When lit, a mobile, you know the frequency it allocated to the CDMA service in the local area. Is tuned for that frequency and the pilot signal. You can find several pilot signals from different base stations, but they can be differentiated because they have different time shifts. The mobile selects the strongest pilot signal and provides time and frequency references from it.
Once this process of selection of the base, the phone begins to demodulate with Walsh code 32 which corresponds to channel synchronization. The synchronization channel contains the future value of the code shift register length (42 bits). The phone loads the value in its register and is synchronized with the base station time.
Additionally it requires that the mobile is registered in the database, thus it knows the mobile is available to receive calls and what is its location. When a mobile moves from one area to another and there is no call in progress, takes a process-state idle handoff. When the user makes a call, the phone tries to connect the base station with an access test. The long code used is based on the parameters of the cell. If a collision occurs there is no answer the phone and wait a random time before trying again.
By establishing contact with the base station is assigned a traffic channel using a Walsh code. From this time the mobile changes the code along with one based on its serial number. The Walsh code is used in the forward-link, while the long code is used in the reverse-link. When a mobile connected to a base driver detects another signal powerful enough, requests a soft handoff process. Mobile is assigned to another code of Walsh and another pilot timing. The mobile should be able to receive both signals and combine them. When the signal of the original base has decreased enough, the phone calls to the soft handoff.
At the end of a call, the channels are released. When the phone is switched off log generates a signal that is sent off to the base to indicate that it is not available for calls.
CDMA. Past, present and future
CDMA technology was a strong driver of the 2G systems at the time of its emergence in the early 90s. Currently under development efforts of 3G systems, CDMA resubmits a leading role, this time in broadband version or W-CDMA (Wideband CDMA) In fact, this technology appears in most of the proposals submitted to ITU for radio interface for third generation. Other technologies are associated with 3G W-TDMA (Wideband-Time Division Multiple Access) and hybrid systems between the two already mentioned.
Conventional CDMA systems based on spread spectrum techniques (spread-spectrum), which are a legacy of defense applications relating to the elimination of interference (anti-jamping), measures the distance (ranging) or encryption. These techniques are based on spread spectrum frequency of a signal at a bandwidth greater than the minimum required for transmission, a situation which is maintained throughout the transfer process. Later, when arriving at the receiver, the signal is recomposed to obtain the initial signal that is intended to convey. Thus, one can obtain a series of links that use the same frequency band simultaneously without generating interference.
