Wireless Characteristics

Wireless Access Networks utilize radio frequency (RF) spectrum, which is generally defined as the frequency range of 300 kHz through 300 GHz. This frequency range sits between the electrical equivalent of audible sound and visible light. Audible sound is transmitted as changes in air pressure. Table 6-1 shows the broad classes of frequency definition.

Table 6-1. Broad Classes of Frequency Definitions

	From	To
Audible Sound	3 kHz	20 kHz
Radio (RF)	300 kHz	300 GHz
Infrared	300 GHz	1014
Visible Light	~ 8 × 1014	~ 8 × 1014
Ultraviolet	1015	1017
X-Rays	1015	1022

The lowest frequencies of the RF spectrum do not propagate readily; the highest frequencies behave more like visible light. Higher frequencies, such as infrared or visible light, can be focused, using reasonably sized antennas. Therefore, a benefit of high frequencies is tighter focusing, which enables sectorized RF transmission, which in turn increases frequency reuse. Lower frequencies also can penetrate household walls, whereas higher RF frequencies are blocked by physical objects, including raindrops, at very high frequencies. Radio frequencies below the AM radio band (500–1600 kHz) cannot be used practically for wireless data. (Note the huge towers required for AM radio transmission). Reasonable frequencies for Access Networks begin at the analog cellular band of approximately 800 MHz and extend upward to 30+ GHz. Otherwise, wireless transmission shares many characteristics of wired transmission, namely attenuation with distance, which ultimately limits range.

The next sections review some basic characteristics useful for comparing wireless Access Networks.

Location

Location refers to where in the RF spectrum the particular service is located. That is, location determines whether the service uses relatively high or low frequency in the RF range.

Amount of Bandwidth

Some Access Networks use as little 10 MHz (for example, block D PCS); others use as much as 1.1 GHz, as for LMDS. Typically, the higher the frequency, the more bandwidth is available. The amount of bandwidth determines the data rate that can be carried.

Modulation

As with wired media, wireless transmitters can permute frequency, amplitude, and phase. Wireless has the added dimension of polarity modulation, which means that waves can be horizontally or vertically polarized to provide an extra degree of freedom. Sectorization adds another degree of freedom: space diversity. These extra two degrees of freedom permit greater frequency reuse.

Footprint

This characteristic refers to the effective radius of service. Some services, such as DBS, have a nationwide footprint or, in the case of Europe, a continental footprint. Broadcast DTV has an effective radius of up to 50 km in relatively flat places. Other services, such as LMDS or PCS, have a relatively small radius, perhaps less than 5 km or so, due to the limited transmitted power available at higher frequencies and the greater attenuation encountered.

The advantage of having a large footprint is that a service can cover many subscribers—even millions—with a single transmitter. For example, a single satellite can beam programming or data to all of the United States or Europe. This reduces cost and has the side benefit of enabling simultaneous reception by every subscriber.

The drawback is that interactive, two-way communications is inhibited as the footprint increases. A larger footprint means that potentially more subscribers would be communicating with the network provider simultaneously. Return-path arbitration becomes increasingly problematic as the footprint increases.

Point-to-Point Versus Point-to-Multipoint

Some wireless communications, such as microwave relay, are point-to-point communications: One transmitter is communicating with exactly one receiver. The alternative is point-to- multipoint (PMP) communication: One transmitter communicates with multiple receivers simultaneously, as in satellite and broadcast television.

The advantages of point-to-point communication are privacy, guaranteed bandwidth, and the absence of a bandwidth arbitration mechanism. On the other hand, point-to-point technology also relies on a transmitter and receiver apparatus for each potential session, which increases cost.

For high-frequency communications, the development of point-to-multipoint technology is seen as an important cost-reduction technique because one transmitter can serve many users. For the return path, however, an arbitration mechanism is required. This makes PMP software more complex, but many believe this is well worth the reduced hardware. Point-to-multipoint communication conserves spectrum and reduces capital costs at the central site.

Providing tow-way PMP capability is the real challenge of wireless access.

Full-Duplex Operation (FDD Versus TDD)

In a point-to-multipoint system, a single transmitter sends bits downstream to multiple receivers. Debate continues to arise between supporters of frequency division duplexing (FDD) or time division duplexing (TDD) to achieve two-way operation.

The established model is frequency division duplexing, which divides the available spectrum into two separate channels: one for upstream and one for downstream. A newer approach called time division duplexing, allows traffic to flow in either direction on the same channel, but in different time slots. TDD is sometimes referred to as ping-pong operation.

FDD has been used for years in cellular telephone. When you talk on a cell phone, you are using frequencies 824 MHz through 849 MHz. When you listen, you are using frequencies 869 MHz through 894 MHz. There is some development of broadband FDD systems by companies such as Netro. Although TDD technology has been used in the Personal Handyphone System (PHS) in Japan and Digital European Cordless Telecommunications (DECT) systems in Europe, it has yet to be commercially deployed in a broadband wireless access system.

At the heart of the debate between FDD and TDD is whether wireless networks will be used for symmetric (voice) and asymmetric (data) applications. The criticism of FDD is inefficient bandwidth utilization, especially for data. With FDD, the upstream and downstream paths are permanently and statically allocated. This means that if traffic is flowing downstream (as in a big Web page download), then the upstream bandwidth is unused. This is not a problem for voice because the amount of bandwidth used (or rather, wasted) is small and because humans tend to talk as much as they listen, so the bandwidth demand is more or less symmetric. For high-speed data and video applications, however, this is not necessarily so. A lot of bandwidth could be flowing downstream, and if half the spectrum is permanently and statically allocated for upstream use, then that upstream bandwidth is underutilized. Because spectrum is so precious, preallocation seems to be an unnecessary waste, at least to TDD supporters.

With TDD, a variable number of time slots flow continuously in both directions. Upstream and downstream users get allocations of time slots into which they can pack data. The number of time slots in both directions can be metered on a fine-grained or coarse-grain granularity. This means that TDD can support bursty transmission in either direction. If a big Web page download is taking place, for example, time slots can be taken from the upstream users and used for downstream traffic.

The issue with TDD systems is that it takes time to turn the line around, and time slot allocation is a tricky software problem. In addition, TDD requires precise timing. All clients in a point-to-multipoint footprint need a common time reference defining time slots. This timing or ranging tends to limit the geographical distance of the point-to-multipoint footprint.

Still, more efficient use of bandwidth for asymmetric and bursty applications has induced new entrants into the broadband TDD space. Broadband TDD systems are under development by WavTrace and Ensemble Communications. The growth of ecommerce and digital entertainment on the Internet may change the traffic patterns of the Internet to benefit either FDD or TDD, but it is too soon to tell how such patterns will evolve.

Licensing

Some spectrum is licensed, and some is not. Licensed spectrum is held by a license holder who is granted exclusive or near-exclusive use of spectrum within a specified geographical area for a specified period of time. Licenses are normally granted by the FCC Mass Media Bureau or the FCC Wireless Transport Bureau, either by auction or by some other waiver.

Unlicensed spectrum can be used by any party that wishes to transmit in the approved passband. Because many transmitters may wish to use some common spectrum, etiquette rules apply, such as limits on transmit power. Such limits reduce the effective radius of the service or otherwise affect the quality of the service.

The obvious advantage of using unlicensed spectrum is the low cost of entry for a service provider, who doesn't pay for bandwidth. The drawback is congestion, even with etiquette rules in place. Moreover, some transmitters may not obey the rules, due primarily to faulty equipment. This is where the government must use its enforcement powers.

This brings us to the broader question of spectrum management.