Conceptos de cdma telefonia celular

CDMA SYSTEM FOR MOTOROLA CELLULARS

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WHAT IS CDMA?
Code Division Multiple Access (CDMA) is a spread-spectrum multiple access technique. This means a large number of users share a common pool of radio channels with any user being able to get access to any channel. Frequency Division Multiple Access (FDMA) is the multiple access technique that is used in traditional analog mobile systemsand Time Division Multiple Access (TDMA) is the technique used in GSM. With CDMA, unique digital codes, rather than separate RF frequencies or channels are used to differentiate subscribers. There are several types of CDMA but the most commonly used variant for cellular is Direct Sequence CDMA (DS-CDMA). In this case, the carrier is quite wide at 1.23 Mhz. The traffic channels are created byassigning each user a separate "code" and spreading each code over the entire carrier. Hence there is no time-division and all users use the entire carrier, all of the time. The best way to visualise this is to imagine a room with several "pairs" of people in it. The pairs only want to talk and listen to themselves and have no interest in the others. If each pair only knows one language and uses it, andall the languages are different, they can all use the air in the room as a "carrier" for their voices and experience little interference from the other pairs. The analogy here is that the air in the room is a very wideband "carrier" and the languages are the "codes". If we incorporate language "filters", people speaking Spanish will not hear people speaking German etc. We continue to add usersuntil the overall "background noise" ( interference from other users ) limits us. By controlling the signal strength of all users to no more than necessary, while maintaining high quality conversations, we get many users per carrier.

Edit And Written By : Tony Cheung

07/22/99

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ADVANTAGES OF CDMA
1.Due to the inherent nature of CDMA, a high capacity and spectral efficiency is achieved.Some operators have confirmed that a CDMA system has 6-10 times the capacity of a traditional analog system. This is probably the bottom line for operators since their profits ultimately come down to system capacity. 2.A lower cost per subscriber is achieved since fewer cell sites and reduced frequency planning are required. Frequency planning places a big drain on an operators resources in termsof time spent planning and maintaining complex frequency plans. A CDMA system requires significantly less planning as it doesn't re-use frequencies which is the main factor driving frequency planning. 3.Improved voice quality. CDMA voice quality is comparable to GSM. 4.Security benefits. Due to the fact that the voice signal is coded and is at a power level down in the noise region, this meansthat the CDMA voice signal is extremely difficult to identify and decode. 5.Variable bandwidth offering ease of implementation. CDMA's inherent multipath resistance and high bandwidth makes it suitable for high bit rate applications. 6.Lower power transmitters. This results in lower hazard and lower costs. It also reduces interference with other systems which is a key factor for operators. 7.Improvedquality on handoffs. Through the soft handoff process, reliability is improved for handoffs. 8.CDMA is one of the systems currently being considered for the Third Generation Mobile Standard.

Edit And Written By : Tony Cheung

07/22/99

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WHAT ARE THE DRAWBACKS TO CDMA?
1.CDMA systems require more complexity in both the Base Stations and the Mobile handsets to handle the unique powercontrol requirements of CDMA and the complicated encoding/decoding mechanisms. 2.The near-far effect is more significant in CDMA systems. This requires complex open/closed loop control mechanisms to eliminate the effects. 3."Rogue Mobiles" i.e. mobiles that don't obey power control commands from the base-station can cause havoc in CDMA systems. Methods such as malfunction timers and lock orders...