortions on the signal received at the wireless receiver. Of course, one of the several challenges inherent in transmit signal pre-compensation is in obtaining downlink channel estimates. Obtaining such estimates can be challenging because the process generally involves obtaining loop-back information from the receiver(s) that actually receive the transmitted signal to which pre-compensation is applied.