Until about 1996, the only economical way to connect LANs was through a wired infrastructure. In the last 5 years, several new wireless broadband LAN infrastructures have been proposed and built. Wireless local loop is a new wireless broadband option and comes under fixed wireless broadband as opposed to mobile. Fixed here refers to a fixed location. It means that the data transmission is wireless and that the stations are fixed, unlike in mobile, where the stations could be moving (assuming that a station is a subscriber). Here, the stations communicate at a very high speed. Dense modulation schemes are also required, and a higher signal-to-noise ratio is required in the fixed wireless broadband scheme.

Advantages of Using Fixed Wireless Broadband Technology for LANs

Some of the various advantages of adopting a fixed wireless broadband paradigm are:

• The entry and setup costs are very small (setup cost is very low, and expansion can always be opted on demand).

• Systems can be set up with great ease and speed. All equipment can be carried and installed with great ease.

• Equipment can be set up only after a customer signs up. This is different from wired systems, because for wired LANs, a complete infrastructure has to be built even before the customers show up.

• The buildout becomes demand-based, which is a major advantage when compared with wired architectures.

• The cost of upgrading can be substantially less, since there is no other infrastructure other than the end equipment. Once the equipment is designed to be upgradable, upgrading becomes very easy.

• There is less overhead for changing the transmission equipment, and many problems of wired LANs, such as damage tracking in transmission equipment, do not exist at all.

• Once the basic infrastructure is handled, quality of service can be achieved.

• Bandwidth reuse is very high because of the cell structure used.

• Network management, maintenance, and operating costs can be very low. However, it is believed to be low at this time, but as the number of users grow, network management and operating costs will increase. Nevertheless, there is no real evidence that in the long run it will be less expensive to manage the networks (software and personnel costs). Maintenance, on the other hand, is definitely lower because there is less physical plant.²

Different Methods Available for Fixed Wireless Broadband Communications

In order to achieve fixed wireless broadband communications, various physical media and equipment can be used—ranging from infrared, to microwave, to radiowave. A major problem with using an infrared signal is that it can be obstructed by physical objects. It is also likely that it can be affected by extreme weather conditions such as heat. Thus there should be an unobstructed path between the communicating equipment, which is not always possible. Microwave systems operate at less than 500 mW of power. For fixed service, broadband wireless access systems are of particular interest. The reasons for this are that they are very quick to install and are economical and cost-effective. Moreover, interconnection of the base station to fixed public switched telephone network (PSTN) is also possible and easy. In using the wireless broadband signal, there are various issues that need to be discussed, one important one being the spectrum that can be used.

Spectrum Allocation and Partitioning

The primary issue that needs attention is the spectrum in broadband that is best suited for fixed wireless needs and the bandwidth required for achieving a high data transmission rate. The FCC made several new bands of wireless spectrum available. In order to create viable opportunities for wireless broadband competition to incumbent local exchange carriers (ILECs) (they built a wired high-speed infrastructure for data transmission), the FCC enhanced the capacity of the existing spectrum licenses. It started a host of omnidirectional wireless high-speed-access (HSA) networks. The new allocations promote bidirectional transport with no receive-site license required.

Now let's discuss the new omnidirectional transmission bands. There are many bands other than the ones just discussed but none with exclusive licensing structure and bandwidth.

Integration with Existing Technology

The FCC has started a host of different omnidirectional high-speed access broadband networks that can be integrated with existing wireless broadband technology. They are:

• The 38-GHz band

• The 28-GHz or LMDS band

• The DEMS band

• The MMDS band

The 38-GHz band is licensed primarily to WinStar. Winstar uses asynchronous transfer mode (ATM)-based equipment and provides plain old telephone service (POTS) and high-speed data. From a cost point of view (starting with point-to-point links and then as the network size increases), switching to an omnidirectional cell site is advisable. However, for a particular network, the shifting overhead is more, so it is better to start with omnidirectional networks.

The 28-GHz or LMDS Band

The 28-GHz band was regulated in 1998, with only a few major companies participating (see sidebar, "Propagation Impairment"). This is called the LMDS band because LMDS operates in this band in the United States.

Note: It could be different for different countries; for example, in Europe, it is the 40-GHz band.

The 28-GHz band has different blocks of bandwidth: the A block with 1150 MHz of bandwidth and the B block with 150 of MHz bandwidth. A high degree of cellularization is required with this band. Cell size is about 2 miles in radius. Various new proposals have been made about the band, and some of these will be discussed later in this chapter.

The DEMS Band

The DEMS band was allocated originally at 18 GHz. It has 100 MHz of bandwidth.

The MMDS Band

The FCC allocated about 200 MHz of spectrum at 2.1 and 2.5 to 2.7 GHz for television transmission. In 1995 and 1998, the FCC allowed for digital transmission with CDMA (code division multiple access), QPSK (phase shift keying), VSB (vestigial side band), and QAM (quadrature amplitude modulation) modulation schemes. This band is licensed primarily to such companies as SpeedChoice and Wavepath.