Friday, January 20, 2012

What Is The Next for Mobile System Design? I: A Single-Cell Model Perspective on Downlinks

Interference Cancellation: A Short Overview
How to Broadcast Multimedia Contents?

[Note] Due to the asymmetry between the uplinks and downlinks of a mobile network, there are different considerations, tradeoffs and techniques for designing each directions. In general, with the recent advance on uplink interference cancellation and management techniques, mobile network is usually limited by downlinks due inter-cell interference, especially when delay is a key part of the equation. On this blog, my focus will be on downlinks. How to evolve mobile system uplinks will be discussed in separated blogs.

Mobile system design usually starts from our understanding of wireless channels and the services customers are demanding. The properties of various wireless channels can help us understand the system design limitation we are facing and the potentials we may achieve.  For example, COST 231 model, which was developed by European COST Action 231. Its variations are the most popular radio propagation models used in almost every wireless standardization body, including 3GPP, 3GPP2 and IEEE. Its modifications include COST 231-Hata Model and COST 231-Walfisch-Ikegami Model. One nice thing of COST 231 channel model is it helps us understand the tradeoff between reception and coverage we are facing in a typical single-cell environment.
Figure 1. Spectral Efficiency and Coverage Tradeoff

As shown in Figure 1, with a 300-meter-tall transmitter antenna, we can see that the path-loss changes 0.66 dB at every 90% coverage change, 1.39 dB at every 80% coverage change, 2.22 dB at every 70% coverage change and 3.18 dB at every 60% coverage change. In general, if you want more coverage, then you may lose some capacity especially on the cell-edge.  Otherwise, you have to shrink your coverage.

Figure 2.  What we want to achieve.
As shown in Figure 2, though there is a fundamental tradeoff between coverage and performance we are constantly facing in our system design, customers always desire their mobile network having both better coverage and higher performance for less. Now the challenge to us is how to push up the system design boundary. There are at least three major approaches available to push the envelope. They are 1) interference cancellation and management, 2) multi-antenna technology and 3) cells cooperation and relay.

Figure 3.  Mobile System Design Options
Interference cancellation (IC) and management are the key ingredients for mobile network to achieve optimal performance. There are many ways to do interference cancellation, linear ICs (decorrelating detector, MMSE IC) and Nolinear ICs (joint detection, decision feedback IC ). Interference management can be done in time, frequency and space domain. OFDMA-liked multiplexing scheme is friendly to interference management.  MIMO can help meet the demand of high data rate and high link quality. It can not only help improve link quality through spatial diversity and beamforming but also help achieve higher data  throughput using spatial multiplexing and multiuser MIMO. The third weapon is heterogeneous transmission and deployment, which can help improve the network throughput as well as cell-edge user experience. Cooperation between cells is not something very new. Starting from 2G/IS-95, there has been soft handoff for macro-diversity. Additionally in 3G, we did it for broadcast multicase service over mobile networks, e.g., CDMA2000 BCMCS or UMTS MBS. However, all these cell cooperations are coordinated by MSC.  More recently, LTE-Advanced standardized X2 interface between eNodeBs belonging to the same MME.  This makes neighboring cells cooperation, such as corrdinated multi-point transmission and reception (CoMP), inter-cell interference coordination (ICIC) and relay, a reality. Similarly in CDMA2000 EV-DO Rev. C, there is a feature called single-carrier multi-link (SCML), which essentially extends the capability of multi-carrier devices in a single-carrier environment.


John said...

Very Useful insight on interference cancellation. Am exploring the possibility of a PhD research in mobile communications and I find ur blog to be inspiring.


Shu Wang said...

John, I am glad to know you like this blog. BTW, I remember I posted one for interference cancellation too ... Interference Cancellation: A Short Overview... I guess you are very familiar with those basic stuff already. FYI.