How to Expand Coverage Area
Hello and welcome to STI’s fourth installment of our LTE eNOTE series. In today’s segment I will discuss the effects of pathloss on data throughput and show how even small sensitivity improvements can benefit the data network.
LTE is designed to accommodate rich data centric applications with data rates much higher than current 3G air interfaces. In response to growing internet use on mobile devices, LTE was purposely architected to favor the downlink. OFDMA was chosen for the downlink to reduce multipath interference. SC-FDMA was selected on the uplink to optimize battery life because it requires lower peak-to-average power ratios. With this and other tradeoffs the end result is that LTE’s downlink pipe has twice the capacity of the uplink pipe.
Recent consumer mobile data trends indicate that social networking services like video sharing will continue to grow. In addition, applications like two-way video-conferencing, and other mobile business services will account for a much greater proportion of data consumption in the near future. These types of applications will require equal data throughput on both links. Since by design uplink capacity is half the downlink capacity, maximum throughput on the uplink pipe is crucial.
One factor which limits data throughput is pathloss. The following example shows how. Suppose I'm on a two-way video conference with a colleague. At this distance, the base station is providing the optimal modulation with maximum data throughput. As I move further away pathloss increases and in turn degrades the signal-to-noise ratio. At this point, the base station cannot support this modulation. So instead of dropping my connection, the BTS switches to a modulation which can use a lower signal-to-noise ratio. This new modulation inherently has a smaller capacity and therefore less throughput. As I continue to move further from the base station this process repeats. The signal-to-noise ratio degrades, the modulation switches and throughput is reduced. Eventually, the quality of my data transmission will degrade to a point where I lose my connection.
So what can we do to improve the uplink signal-to-noise ratio and throughput? Using the same approach illustrated in LTE eNOTE #2, let’s compare a conventional sharp filter with STI’s solution. By adding SuperLink, which is a low-loss sharp filter and a low-noise amplifier, carriers can improve signal-to-noise ratio by greater than 4 dB. Even in a scenario with the same net gain, STI’s solution can still achieve an improvement of over 1.5 dB. As indicated in this table for WCDMA, the improved coverage area could be 10 to 15% for every 1 dB of a receiver’s signal-to-noise improvement. Since the dominant factor in this analysis is pathloss, and assuming a similar base station coverage area, it is reasonable that LTE will show similar benefits. Therefore, for the example shown, STI’s solution could increase LTE coverage area for every modulation scheme by over 10%. This would help carriers get the most out their uplink pipe.
To summarize, LTE was designed asymmetrically to handle high downlink throughput load. As symmetric mobile applications grow there is a potential for the uplink to be stressed. STI’s approach could increase the coverage area by 10% or more, helping carriers deliver on the promise of LTE. We hope you find this approach to be insightful, and appreciate your time. Please feel free to contact us with any questions or comments.
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