The use of multiple input multiple output (MIMO) technology has attracted increased attention for use in wireless communications systems because MIMO offers significant increases in data throughput and link range without requiring additional bandwidth or transmit power. The increased performance afforded by MIMO technology stems from higher spectral efficiency (greater number of bits transmitted per second per Hertz of bandwidth), as well as greater link reliability or diversity. Accordingly, MIMO forms an important part of modern wireless communications standards including 3GPP Long Term Evolution (see 3GPP TS 36.213, section Technical Specification Release 10, June 2011, 3rd generationPartnership Project), IEEE 802.11n (WiFi), 802.16 (WiMAX) and HSPA+.
One area of concern is the ability to provide robust rank adaptation. Rank adaption refers to the dynamic control of rank according to changing channel conditions. The channel conditions may be determined by such parameters as signal to interference and noise ratio (SINR) and fading correlation between antennae in a MIMO system. With the use of spatial multiplexing, a base station (or eNodeB, or eNB) may send multiple data streams or layers to UEs in a downlink transmission using the same frequency. The number of such layers or streams is defined as the rank. The UE may periodically measure a channel and send a recommendation of the rank to the eNB. The so called rank indicator (RI) may be sent periodically or aperiodically in different schemes. Because the RI reported to the eNB may change with time, the eNB may adjust the number of data streams used in a downlink transmission, based upon the changing RI received from the UE. However, several factors may render this process less than ideal. In some circumstances, the interference levels that may affect channel quality can change substantially between two successive RI reports, in which case, the eNB has no occasion to adjust the rank even though the last reported rank may not be appropriate due to the changed interference conditions. In other circumstances, when a so-called wideband rank is used, the rank indicator reported may be based upon an entire transmission band (wideband), which may be composed of a group of frequency sub-bands used for communications between the UE and eNB. In many cases, the interference conditions may vary substantially between different sub-bands within the wideband, thereby compromising the validity of a wideband RI reported by the eNB for individual sub-bands.
Another concern has been raised regarding the use of multiuser MIMO (MU-MIMO) where a UE may transmit precoding matrix indicator/channel quality indicator (PMI/CQI) reports with too high a rank to effectively support the most efficient MU-MIMO scheduling: In MU-MIMO an eNB may schedule multiple different UEs for transmission over the same transmission band. The reporting of an excessively high rank may arise as a consequence of the fact that the UE can only evaluate its own link performance, and in general is not aware of any co-scheduling candidates in the MU-MIMO scheme. As a consequence, even though the link performance to a UE may be maximized by a high rank single user MIMO (SU-MIMO transmission), the system performance may very well be higher if the UE and a second terminal (unbeknownst to the UE) are co-scheduled using lower rank transmissions. The reported information including PMI/CQI/RI may therefore be ill-matched to a MU-MIMO allocation. In particular, the terminal may often select a too high transmission rank to be beneficial for MU-MIMO scheduling.
In other circumstances, where the eNB prefers operating under SU-MIMO transmissions, the UE may report a rank of 1, although the eNB may prefer the UE to report an adapted rank, rather than always reporting a rank of 1.
It is with respect to these and other considerations that the present improvements have been needed.