Frequency Reuse is a well known concept that has been applied to wireless systems over the past two decades e.g. in GSM systems. As the name suggests Frequency Reuse implies using the same frequencies over different geographical areas. If we have a 25MHz band then we can have 125 GSM channels and 125*8=1000 time multiplexed users in a given geographical area. Now if we want to increase the number of users we would have to reuse the same frequency band in a geographically separated area. The technique usually adopted is to use a fraction of the total frequency band in each cell such that no two neighbor cells use the same frequency. Typically the frequency band is divided into 3 or 7 cells.
The division of the frequency band in to smaller chunks reduces the system capacity e.g. one cell with 25 MHz bandwidth would have much higher capacity then 7 cells having 3.5 MHz each. To overcome this problem a frequency reuse of 1 has been proposed i.e. each cell has the full system bandwidth (nearly). The problem of co-channel interference at the cell boundaries is resolved by dedicating a small chunk of the available spectrum for the cell edges.
In Soft Frequency Reuse (SFR) the cell area is divided into two regions; a central region where all of the frequency band is available and a cell edge area where only a small fraction of the spectrum is available. The spectrum dedicated for the cell edge may also be used in the central region if it is not being used at the cell edge. The lack of spectrum at the cell edge may result in much reduced Shannon Capacity for that region. This is overcome by allocating high power carriers to the users in this region thus improving the SINR and the Shannon Capacity.
1. The Signal to Interference and Noise Ratio is given as:
SINR=Signal Power/(Intercell Interference+Intracell Interference+AWGN Noise)
2. Typically the term capacity was used to describe the number of voice channels (or users) that a system can support. But with modern digital communication systems it usually refers to the Shannon Capacity that can be achieved (in bits/sec/Hz).
 Yiwei Yu, Eryk Dutkiewicz, Xiaojing Huang, Markus Mueck and Gengfa Fang, “Performance Analysis of Soft Frequency Reuse for Inter-cell Interference Coordination in LTE Networks”, ISCIT 2010.
Since our last post on Verizon LTE coverage within California, Verizon has removed the LTE Coverage Map from its site. Now it only gives a list of cities that have 4G LTE service (just like T-Mobile). So we now move from the West Coast to the East Coast i.e. Virginia. The state that is home to Virginia Tech, one of the finest schools in the country and a breeding ground for Wireless Engineers. It is thus somewhat of a shock to see that Verizon Wireless has no 4G LTE footprint in the state of Virginia. The only place that it intends to deploy 4G in near future is Bristol Virginia. It claims that by the end of 2013 it would have 4G coverage throughout the US where 3G service is currently available.
As in California T-Mobile has a much wider coverage with many smaller cities getting 4G service. The list includes: Alexandria, Mclean, Newport News, Norfolk, Petersburg, Portsmouth, Reston, Richmond, Roanoke and Lychburg. So although Verizon might be winning the speed race it is definitely not winning the coverage race (at least in CA and VA). And with AT&T T-Mobile merger also a possibility early next year Verizon is set to face some stiff challenge.
Given below are the results of a 4G speed test conducted by PC Magazine in the Northeast.
4G LTE Speed Test
The above results show that in areas where 4G coverage is available Verizon allows for average download speeds that are twice that of T-Mobile. The upload speeds are somewhat similar. Overall Verizon is by far the best in terms of the Mobile Speed Index, with T-Mobile in second spot and AT&T at third.
We have previously looked at the birds eye view of 4G LTE coverage within the US. We know that Verizon 4G services are now available to more than 50% of the US population. However, geographically, the service is only available in very small islands of population. Now, we take a closer look at 4G LTE coverage within California.
LTE Coverage in CA
We see that the coverage is available in most of the population centers such as Sacramento, San Francisco, Oakland, San Jose, Fresno and Bakersfield. Further south the coverage is also available in areas around Los Angeles and San Diego. But what is not shown on this map is that there is no coverage in many smaller cities such as Stockton, Modesto, Santa Rosa and Visalia. Also there is no coverage on the highways connecting these cities e.g. there is no coverage on I-5 which runs along the length of the state.
Bottomline: You may get very good LTE coverage when you are at home but don’t expect the same when you are on the highway. You will most probably have to fall back to 3G.
Verizon Wireless 4G LTE is now available to 160 million people with coverage in 117 cities within the US. This has been achieved within eight months of the initial deployment. Verizon hopes to increase the coverage to 185 million people by the end of 2011. The company claims that with its current deployment strategy users can experience data rates of 5-12Mbps on the downlink and 2-5Mbps on the uplink. When users do not have access to the 4G LTE network the phones will automatically switch to 3G which is available around most of the US.
This push to 4G creates a big gap between the developed and underdeveloped parts of the world where many nations have still not migrated from 2G to 3G.