DUBLIN, Ireland-Capacity constraints are an issue for almost every cellular operator, and some have turned to frequency-hopping techniques as an answer. But while these systems have increased capacity, their effect on data traffic has been less widely investigated.
Any system that claims to increase the amount of traffic that can be handled over a network many times over will engage the attention of cellular operators. Frequency hopping has proved particularly effective in towns and cities because it is optimized for slow-moving traffic.
Irish operator Esat Digifone began using Nortel Networks’ frequency-hopping technology in early 1998. “We were taking on subscribers faster than we could roll out the network, and because sites were hard to come by, we needed to maximize the capacity of our existing sites,” explained Esat Digifone’s Technical Director Paul Craig. “The Nortel product works by using almost all the frequencies in each cell.”
The carrier implemented frequency hopping across 100 sites over a six-week period. Craig estimated Esat Digifone’s network capacity increased five-fold after the product-one-by-one fractional reuse frequency hopping-was implemented.
“The technology was particularly suited to our network at the time, particularly since an increase in the number of transceivers in the network was leading to interference issues,” he said. “It works particularly well with slow-moving traffic, which makes it ideal for just about any city.”
Unfortunately, interference is bad news for mobile data customers, and the frequency-hopping process impacts the carrier regarding interference calculation in the same way as any other technique that employs fractional reuse, according to Craig.
However, Craig noted that frequency hopping technology is only one element that could create interference, and if a network is properly planned, the effects are minimized. As a result, operators that implement frequency hopping and also offer data services are forced to closely manage the parameters of their network.
“Frequency hopping is one of the elements that has an impact on data, but I wouldn’t describe it as the key element,” Craig said. “Interference is also to do with bandwidth or spectrum-the more bandwidth, the greater the distance between frequencies. This means that all networks have to become far stricter with their engineering criteria.”
There is no difference between voice and data services in terms of physical coverage-a short message service (SMS) call covers the same distance as a voice call. However, because data is much more sensitive to interference, there is a perception that it is less robust.
A voice call can be maintained even with fairly high levels of interference provided each person involved can understand what the other is saying, but that is not true for data traffic. Even low levels of interference will disrupt data calls, and obviously, if that data is affected in any way, the content will be rendered useless.
“The coverage of a cell will not change regardless of the service used,” said Craig. “What will change is the respective quality of service (QoS), which is an essential component for corporate communications. What would be considered a small amount of interference on a voice call has a greater impact on data and networks will have to plan their infrastructure carefully in order to offer guaranteed levels of service.”
Part of the problem is that most of the signal-processing technologies developed for GSM were created to improve call quality rather than facilitate data traffic.
Another frequency-hopping software system was designed by the Central Scientific and Industrial Research Organization (CSIRO) in Australia in 1995. Damian Thomas was part of the Frequency Assignment by Stochastic Evolution (FASE) project and claimed that the technology had no negative impact on data. Indeed, the organization claims significant traffic capacity increases on networks using FASE, including the Telstra Mobilenet analog network in Melbourne, Australia, where overall traffic capacity increased by 30 percent. The system initially was developed with Telstra.
“Mathematical optimization is used to steer a path through the maze to find good solutions, aiming to minimize signal interference caused by the same frequencies being re-used in cells too close together,” Thomas explained. “At the same time, every cell must have enough frequencies to meet demand, and correct gaps between frequencies in the same and neighboring cells must be ensured.”
