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<br />DRAFT 2.0 <br />04/30/97 <br /> <br />Khursheed Khan <br />51. Paul, Minnesota <br /> <br />briefly that at the receiver, a similar type of code is used to demodulate this coded <br />signal such that only the voice call that is intended to be received is demodulated and <br />all other signals are untouched. Basically, all but the intended signals are considered as <br />noise at the receiver. Since any voice conversation that uses more signal strength than <br />required (to support a voice call with acceptable quality) would cause undue <br />interference to all users in the network, a CDMA system uses sophisticated power <br />control mechanism to ensure that all voice conversations are operated with the <br />minimum required energy. This concept tied in with the concept of all users sharing the <br />same CDMA channel leads to the unique relationship between the coverage area and <br />capacity of a CDMA system. <br /> <br />Unlike FDMA or TDMA systems where the coverage area is not (directly) a <br />function of the maximum number of simultaneous users in the system, CDMA system <br />coverage area IS a function of the load (number of simultaneous users) of the system. <br />In the previous paragraph the concept of noise associated with users was introduced <br />(while discussing about demodulation of signals), as all but the intended signal is <br />considered as noise at the particular receiver. Consequently, the coverage area of any <br />CDMA system in the reverse link (from mobile to base station) is a function of the <br />number of users in the system as well as the maximum allowable output power from the <br />mobile station. Since everyone uses the same channel, the level of interference <br />increases with the increase in the number of users in the system. So, to meet the call <br />quality objective, the transmit power level from the mobile station needs to be <br />increased. However, since all mobile stations are limited by the amount of power they <br />can transmit, if we want to keep the same coverage foot print we are quickly faced with <br />this (mobile station) power limitation obstacle. On the other hand, if we want to carry <br />the increased users in the system, we could probably accommodate these additional <br />users by covering a smaller area than which was covered earlier. This would mean <br />dropping users on the boundary of cell sites (from the perspective of the base station in <br />the forward link). Therefore, we notice a complementary relationship between the <br />coverage area and the capacity of a CDMA cell site. Everything remaining equal, at <br />low loads the cell site will cover a greater area, while at high loads it will cover a <br />smaller area. This effect particular to CDMA cells is known as cell breathing. Similar <br />arguments can be presented to show the complementary nature of forward link (from <br />base station to mobile station) coverage area. Putting the forward and reverse link <br />coverage areas and the capacity notions on the same plane, we come out with the <br />concept of capacity versus coverage in a CDMA system. <br /> <br />Getting back to the voice conversation initiated at the mobile end, now that a <br />particular channel access mechanism has been chosen to carry the voice call over the <br />airwaves, we need to consider the demodulation of these transmitted signals and <br />obviously their subsequent reception. With FDMA systems, the signals are <br />demodulated by using an inverse technique as that employed in the modulation process; <br />basically the upbanded signal is filtered and transferred to baseband for subsequent <br />reception at the receiver. The signal processing for the TDMA system is somewhat <br />different and a little involved, and as such is not discussed in this document. However, <br /> <br />5 <br />