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DRAFT 2.0 04/30/97 Khursheed Khan St. Paul, Minnesota In FDMA (Frequency Division Multiple Access) each voice call is modulated onto a carrier in the appropriate transmissions band (on a particular logical channel) and transmitted over the airwaves. Previously we talked about speech being sampled and after "some processing" transmitted. Modulation falls into the "some processing" part of the whole process whereby baseband (nom1al sampled speech at lower :trequencies) signals are "upbanded" to be transmitted on the higher (900 MHz or 1800 MHz) frequencies. Nonnally channels are logically defined as portions of the airwaves in which a voice call is transmitted and received without undue interference from any outside source. These entities are of (nonnally) 25 KHz ban.dwidth with some additional space for guard bands. In this particular access method, each user of a cellphonelPCS phone carries out voice conversation via the use of a logical channel. These channels are spaced sufficiently apart such that they do not interfere with (nor do they accept interference :trom) other users of the same network operating in the same frequency range using a different logical channel. Hence, this system is limited by the maximum number of logical channels that could be used in voice conversations. It should be kept in mind that a limited number of additional channels are needed to support various other functionality when a voice call is in progress and are thus kept aside from the total number of available channels. When the signal is sent out on the airwaves from a mobile station, it is first routed via the base station (cell site) to the intended receiver. The mechanism of routing is quite involved and will therefore be omitted from this discussion. But one thing to keep in mind is that the base station can only "hear" signals from transmitting mobiles (handsets) if the strength of the signals reaching it are above a certain threshold. This limit is known as base station receiver sensitivity. Any signal reaching the base station below this threshold is registered as noise only and is not used for any subsequent processing. Finally, we point out that in this technology, the area of coverage (or the footprint) of a cell site is limited by the SNR (signal to noise ratio), and the maximum available output power from mobile stations. SNR is defined as the ratio of the strength of the transmitted signal from the base station, to the strength of all the interference (adjacent channel, co-channel, and thermal noise). To meet a particular quality of service (usually 2% blocking of voice calls and 1 % :trame error rate FER) the SNR is taken around 17 or 18 dB. It can be shown that the coverage area of a system using the FDMA technology can be increased by increasing the output power from the mobilelbase stations. This phenomenon is also true for the next technology type to be discussed (TDMA). Therefore, provided sufficient channels are available to carry on acceptable voice conversations, the coverage area of an FDMA system will not be a function of the number of simultaneous users in the system. This technology is usually used by the cellular operators (usually in the 800 MHz band) and is colloquially known as the analog system. TDMA (Time Division Multiple Access) is the second of the multiple access technologies that is used by certain providers in the Minneapolis - St. Paul market. It is derived from the basic FDMA multiple access philosophy. Ifwe think of the FDMA 2