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Saturday, July 2, 2011

The Concept of Network MIMO

Cellular communication systems provide wireless coverage to mobile users across potentially large geographical areas, where base stations (BSs) provide service to users as interfaces to the public telephone network. Cellular communication is based on the principle of dividing a large geographical area into cells which are serviced by separate BSs. Rather than covering a large area by using a single, high-powered BS, cellular systems employ many lower-powered BSs each of which covers a small area. This allows for the reuse of the frequency bands in cells which are not too close to each other, increasing mobile user capacity with a limited spectrum allocation. Traditional narrowband cellular systems require the cochannel interference level to be low.
Careful design of frequency reuse among cells is then crucial to maintain cochannel interference at the required low level. The price of low interference, however, is a low frequency reuse factor: only a small portion of the system frequency band can be used in each cell. More recent wideband approaches allow full frequency reuse in each cell, but the cost of that is increased intercell interference. In both approaches, the capacity of a cell in a cellular network, with six surrounding cells, is much less than that of a single cell operating in an intercell interference-free environment. In a conventional cellular system, each mobile user is serviced by a single BS, except for the soft-handoff case – a temporary mode of operation where the mobile is moving between cells and is serviced by two base stations. A contrasting idea is to require each mobile station to be serviced by all BSs that are within its reception range. In this approach all the BSs in the cellular network are components of a single transceiver with distributed antennas, an approach known as “network multiple-input multiple-output (MIMO).”

Network MIMO requires cooperation between BSs. On the uplink, the BSs must cooperate to jointly decode the users, whilst on the downlink, the BSs must cooperate to jointly broadcast signals to all the users in the network.

The question then arises: how can such cooperation be realized in practice? It is natural to conceive first a centralized system in which a central processor is connected to all the base stations, so that the network is operated as a single cell MIMO system, but with distributed antennas. Such an architecture is, however, expensive to build, has a single point of failure, and does not satisfactorily address issues of complexity and delay. A more feasible and desirable solution is to distribute the processing among the base stations.
For distributed processing, communication among the BSs is mandatory. The desired properties of a feasible distributed method are: (1) communication should only be required between neighboring BSs, as opposed to message passing among all BSs, and (2) the processing per BS and message passing delay should remain constant with increasing network size.

The network multiple-input multiple-output (MIMO) technique has become a hot topic which aims to mitigate the intercell interference by coordinating the multicell transmission for downlink or reception for uplink among a few geographically separated antennas (base stations, BSs). To effectively reduce the intercell interference, the network MIMO requires a reliable and high-speed backhaul connection between BSs to pass the channel state information (CSI) and mobile messages between those cooperating cells. With full BS cooperation, the downlink transmission can be modelled as a multiuser MIMO broadcast system. A fundamental question for network MIMO is: how many cells should be coordinated to provide adequate SINR performance.

Most of the studies on network MIMO assumed a global coordination which can eliminate the intercell interference completely. However, it is impractical to have cooperation (or coordination) among too many cells. The huge computational complexity and synchronization needed with a large number of cells are quite challenging. In practice, only a limited number of BSs can coordinate and jointly process the received or transmit signals.The video beslow provides an overview on MIMO.

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