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DRAFT 2.0 04/30/97 Khursheed Khan St. Paul, Minnesota channels as non-overlapping, non-interfering logical entities spread out in the frequency domain then we know that as long as the voice conversation is transmitted on the assigned logical channels, there is no cause of interference and the system will support as many simultaneous voice calls as the nwnber of available logical channels (approximately, barring the minimal requirement for control channels). Hence the capacity of the FDMA system is limited. In order to support a greater number of simultaneous users on the system, each of the logical FDMA channels are further divided in time (widely known as time-slots; nonnally 3 slots are grouped onto one logical FDMA channel). Conceptually, this is done by time sharing each channel such that (let's say) 3 users (voice calls) use the 22 mill i-second voice frame (that is transmitted on the logical FDMA channels) one after the other for one third of the time. This process is repeated so that the first user always gets the first slot on the frame, the second user gets the second slot, and the third user gets the third slot. So, each user can transmit (approximately) 7 milli-second of data on each frame. Since the time difference between subsequent transmissions should not be perceptible to the end user, a lot more different types of processing is carried out on the background to ensure that proper quality of service is achieved. This method of multiple access is known as Time Division Multiple Access (TDMA), and obviously we can get 3 times more capacity using this teclmology over that of the standard FDMA teclmology, at least in theory. If we think of a railway station with multiple tracks, then the tracks would correspond to the logical channels of the FDMA system, whereas with trains on the tracks - the bogies/cars would correspond to the time slots of the TDMA system. Naturally for our hypothetical train station, none of the trains would have more than 3 cars/bogies. As a matter of fact, ALL the trains will have 3 and only 3 cars/bogies. As mentioned earlier, the coverage area of a TDMA system is detennined in a similar fashion as done in the FDMA system and is not a function of the number of simultaneous users (in theory). So the footprint of a FDMAlTDMA system is quasi-static and can be easily calculated before a system is actually built. We need to keep this point in mind to compare this characteristic to that of the CDMA teclmology. Colloquially this TDMA teclmology is known as the digital teclmology and can be used either in the 800 MHz (Cellular) or in the 1800 MHz (PCS) band. Now we come to the multiple access teclmology used by US West Wireless Communications in the Minneapolis - S1. Paul greater metropolitan area. This access method is known as CDMA (Code Division Multiple Access). In this teclmology, the particular frequency band of operation (800 MHz or 1800 MHz) is used by ALL users in a specific system. Therefore, all voice conversations take place within a 1.23 MHz CDMA channel (1.23 MHz for transmission and 1.23 MHz for reception). Consequently the concept of logical channels and time slots are not applicable here. One would have to approach it from a different perspective. 3