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Code Division Multiple Access (CDMA) is a relatively new technology that has its roots in spread-spectrum radio developed for secure military use. CDMA is in place in numerous trials, but is not widely deployed. In fact, the first commercial CDMA system was placed in service in Hong Kong only recently [12-5], where cellular phones are considered almost a necessity and where the networks have suffered terrible congestion. CDMA is a wideband radio technology, providing the full carrier channel to all terminals through a frequency-hopping technique, in tune with the base station. Each terminal has its own unique 10-bit code, which is applied to the channel; that code allows the terminal and base station to recognize data intended for it, rejecting all other data [12-3]. Each of a great number of transmissions is spread across the entire available spectrum and is denoted by a unique code which is imprinted on the packet radio transmission (Figure 12.4). In this manner, each transmission can be identified from all others, as well as from background noise; error performance is excellent.
Figure 12.4 Code Division Multiple Access (CDMA).
CDMA improves bandwidth utilization, as a great number of users can share the same wideband radio frequency channel. Improvements of up to 2000% (20x) are predicted, compared to AMPS. CDMA also provides excellent security as it is virtually impossible to intercept more than a small portion of a transmission; encryption can be used to provide additional security [12-7].
CDMA was perfected by Qualcomm, which develops, manufactures, markets and licenses CDMA products. CDMA recently has been licensed by a great number of manufacturers and providers of cellular, PCS, Wireless LAN and other systems and networks. CDMA is being trialed by Ameritech Mobile, Bell Atlantic Mobile, GTE Mobilnet, Southwestern Bell Mobile Systems, and other cellular providers [12-5]. NEC has also licensed the technology.
The first experimental two-way mobile radio system was placed into service by the Detroit Police Department in 1921, operating in the 2-MHz band, followed by the Bayonne, New Jersey police department in the early 1930s. While this AM (Amplitude Modulation) radio application grew quickly, even as late as 1937 there were only 40 channels allocated by the FCC. In the late 1930s, FM (Frequency Modulation) replaced AM as the method of choice due to its improved quality of reception and lower power requirements. (FM receivers tend to lock in on the stronger competing signal, whereas AM recognizes all competing signals.) In 1949 the FCC recognized two-way mobile radio as a new class of service, and began to allocate spectrum and regulate its use [12-8] and [12-2].
Commercial applications were first offered in 1946, when AT&T was granted the first license for two-way, mobile FM service in St. Louis, MO. This service offered the additional advantage of being connected to the PSTN WAN. The transmitters had a range of approximately 50 miles. As the costs were reasonable, the service grew in popularity and the systems were soon oversubscribed. In fact, it was not uncommon for a provider to load as many as 100 subscribers per channel; as a result, service was horrible. In 1976, by way of example, service in the New York metropolitan area (20 million population) consisted of 20 channels supporting 543 subscribers; there was a waiting list of approximately 3,700 [12-8].
Switched Mobile Radio (SMR), also know as Trunk Mobile Radio (TMR) entered the scene in the 1960s marketed as Improved Mobile Phone Service (IMTS). This service made better use of FM bandwidth through narrowband communications (smaller frequency ranges). IMTS also allowed users to search multiple frequencies on a manual basis. Shortly thereafter, intelligent mobile sets were developed that searched channels automatically. The concept of SMR/TMR remains much the same as it was originally. The provider places a radio tower and omnidirectional transmit/receive antennae on the highest possible point in the area and blasts the signal at the maximum allowable power level. As illustrated in Figure 12.5, this approach provides a coverage area of 50 miles or more, depending on topography.
Figure 12.5 Switched Mobile Radio (SMR) network.
SMR/TMR largely has been supplanted by cellular service offerings, although it still is widely used in dispatch and fleet applications such as police, fire, and emergency vehicles; taxi fleets, utility fleets (e.g., telephone companies, electric utilities, and CATV providers), and courier services. In the United States, 80 MHz has been allocated for SMR.
Enhanced Switched Mobile Radio (ESMR) is a technique developed by Nextel and Geotek Communications for the development of a voice and data, cellular-like network using embedded SMR networks operating at 800-900 MHz. For instance, Nextel acquired and linked a large number of SMR networks throughout the United States. Through the use of digital technology, each frequency channel is divided into multiple time slots to support multiple conversations; call hand-off is supported. MCI in 1995 withdrew its offer of US$1.3 billion for 17% of Nextel, without citing any reasons. It seems apparent that MCI had second thoughts about committing such a significant amount of money and putting its reputation on the line for a technology which is largely unproven. The Nextel network, which offers data throughput of 7.2 Kbps, is yet to be fully deployed [12-9].
The concept of a paging system to cover a local or metropolitan area was first introduced in 1950 in the New York, and involved proprietary solutions including the GOLAY standard from Motorola [12-8]. During the 1970s, the British Post Office (BPO) developed a standard set of code and signaling formats, which evolved into the POCSAG (Post Office Code Standardization Advisory Group) code. The POCSAG standard provides for transmission speed of 2400 bps, using bandwidth of 25 kHz. The CCIR (now ITU-R) standardized that code, internationally, in 1981. With the exception of the Japanese (not surprisingly), all developed nations have conformed to this standard. According to estimates from the Personal Communications Industry Association, at year-end 1995 there are well over 34 million pager subscribers, worldwide; it is estimated that there are over 27 million subscribers to over 2,000 paging services in the United States, alone [12-10] and [12-11].
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