Cellular and PCS subscribers are high-mobility consumers. Users expect, and pay for, the ability to communicate anytime, anywhere. This is the major market for wireless communications, but another type of consumer is beginning to reap the benefits of a fast-moving wireless industry.
Some people want an inexpensive wireless communications link for use in their home, office, or public locations such as shopping malls and grocery stores. These low-mobility applications demand low cost, reliable solutions that can provide adequate service in specific locations. Currently these solutions involve cordless telephones, but future developments in the PCS bands may bring about inexpensive wireless voice and data connections for the average consumer.
The first generation of cordless phones operate in the 46-49 MHz frequency band, somewhat below commercial FM radio (88 MHz to 108 MHz). These units are still being sold in many places, often at a significant discount, but must share the crowded band with other devices, such as baby monitors, low-power walkie-talkies, remote stereo headphones, and the like. These other devices also make it clear that there is no privacy on these phones, since it is very simple to use one and accidentally overhear other people's phone conversations. Some phones offer "voice security" or "voice privacy," but the schemes to protect these signals are often easy to defeat.
Table 1: 49 MHz cordless telephone frequencies.
The second generation of cordless phones operate up in the 902 - 928 MHz band and are often referred to as "900 MHz phones." This band is far less crowded than the 46-49 MHz band, and the FCC allows devices here to use more power. This has also become the home of "digital" phones, a term which has quickly become a marketing tool.
The word digital has at least three different meanings in the world of cordless telephones, and not all "digital" cordless phones are alike. Almost all cordless phones now have a digital security code, which is a simple combination code shared by the handset and the base to prevent other handsets on the same frequency from fraudulently using the telephone line.
A "traditional" digital phone transmits audio information between the base and the handset as a stream of digital bits instead of a continuous analog signal. Since the digital signals have a fixed format and usually make use of error correction, they are less susceptible to interference since they can be reconstructed at the receiving end. It also makes eavesdropping much more difficult, as baby monitors and other retail devices can't readily decode the digital information.
A "spread spectrum" digital phone not only sends digital bits instead of an analog signal, but those bits are "spread" during transmission using one of two methods. Frequency hopping spread spectrum phones rapidly switch between a number of different channels during a conversation, spending very little time on any one channel. The pattern of channel hopping is shared between the base and the handset, but if an eavesdropper doesn't know the pattern the conversation will be very difficult to intercept. In addition, any interference will probably be localized to just a few channels, so a majority of the data will be received without difficulty.
Direct sequence spread spectrum phones "spread" the signal across a wide bandwidth during transmission. This spreading is done according to a pseudo-random noise code (PN code), which is used by the receiver to "despread" the signal and recover the digital bits. Again, an eavesdropper without the PN code will have an extremely hard time intercepting the conversation. More details on direct sequence, also known as Code Division Multiple Access (CDMA), are available in the February, 1997, PCS Front Line column.
Spread spectrum systems have an additional advantage. Because they use the available spectrum more efficiently, the FCC allows these devices to transmit at up to one watt of power, giving some phones a range of half a mile or more. Non-spread phones in the 900 MHz are limited to about a milliwatt, and phones in the 46-49 MHz band even less, giving them a correspondingly shorter range.
I have owned a V-Tech 900 MHz phone for a few years now. The Tropez 900DX uses twenty channels in the 902 - 928 MHz band, each spaced 100 kHz apart. The base transmits on the lower frequency and the handset transmits on the higher frequency. To test V-Tech’s claims of digital security and resistance to casual eavesdropping, I used an Optoelectronics Scout to read the frequency output of the base at 905.81 MHz and the handset at 925.81 (channel 3) while the phone was in operation. A scanner tuned to either frequency revealed nothing but digital "hash," and no audio signal was discernible.
Table 2: Frequencies for V-Tech Tropez 900DX.
In the Report and Order that created Personal Communications Services, the FCC allocated 20 MHz of spectrum between 1,910 and 1,930 MHz (see PCS Front Line, October 1996) for use by unlicensed, low power devices. Spelled out in technical detail in Title 47 of the Code of Federal Regulations, Part 15 subpart D, devices that transmit in these frequencies must follow an "etiquette" intended to allow a wide variety of equipment to share the same spectrum.
Each device must follow three basic rules, very similar to the rules children learn in kindergarten: listen before talking, talk for a short amount of time so others have a turn, and don't shout.
An unlicensed PCS (U-PCS) device is required to listen to the "channel" it wants to use before transmitting. If the channel is being used, the device must either wait for it to become idle, or select another channel.
Devices must not monopolize the channel. The transmission should be limited to only the amount of time needed to send the message, and the device must stop transmitting at specific intervals in order to give other devices the opportunity to use the channel.
Each transmitter is also limited to fairly low power, again to avoid interference with other devices. Power limits are determined primarily by power spectral density, and the rules are available in Part 15 for anyone who wants the nitty-gritty details.
The unlicensed PCS band is divided into two 10 MHz bands. The space from 1,910 to 1,920 MHz is referred to as the Asynchronous band, and is intended primarily for data applications, such as wireless local area networks (LANs) and wireless data exchange. Devices operating in this band will typically send high-speed bursts of data whenever necessary, and will not follow a regular pattern of transmission. Most bursts will be one-way, and may or may not generate a response from the receiver. The minimum bandwidth for a burst is 500 kHz.
The space between 1,920 MHz and 1,930 MHz is called the Isochronous band. Devices operating in this band are expected to transmit information at regular intervals and carry on a two-way "conversation" with another device, such as would be needed for a cordless telephone. This band is subdivided into eight "channels," each of which is 1.25 MHz wide. The minimum emission bandwidth is 50 kHz, and it must fit entirely within one of these eight channels. Isochronous devices may further subdivide the channel for their own uses, but each channel must be at least 50 kHz wide.
Northern Telecom was the first company to receive FCC approval for a U-PCS device, and began marketing their wireless voice system to businesses. Similar systems are now available from other suppliers, and allow workers to place and receive calls anywhere inside their office building or warehouse.
PCS frequencies were formerly licensed to the fixed microwave service by the FCC. Part of the PCS rules require that these microwave operators relocate to other parts of the spectrum, and that operations in the PCS bands be coordinated by a third party. For U-PCS, UTAM Inc. is designated to coordinate the microwave relocation, negotiate costs and payments, ensure that adequate facilities are available, and resolve any interference disputes. UTAM and the relocation effort will be funded, in part, by fees gathered from the sale of U-PCS devices.
On July 9 a McDonnell-Douglas Delta II rocket carried five more IRIDIUM satellites aloft from Vandenberg Air Force Base in California, bringing the total number of IRIDIUM satellites in orbit to 17. The first IRIDIUM launch on May 5 brought five space vehicles into orbit, and a June 18 launch from the Baikonur Cosmodrome in Kazakstan using a Proton rocket added seven to the constellation.
These satellites will be moved to the proper location in their respective orbital planes and undergo systems testing. Voice uplink and downlink frequencies are between 1,616 and 1,626.5 MHz with a TDMA/FDMA access scheme, so if you're monitoring in that band and detect some new digital signals from overhead, it may be an IRIDIUM bird.
That's all for this month, and as usual more information is available at http://www.grove.net/~dan PCS Front Line is now a year old, and I welcome questions, comments, and suggestions on what you've seen and what you'd like to see in this column. I am reachable via electronic mail at firstname.lastname@example.org, and look forward to hearing from you. Until next month, happy monitoring!
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Updated May 1, 2003