Реферат: Что является CDMA (Разделение Кодекса Многократный Доступ) (?)

codes are applied to the data bits associated with users in a cell. These data bits are transmitted along with the signals of all the

other users in that cell. When the signal is received, the codes are removed from the desired signal, separating the users and

returning the call to a rate of 9600 bps. Traditional uses of spread spectrum are in military operations. Because of the wide

bandwidth of a spread spectrum signal, it is very difficult to jam, difficult to interfere with, and difficult to identify. This is in

contrast to technologies using a narrower bandwidth of frequencies. Since a wideband spread spectrum signal is very hard to

detect, it appears as nothing more than a slight rise in the "noise floor" or interference level. With other technologies, the power of

the signal is concentrated in a narrower band, which makes it easier to detect. Increased privacy is inherent in CDMA

technology. CDMA phone calls will be secure from the casual eavesdropper since, unlike an analog conversation, a simple radio

receiver will not be able to pick individual digital conversations out of the overall RF radiation in a frequency band.

Synchronization

In the final stages of the encoding of the radio link from the base station to the mobile, CDMA adds a special "pseudo-random

code" to the signal that repeats itself after a finite amount of time. Base stations in the system distinguish themselves from each

other by transmitting different portions of the code at a given time. In other words, the base stations transmit time offset versions

of the same pseudo-random code. In order to assure that the time offsets used remain unique from each other, CDMA stations

must remain synchronized to a common time reference. The Global Positioning System (GPS) provides this precise common time

reference. GPS is a satellite based, radio navigation system capable of providing a practical and affordable means of determining

continuous position, velocity, and time to an unlimited number of users.

"The Balancing Act"

CDMA cell coverage is dependent upon the way the system is designed. In fact, three primary system characteristics-Coverage,

Quality, and Capacity-must be balanced off of each other to arrive at the desired level of system performance. In a CDMA

system these three characteristics are tightly inter-related. Even higher capacity might be achieved through some degree of

degradation in coverage and/or quality. Since these parameters are all intertwined, operators cannot have the best of all worlds:

three times wider coverage, 40 times capacity, and "CD" quality sound. For example, the 13 kbps vocoder provides better

sound quality, but reduces system capacity as compared to an 8 kbps vocoder.

CDMA Benefits

When implemented in a cellular telephone system, CDMA technology offers numerous benefits to the cellular operators and their

subscribers. The following is an overview of the benefits of CDMA. 1.Capacity increases of 8 to 10 times that of an AMPS

analog system and 4 to 5 times that of a GSM system 2.Improved call quality, with better and more consistent sound as

compared to AMPS systems 3.Simplified system planning through the use of the same frequency in every sector of every cell