Mimo and Comp in Lte-advanced

CoMP is considered for LTE-Advanced as a way to enhance the coverage area of high data rate communications, the cell-edge throughput for the MTs, and as a way to increase the system throughput in varying load conditions. CoMP is used to coordinate the transmissions with several cells and to reduce the interference from neighbouring cells and thus reducing the power required to hold a certain QoS.

In the article, the authors analysed the energy saving performance by turning off certain BSs with average outage constraint in three typical cooperation schemes: single BS transmission, BS cooperation and wireless relaying. They also investigated the effect of the system parameters (traffic intensity and network density) on the energy efficiency performance.

In Ismail and Zhuang both network cooperation for large and small-scale traffic fluctuation was modelled and analysed. For large-scale fluctuation, the networks with overlapped coverage could alternately switch their BSs on or off according to the long-term fluctuations in the traffic load. On a small scale, each active BS can switch its wireless channels on and off according to the short-term traffic load fluctuations.

WMNs are also developing rapidly, and they are expected to resolve the limitations of ad-hoc networks, WLANs, WPANs and WMANs, as well as to crucially enhance their performance. WMNs are able to deliver a wide variety of wireless applications in everyday life in public and in private use. Even with all of the recent developments, much work still remains to be done in the various WMN protocol layers. Due to the possibility to deploy WMNs over the existing wireless technologies, some companies have already launched their rather pioneering WMN products for sale.

Practical experience however tells that WMNs can, and ought to be improved in several areas:

• • Scalability

The overall network performance indicators (throughput, end-to-end delay and fairness) are not scalable with the amount of nodes or the network hops. The issue can be eased somewhat by increasing the capacity in the network nodes, for instance by utilising multiple radios or channels in a node, or by utilising radios with higher communications speeds. These enhancements will not however completely solve the problems – the relative performance over the increased network capacity is not affected. New MAC, routing and transmission protocols however will.

• • Self-organisation and self-configuration

These imply that all of the WMN protocols were fully distributive and collaborative this is not however currently true.

• • Security

Current security procedures leave WMNs partially unprotected against security attacks in different protocol layers.

• • Network integration

WMNs are currently fairly limited in their capability in integrating heterogeneous wireless networks, due to the problems in incorporating multiple wireless interfaces and their corresponding gateway/bridge functions in the same WMN router. Software radios may be the answer to this issue.

Performance

WMNs still lag behind wired networks in terms of throughput and delays. The most apparent reason for low performance in WMNs (e.g. poor throughput, high network latency), is mainly due to the insufficiently planned wireless networks. Some research work has been done to diagnose the performance bottlenecks and problems in WMNs, such as:

• Multi-path interference
• Link slow down because of congestion or voluntary or involuntary packet dropping
• Large co-channel interference
• External RF noise
• Misconfigurations
• Hardware or software bugs in clients or in the APs.