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Thursday, June 23, 2011

IEEE 802.16m/LTE: How MULTI-BS MIMO improves cell throughputs

Multi-BS MIMO techniques are used for improving sector throughput and cell-edge throughput through multi-cell collaborative precoding, network coordinated beam-forming, or inter-cell interference nulling. Both open-loop and closed-loop multi-BS MIMO techniques are supported in the IEEE 802.16m standard. For closed-loop multi-BS MIMO, CSI feedback via codebook-based feedback or sounding channel is used. The feedback information may be shared by neighboring base stations via a core network interface. Mode adaptation between single-BS MIMO and multi-BS MIMO can also be utilized.

To understand the concept of multi-BS MIMO consider a mobile station is at the cell edge, it may be able to receive signals from multiple cell sites, and the mobile station’s transmission may also be received at multiple cell sites as shown below.

If the data transmission and signaling from multiple cell sites can be coordinated, the downlink performance can be significantly improved. This coordination can be similar to the interference avoidance techniques or the case where the same data is transmitted from multiple cell sites. In the uplink, since the signal can be received by multiple cell sites, the system can take advantage of coordinated multiple reception to significantly improve the link performance.

The Coordinated MultiPoint (CoMP) transmission used in both the 3GPP LTEAdvanced and the IEEE 802.16m is a method of MIMO transmission for interference reduction which enables features such as network synchronization, cell- and user-specific pilots, feedback of multi-cell channel state information, and synchronous data exchange between the base stations that can be used for interference mitigation and to achieve macro diversity gain. The collaborative MIMO and the closed-loop macro diversity schemes are optional features supported in the IEEE 802.16m standard. For downlink collaborative MIMO, multiple base stations perform joint MIMO transmissions to multiple mobile stations located in different cells. Each BS performs multi-user precoding when transmitting to multiple mobile stations and each MS benefits from collaborative MIMO by receiving multiple streams from multiple base stations. When collaborative MIMO is enabled, several mobile stations are jointly served by multiple coordinated base stations through MU-MIMO scheduling and precoding, whereas when closed-loop macro-diversity is enabled, a single MS is served jointly by multiple coordinated base stations.

In terms of downlink CoMP, two different approaches are under consideration in 3GPP LTEAdvanced: coordinated scheduling and/or beamforming; and joint processing/transmission. In the first category, the downlink transmission to a single UE is sent from the serving cell, exactly as in the case of non-CoMP transmissions. However, the scheduling, including any beamforming functionality, is dynamically coordinated between the cells in order to control/reduce the interference between different transmissions. In principle, the best serving set of users will be selected so that the transmitter beams are constructed to reduce the interference to other neighboring users, while increasing the served users’ signal strength. For joint processing/transmission, the transmission to a single UE is simultaneously transmitted from multiple transmission points, in practice cell sites. The multipoint transmissions will be coordinated as a single transmitter with antennas that are geographically separated. This scheme has the potential for higher performance, compared to coordination in scheduling only, but at the expense of more stringent requirement on backhaul communications.

Network and collaborative MIMO have been studied for the 4th generation of cellular systems. Their application depends on the geographical separation of the antennas, coordinated multipoint processing method, and the coordinated zone definition. Depending on whether the same data to a UE is shared at different cell sites, collaborative MIMO includes single-cell antenna processing with multi-cell coordination, or multi-cell antenna processing. The former technique can be implemented via precoding with interference nulling by exploiting the additional degrees of spatial freedom at a cell site. The latter technique includes collaborative precoding and closed-loop macro diversity. In collaborative precoding, each cell site performs multi-user precoding towards multiple UEs, and each UE receives multiple streams from multiple cell sites. In closed-loop macro diversity, each cell site performs precoding independently, and multiple cell sites jointly serve the same UE.

As shown in the illustration above, uplink coordinated multipoint reception implies reception of the transmitted signal at multiple geographically separated points. Scheduling decisions can be coordinated among cells to control interference. It should be noted that in different instances, the cooperating units can be separate base station’s remote radio units, relays, etc. Moreover, since uplink CoMP mainly impacts the scheduler and receiver, it is primarily an implementation issue.

As shown in the illustration, the single-BS precoding with multi-BS coordination is mainly an interference mitigation and coordination mechanism.

These interference mitigation techniques are applicable to downlink MIMO modes 2 and 4 with codebook-based feedback mode with additional inter-BS coordination mechanisms and interference measurement support. The inter-BS coordination mechanisms do not require data forwarding between different base stations. Two types of single-BS precoding techniques with Multi-BS coordination are supported in IEEE 802.16m: (1) PMI coordination, supported by codebook-based feedback; and (2) interference nulling, supported by codebookbased feedback or uplink sounding. Single-BS precoding with multi-BS coordination may be enabled by the BS for one or several mobile stations when closed-loop MIMO precoding with downlink MIMO modes 2 or 4 are used in the serving and neighboring cells. The inter-cell interference can be mitigated by coordinating the precoders applied in the neighboring cells using higher layer signaling and based on feedback from mobile stations to their respective serving base stations.

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