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The FDMA bandwidth spectrum efficiency is 1.5 bps/Hz for a 4-QAM modulation. For 16-QAM and 64-QAM modulation, the bandwidth spectrum efficiency is 3.5 and 5 bps/Hz, respectively. The TDMA band does not use 64-QAM modulation. For the other modulations, it has a reduced data rate.

Maximum Number Of Customer Premises Sites

In FDMA (assuming an x-MHz spectrum with a reuse frequency of r), the LMDS system provides an x/r-MHz usable spectrum per sector. If we assume the downlink spectrum to be d times the uplink spectrum, the downlink will have a d(x/r)/(d + 1) spectrum, and the uplink will have an (x/r)/(d + 1) spectrum. If the channel bandwidth is assumed to be b, then the maximum number of customer premise equipment would be:

(x/r)/(d + 1)b

The TDMA for a given (x/r)/(d + 1) spectrum assumes about 16 DS0 connections possible with 1 MHz. The total number of simultaneous users would then be 16(x/r)/[(d + 1)b]. If the values of concentration over the entire sector and cell are assumed to be in the ratio 1/s, then the total connections would be s16(x/r)/[(d + 1)b]—which would be very high when compared with what is possible with FDMA.

Network Planning

Network planning for LMDS includes a cell design, where the design of an LMDS cell is discussed. Then the issue of planning the frequency comes in. After planning the use of a frequency (a very major issue, which could make a very big difference when it comes to data transmission speeds), the issue of cell reuse and reuse optimization should be discussed. Each of the preceding issues are discussed briefly next.

Cell Design Issues

The attributes that require attention while designing an LMDS cell are:

• Cell size selection. Based on the desired reliability level, the cell size has to be decided.

• Cell overlap. This is an issue that has to be taken into consideration while designing the cells.

• Subscriber penetration. This is the number of subscribers having the required signal level to achieve quality of service.

• Number of cells. The number of cells in a sector depends on the cell size decided.

• Traffic capacity. Based on the traffic capacity of the area, the cell size and properties are fixed.

• Quality of service. Cell overlaps that exceed the allowed normal cell area (ca) affect the quality of service.

• Link budget. This is an estimation of the maximum distance that a user can be located from the cell while the cell is still achieving an acceptable service reliability.

• Capital cost per cell. This is used to estimate the network capital requirement.¹⁷

Frequency Planning

The channel spacing that is usable by the operators in Europe is 112, 56, 28, 14, 7, and 3.5 MHz. These are obtained by successive division of 112 by 2. The capacity in upstream and downstream locations usually differs because even if the bandwidth allocated is the same, the physical layer functions of both the channels are different. Thus, even if the bandwidth is equally distributed among the upstream and downstream channels, it is not possible to get the same capacity. Therefore, physical layer issues such as channel coding and filtering have to be taken into consideration when planning channels, especially if equal capacity for downlinks and uplinks is desired.

Reuse Schemes

A very important issue that can substantially change the speed of transmission and use of bandwidth is frequency reuse. In a given geographic area, how effectively can the frequencies be reused? The first possibility is to use a hexagonal cellular pattern (same old mobile cells). As illustrated in Figure 7-11, this frequency-allocation scheme requires three times the bandwidth allocated to one cell.¹⁸

• Maximization of isolation between adjacent sectors through the use of polarization
• Maximization of the directivity of the cell antennas by sectoring the distribution system
• Minimization of cross-polarization and multipathing²⁰

Modulation schemes can tune the data rate to some extent. Low-density modulation allows greater distance at a given power but sacrifices data throughput rates. LMDS, however, uses QPSK; therefore, it realizes about 1.8 Gbps of raw capacity even though it has five times the MMDS bandwidth.

Note: MMDS can give 1 Gbps using 64-QAM for its downstream links.

Recently, broadband developers have been taking more risk in employing advanced coding methods to achieve efficient use of bandwidth. Thoughts of using coding techniques such as orthogonal frequency division multiplexing (OFDM) for LMDS have been put forth. Another new coding scheme, called frequency-domain reciprocal modulation (FDRM), has been proposed as an alternative to OFDM.

Finally, the use of turbo product codes for LMDS applications is also feasible. Radio developers could cut the number of base stations necessary for the LMDS Internet access system potentially by reducing the rain fade common to such broadband systems and by using their turbo product codes.

The FCC and its auctions were mentioned briefly in the preceding section. Now let's talk about them a bit.

Coming Up Next: The FCC and Auctions

2, 6, 13, 16, 17, 18, 19, 20 -   Vinod Tipparaju, "Local Multipoint Distribution Service (LMDS)," Department of Computer and Information Science, The Ohio State University, 2015 Neil Avenue, DL 297, Columbus, OH 43210-1277, 2000.