The system capacity for LMDS systems can be measured in terms of data rate and maximum number of customer premises sites.

Data-Rate Capacity—FDMA Access

For data-rate calculations, LMDS system capacity is equal to the number of cell sites within the system multiplied by the capacity per cell site. The cell site capacity is equal to the number of sectors within the cell site times the sector capacity. To provide basic examples, assume the values in Table 3 for spectral efficiency. Spectral efficiency is measured in bits per second per Hertz (b/s/Hz) and is a basic figure of merit for different modulation schemes.

Modulation	Spectral Efficiency
4–QAM	1.5 b/s/Hz
16–QAM	3.5 b/s/Hz
64–QAM	5 b/s/Hz

Table 3. Spectral Efficiencies

Using these spectral efficiencies, and assuming 1,000 MHz of useable spectrum with a frequency reuse of 2, the LMDS system provides 500 MHz of useable spectrum per sector. Assuming symmetrical upstream and downstream links, there is 250 MHz in each direction per sector. The sector capacities are shown in the following examples:

Example 1
If each customer-premises site uses 5-MHz FDMA links at QAM–4 modulation, this provides 5 x 1.5 = 7.5 Mbps per customer site. There are (250/5) = 50 of these links, providing a total of 375 Mbps upstream. The downstream links also use 4–QAM modulation, providing 375 Mbps.

Example 2
If each customer-premises site uses 5-MHz FDMA links at 16–QAM modulation, this provides 5 x 3.5 = 17.5 Mbps. There are 50 of these links, providing a total of 875 Mbps. The downstream links also use 16–QAM modulation, providing 875 Mbps.

Example 3
If each customer-premises site uses 5-MHz FDMA links at 64–QAM modulation, this provides 250 x 5 = 1250 Mbps upstream. The downstream links also use 64–QAM modulation, providing 1250 Mbps.

It is possible to have more capacity per sector than can be used within the coverage areas. For this reason, LMDS systems will probably be range-limited rather than capacity-limited. One option to increase range is to move to lower-modulation constellations. The values provided are examples only and may not reflect the full scope of equipment capability.

Maximum Number of Customer-Premises Sites—FDMA Access

In the previous calculations it was assumed that the FDMA channel bandwidth was 5 MHz. Using this assumption to calculate the total number of users, there are 250 MHz/5 MHz = 50 customer sites per sector. The number of sectors dictates the number of total customer sites per cell site. The customer site may be a large office building with many offices, all connected to the base station through the same 5-MHz channel.

Data-Rate Capacity—TDMA Access

TDMA systems have a reduced data-rate capacity compared to FDMA systems in the range of 80%. Also, TDMA systems do not use 64–QAM modulation; as a result, the very dense data rates achievable in FDMA systems are not available. However, 64–QAM modulation is useful only on shorter links as a result of the increased signal levels required for its operation. Therefore, 64–QAM FDMA access is only useful when the dense data-rate customers are close to the base station site.

Maximum Number of Customer-Premises Sites—TDMA Access

TDMA systems are the best choice when many low-data rate users must be serviced. For example, it is assumed that a 250-MHz upstream bandwidth is available within the LMDS system (same as before) and that 5-MHz TDMA channels are used. Each 5-MHz TDMA channel can provide approximately 80 DS–0 connections simultaneously. The total number of simultaneous DS–0 users on the TDMA system per sector are 80 DS0s per channel x (250/5) = 4,000.

The total number of simultaneous DS–0 users over the cell depends on the number of sectors. If typical values of concentration over the entire sector and cell are assumed to be in the range of 5:1, this TDMA system allows for a total of 20,000 DS–0 connections per sector within the blocking level probabilities consistent with telecommunications system design. We have assumed a 5:1 concentration level to reflect some Internet usage (telephone modems) over these DS–0 connections. There may also be some fax lines.

As in the case of the earlier FDMA example for data rate, 20,000 DS–0 lines per sector is excessive with respect to the coverage area of these LMDS systems. If 10 sectors were used, this would imply that 200,000 DS–0 lines could be supported. The typical coverage distances for LMDS systems are in the range of 3 km to 5 km for 99.99% service in the various rain regions, so 200,000 lines is too many.

Based on these numbers, it is important to look at the combined effect of TDMA and FDMA access methods in order to address all data rate and customer site requirements.