Downlink multi-user multiple-input, multiple output (MU-MIMO), also known as transmit spatial division multiple access (Tx-SDMA), is a method that enables multiple mobiles to share the same time-frequency resource through downlink beamforming using an antenna array at the base. This is illustrated in FIG. 1. As shown in FIG. 1, MU-MIMO transmit beamforming increases the system-level capacity by transmitting to two mobiles on the same time-frequency resource by creating two transmit beams patterns 101 and 103. Transmit beam pattern 103 maximizes the power to mobile 1 and transmits little power toward mobile 2 whereas transmit beam pattern 101 maximizes the power to mobile 2 and transmits little power toward mobile 1. This type of beamforming is accomplished by employing multiple antennas 102 at the transmit site and weighting each antenna such that the combined transmissions result in a beamformed pattern that enables two data streams to be transmitted on the same time-frequency resource where one data stream is destined for mobile 1 and the other data stream is destined for mobile 2.
To enable downlink MU-MIMO, channel information for multiple users is needed at the base station. In time division duplexing (TDD) systems where the uplink and downlink use the same carrier frequency, the channel information is easily obtained by the mobile sounding the uplink and exploiting channel reciprocity. However, in frequency division duplexing (FDD) where the uplink and downlink are on different carrier frequencies, the channel information cannot be obtained with uplink sounding and thus the mobiles have to use some sort of feedback mechanism in order to provide channel information to the base. One method of obtaining this feedback is codebook feedback which is known in the art. In codebook feedback, the base station and the mobiles all use a same codebook which is a collection of B vectors or matrices. A mobile will measure the downlink channel from all base station antennas and then will choose the best codebook vector or matrix that matches the downlink channel. The mobile will feed back this best codebook choice to the base station which can then use this selection to determine the transmit weights on the downlink.
In addition to beamforming, many communication systems may utilize multiple Modulation and Coding rates (MCRs). Particularly, the MCR of a transmitted data stream for a particular receiver can be tailored to predominantly match a current received signal quality (at the receiver) for the particular frame being transmitted. The MCR may change on a frame-by-frame basis in order to track the channel quality variations that occur in mobile communication systems (this method of choosing the MCR is called adaptive modulation and coding). Thus, streams with high quality are typically assigned higher order modulations rates and/or higher channel coding rates with the modulation order and/or the code rate decreasing as quality decreases. For those receivers experiencing high quality, modulation schemes such as 16 QAM or 64 QAM are utilized, while for those experiencing low quality, modulation schemes such as BPSK or QPSK are utilized. Multiple coding rates may be available for each modulation scheme to provide finer MCR granularity, to enable a closer match between the quality and the transmitted signal characteristics (e.g., coding rates of ¼, ½, and ¾ for QPSK; and coding rates of ½ and ⅔ for 16 QAM, etc.).
Besides choosing the best codebook index, the mobile needs to also select the best MCR which is typically fed back in a channel quality indication (CQI) message. Because MU-MIMO has multiple users sharing the same time/frequency resource, it may be difficult for a mobile to know its channel quality prior to users being assigned their time/frequency resource. In other words, because a mobile will not know its possible interferers (i.e., those using the same time/frequency resource), it is difficult for a mobile to determine an MCR that will match the channel quality when all mobiles are assigned their resources. Therefore a need exists for a method and apparatus for choosing a modulation and coding rate in a MU-MIMO communication system that alleviates the above-mentioned difficulties.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.