Coverage & Capacity

Additional receive sensitivity improves the system power budget, thereby increasing the allowable uplink path loss between the mobile handset and the base station. When designing a network, this additional available path loss can nominally be allocated to one or more link budget items, such as in-building coverage margin, interference degradation margin or area coverage margin. Whilst the specific allocation of this extra link budget may have some bearing on the choice of locations of the base stations, once the base stations are installed, the signals will propagate as the physics dictates, not according to any design philosophy.

The extra path loss margin will therefore simultaneously support a greater potential coverage area from each site, an improved quality of coverage within the nominal coverage footprint and better in-building performance. For improved symmetrical performance, the base station transmit power must be sufficient to ensure that the system remains balanced or downlink limited.

Worked Example 1: Large Cell Coverage Improvement

For a large cell with an effective base station height of 30m the Path Loss Exponent for the COST 231 Hata-Okumura model is 35.2. That is, the signal strength decays at a rate of −3.52 d. A 3dB improvement in the allowable path loss corresponds to an increase in achievable range of 21.7%. The additional coverage possible from each base station is then 48%, which means that to achieve the same coverage just 67.5% of the number of base sites is required. A 6dB improvement would mean just 45.6% of the number of base sites would be required to achieve the same coverage.

Worked Example 2: Small Cell Coverage Improvement

For a small cell with an effective base station height of 17m (assuming the median building height is 15m, average building separation is 40m and average road widths are 20m) the Path Loss Exponent for the COST 231 Walfisah-Ikegami model is 38.0 [ETSI]. That is, the signal strength decays at a rate of −3.8 d. A 3dB improvement in the allowable path loss corresponds to an increase in achievable range of 19.9%. The additional coverage possible from each base station is then 43.8%, which means that to achieve the same coverage approximately 30% less base sites are required.

Worked Example 3: Road/Rail Coverage Improvement

For a large cell with an effective base station height of 30m the Path Loss Exponent for the COST 231 Hata-Okumura model is 35.2. A 3dB improvement in the allowable path loss corresponds to an increase in achievable range of 21.7%. The additional road/rail coverage possible from each base station is then 21.7%, which means that to achieve the same coverage just 82.2% of the number of base sites is required. A 6dB improvement would mean just 67.5% of the number of base sites would be required to achieve the same coverage.

In a mature network, there is generally contiguous voice coverage across large regions, so retrofitting an HTS TMA to an existing network will help to fill in holes in the coverage and extend the perimeter of the coverage.

Capacity

In CDMA uplink system capacity is effected by receive sensitivity. There is a theoretical maximum capacity for the base station sector, called the pole capacity, which can never be attained in practice. The percentage of pole capacity that can be realized is a function of many variables, including the maximum allowable path loss [Holma and Toskala]. There is a trade off between the allowable path loss and the uplink data capacity improvement. Hence, with an improvement in receive sensitivity, for the same range, a cell will support more capacity, or for the same capacity a cell will provide greater range.