A normal LMDS setup has a central facility with a fiber-linked PSTN and Internet connections that relay a signal via point-to-point microwave links, which in turn pass the signal along to hubs located on rooftops or as stand-alone towers for point-to-multipoint (PMP) transport to the end site. Basically, the four parts in the LMDS architecture are:

1. Network operations center (NOC)

2. Fiber-based infrastructure

3. Base station

4. Customer premises equipment and NOC designs¹⁴

The conversion from fibered infrastructure to a wireless broadband infrastructure happens at the base stations. Interface for fiber termination, modulation and demodulation functions, and microwave transmission and reception equipment are a part of the base station equipment. Local switching also can be present in the base station. If local switching is present, then customers communicating in the same base station can communicate with each other without entering the fiber infrastructure.

The customer premises equipment varies widely from vendor to vendor. All configurations include indoor digital equipment and modulation and outdoor mounted microwave equipment. The customer premises equipment may attach to a network using time-division multiple access (TDMA), frequency-division multiple access (FDMA), or code-division multiple access (CDMA). Different customer premises equipment requires different configurations. The customer premises will run the full range from DS0, POTS, 10baseT, unstructured DS1, structured DS1, Frame Relay, ATM25 serial, ATM over T1, DS-3, OC-3, to OC-1. And the customer premises locations can range anywhere from malls to residential locations.

Architectural Options

There is one commonly discussed architecture with rf planning. Typically, the rf planning for these networks uses multiple-sector microwave systems. In this transmit and receive sector, antennas provide service over a 90-, 45-, 30-, 22.5-, or 15-degree beam width. The idealized circular coverage area around the cell is divided into 4, 8, 14, 16, or 24 sectors. Alternative architectures include connecting the base station indoor unit to the multiple remote microwave transmission and reception systems with an analog fiber interconnection between the indoor data unit and the outdoor data unit.

Manufacturers such as Ensemble Communications have come up with different approaches. One idea from Angel Technologies is to have an aircraft transmitting signals from overhead. The company called it HALO (high-altitude long-operating). This idea has various problems ranging from air traffic control to cost for medium-sized cities.

When developing an architecture, a standard issue that is considered is point-to-multipoint (PMP) communication. The question that arises is whether PMP is actually required. PMP allows multiple microwave paths— allowing spectrum and capacity to be shared as needed. Thus, when high bandwidth is required, the PTP (point-to-point) connection may be the best; otherwise, however, if a bandwidth on demand is the case, then PMP is well suited. A new model that is ramping up quickly is called invisible fiber unit (IFU) (see Figure 7-9).15 Two IFUs are set up in a line-of-sight link and placed back to back with other links. Thus, in an IFU transmit and receive, a link should be created between source and destination.

The customer premises equipment has one outdoor unit with a transmitter and receiver antenna and an indoor unit, which, in turn, communicates with subscriber equipment such as telephones and PCs. The indoor unit accepts the signal from the outdoor unit, demodulates and demultiplexes it, and then interfaces with the connected subscriber equipment. The downstream intermediate frequency in LMDS is the satellite intermediate frequency (950–2050 MHz). A major design issue for a receiver could be to achieve a large frequency-acquisition range in the carrier recovery loop.

Various Options in Access Methodologies

For any wireless broadband upstream link, there can be three access methodologies: TDMA, FDMA, and CDMA. In the downstream direction from base station to customer premises, most companies supply time division multiplexed (TDM) streams either to a particular user (PTP) or shared among various user sites (PMP). Figure 7-10 shows both the TDMA scheme and the FDMA scheme.¹⁶

Code division multiple access (CDMA) supports a significantly smaller number of users than TDMA. Two classes of CDMA are available; one is orthogonal CDMA (OCDMA), and the other is the nonorthogonal CDMA. Systems often use a combination of the two. OCDMA is said to have identical capacity with TDMA. OCDMA allocates using a mutually orthogonal spreading sequence. The other class of CDMA, which is pseudonoise CDMA (nonorthogonal), is where all users interfere with each other, and the capacity depends on how much interference one is prepared to tolerate. Both CDMA and TDMA once again have case-based advantages, and both can be shown to be good in particular situations. When "smart" antennas are used, TDMA has an advantage.

"Smart" antennas use an adaptive array to cover a sector instead of fixed-beam antennas. With the help of sensor locations, the beam can be moved dynamically in the direction of the user. By changing the coefficients in the adaptive array, the beam can be moved horizontally or vertically. These "smart" antennas implement what is called space division multiple access (SDMA). Since the users in the TDMA are sequentially using the channel, it is well suited for the SDMA and "smart" antennas—whereas in CDMA the simultaneous access makes this complicated.

In discussing the data-rate capacity in both the access methods, you should use the bits per second per hertz measurement unit. For the various modulation schemes, this rate varies. Two areas where comparisons can be made would be data-rate capacity and the maximum number of customer premises sites.