Research On Key Technologies In CoMP Downlink System

Coordinated Multi-Point (CoMP) technology, which can enhancemulti-user diversity and reduce the interference from the inter-cell at the sametime, improve the throughput of system. Therefore, it has now been qualifiedas one of the key technologies of4G in Wireless communication system.However, after introducing CoMP, the traditional precoding scheme whichhas been used for MIMO system will not meet the requirements in Multi-Cellscenario. Naturally, the novel precoding scheme which is designed forMulti-cell becomes one focus of attention in recent years. At the same time,there will be myriad demands for exchange of user data and user channel stateinformation among BSs due to the fact of introducing joint processing inMulti-cell Inevitably, the backhaul overhead would increased. Consequently,how to reduce this backhaul overhead turns to be a serious problem thaturgently needs to be solved. In addition, the BSs need to know the downlinkCSI before performing the resource allocation, precoding and user selection.For FDD system, the only way to get CSI is by limited feedback. Sincefeedback channel is limit, how to make use of this channel efficiently andhow to feedback CSI accurately, is the most essential work for thestandardization of4G wireless communication system.This paper models the CoMP in LTE-A system, and makes a thoroughresearch for precoding scheme and limited feedback in CoMP downlinksystem.The interferences from inter-cell have been suppressed effectively inCoMP downlink system. At the same time, joint processing among BSs formsvirtual MIMO array, increases the diversity and further ameilirates the systemthroughput efficiently. For multi-cell system, using the same precoding scheme in different cell will cause rate loss since there are different channelstate informations in different cells. This paper proposes adaptive precodingscheme in each cell, and choosing this precoding scheme that can ensure thesystem to achieve a better performance. As a result, backhaul overhead willincrease due to joint processing requirement. Besides, CSI is composed bytwo parts named CQI and CDI, and they are mutually independent. CDIvaries slowly in slow fading environment. As precoding scheme is onlyrelated to CDI, we propose using1-bit indicator to replace the precoding itselfto exchange between BSs. Simulation results and analysis demonstrate thatthe proposed scheme not only improve the system sum-rate, but also reducethe backhaul overhead.Considering the model which only channel state information will beexchanged among BSs in CoMP system and the number of users is larger thanthe number of antennas in BS. To get multi-user diversity, users not onlyneed to feedback CDI, but also CQI. This paper only considers SINR as CQI,studies the performance of limited feedback in this system model, theexpressions for SINR distribution and the sum-rate are derived. Tradeoffbetween the number of feedback bits, the number of users, and SNR areobserved. Analysis shows that for a target performance, having more userswill reduce the feedback overhead.