Home Networking Basis: Transmission Environments and Wired/Wireless Protocols

The use of radio frequency is generally regulated by federal or international authorities. In the United States, the usage of radio frequency is regulated by the Wireless Telecommunications Bureau [1] under the Federal Communications Commission. Specifically, radio frequency domestic and international usages have been defined in detail by Part 2 (Frequency Allocations and Radio Treaty Matters: General Rules and Regulations) of the FCC rules and regulations, which is labeled as Code of Federal Regulation title 47. In Part 2 of title 47, the available radio frequency is divided into nine bands as shown in Table 5.1.

Detailed frequency allocation is available in a tabular format for frequencies from 9 kHz to 400 GHz. Table 5.1 has frequency usage allocations for the United States as well as international regions. To facilitate the international allocating of the radio spectrum, the International Telecommunication Union (ITU) has divided the world into three regions as shown in Figure 5.1. Region 1 includes the area limited on the east by line A and on the west by line B. Region 2 includes the area limited on the east by line B and the west by line C. Region 3 includes the area limited on the east by line C and on the west by line A. The United States reside in region 2.

Table 5.2 shows a portion of the Part 2 title 47 radio frequency usage allocation table for frequencies between 2402 and 2500 MHz. In these three leftmost columns, radio frequency utilization is authorized for applications according to individual frequency bands and geographic regions. In the next two columns near the center of the table, radio frequency bands are allocated for government and nongovernment U.S. utilization. Corresponding FCC rules and regulations part numbers as well as special-use frequency notification are listed in the last two rightmost columns. There are several ways to categorize how these radio frequencies are used. For two-way communications purposes, radio frequencies are allocated for land, mobile, maritime, aeronautical mobile, and aviation applications. Radio frequencies are allocated for broadcast radio and TV channels, as well as for broadcasting standard time and frequency signals. There are radio frequency usages for satellite communication, radio astronomy, meteorological, and navigation purposes. Amateur radios are also authorized for use at certain frequency bands.

The primary type service for a particular band is annotated by all capital letters, the secondary service annotation uses normal letters, and permitted services are annotated also by capital letters but enclosed in oblique strokes (e.g., /RADIOLOCATION/). Permitted and primary services have equal rights. However, in the preparation of frequency plans, the primary services, as compared with the permitted services, have prior choice of frequencies. Stations of a secondary service should not cause harmful interference to stations of primary or permitted services to which frequencies are already assigned or to which frequencies may be assigned at a later date. They cannot claim protection from harmful interference from stations of a primary or permitted service; however, they can claim protection from harmful interference from stations of the same or other secondary services to which frequencies may be assigned at a later date.

Radio frequency usages from 4 kHz to 400 GHz are all defined in detail with no spare gap at all. Defined details are noted in Table 5.2 by either the new "S" numbering scheme or the old numbering scheme. These notes are included in the text of the CFR title 47 document in Part 2. For example, S5.150 says that: "The following bands: 13533 13567 kHz (center frequency 13560 kHz), 26957 27283 kHz (center frequency 27120 kHz), 40.66 40.70 MHz (center frequency 40.68 MHz), 902 928 MHz in Region 2 (center frequency 915 MHz), 2400 2500 MHz (center frequency 2450 MHz), 5725 5875 MHz (center frequency 5800 MHz), and 24 24.25 GHz (center frequency 24.125 GHz) are also designated for industrial, scientific and medical (ISM) applications. Radio communication services operating within these bands must accept harmful interference which may be caused by these applications."

These notes specify frequency bands and service types. Technical specifications of service types such as signal level, bandwidth, and modulation method are usually also defined in detail in referenced documents. Licenses issued by government authorized agencies are usually required for operators of equipment for these services. The combined regulation and licensing procedure ensures that only authorized services are operational at their desired performance level within particular frequency bands. Regulated radio communication applications have been developed since the early 1940s. For historical reasons, many authorized applications are based on earlier analog technologies. Recent development of digital transmission technologies have opened up a tremendous opportunity window for further realization of the potential of radio frequency spectra. A process involving experimenting, petition, regulation, and licensing is usually required for the introduction of a new service within a certain radio frequency band.

Broadband data communication using radio frequency in the form of a Local Area Network is very convenient and cost-effective to link computers and peripheral devices together. However, the characteristics of radio signal for high throughput data traffic might be incompatible with most previously defined services. On the other hand, ISM radio frequency bands are ideal for experimenting with broadband data communication services. CFR title 47, Part 15 governs the use of unlicensed transmitters in 902 928, 2400 2483.5, and 5725 5850 MHz ISM bands. To minimize the effect of interference to others, only a broadband low power density signal is allowed in the ISM band in the format of spread spectrum technology. Two formats of spread spectrum technologies are defined in CFR title 47, Part 15. The first is frequency hopping and the second is direct sequence.

For the case of frequency hopping, the instantaneous signal power density is much higher than the background noise if the transmitter and receiver are in sync. Interference between different frequency-hopping systems could occur occasionally if some of their hopping frequencies are the same. Interference could be minimized or avoided if hopping sequences can be coordinated among different frequency-hopping systems. For the direct sequence case, the received signal-to-noise ratio is enhanced by the reverse of the spreading process. The amount of signal-to-noise ratio enhancement is called spread gain. Different direct sequence spread spectrum systems can occupy the same frequency band. The desired signal is recovered through the despreading using a correlation process of the correct sequence code from the rest of other sequence coded signals. The effective signal-to-noise ratio decreases when more direct sequence spread systems are occupying the same frequency band.

General transmission parameters of spread spectrum systems are also defined by Part 15. For frequency-hopping spread spectrum systems operating in the 902- to 928-MHz band, the minimum number of hopping channels should be at least 25 or 50 depending on if signal bandwidth is more or less than 250 kHz. The maximum bandwidth of each hopping channel is 500 kHz. The average time of occupancy at each hopping channel should be less than 0.4 second. The limit for transmit power is 1 W for systems employing at least 50 hopping channels and 0.25 W for systems employing fewer than 50 hopping channels. Frequency-hopping systems operating in the 2400 2483.5 MHz and 5725 5850 MHz bands should use at least 75 hopping frequencies. At these frequency bands, the maximum bandwidth of the hopping channel is 1 MHz. For direct sequence spread spectrum systems, the minimum bandwidth is 500 kHz. The transmit power limit is 1 W, and the peak power spectral density at the antenna interface should be less than 8 dBm in any 3-kHz band. The processing gain of a direct sequence system should be at least 10 dB. CFR title 47, Part 15 also specifies that the transmit field strength measured at a distance of 3 m should be less than 94 dBµV/m at these ISM frequency bands.

Because of their relatively broad available bandwidths, many indoor high-throughput data transmission systems utilize frequency bands at 902 928 MHz, 2.4 2.4835 GHz, or 5.725 5.850 GHz after initial trial experiments. Since these are unlicensed frequency bands, the transition from experimenting to productization can be relatively efficient as long as FCC regulations are followed. According to CFR title 47, Part 15, these systems have to rely on spread spectrum technologies. Such developed radio frequency data transmission systems are in the secondary category because they cannot claim any interference protection from other systems.