CDMA has been adapted for use in cellular communications with the addition of several system features that enhance efficiency and lower costs. Mobile Station Sign-on On power-on, the mobile station already knows the assigned frequency for CDMAservice in the local area and will tune to that frequency and search for pilot signals. Multiple pilot signals typically will be found, each with a different time offset. This time offset distinguishes one base station from another.
The mobile station will pick the strongest pilot and establish a frequency reference and a time reference from that signal. Once the mobile station becomes synchronized with the base station’s system time, it can then register. Registration is the process by which the mobile station tells the system that it is available for calls and notifies the system of its location.
The user makes a call by entering the digits on the mobile station keypad and hitting the “Send” button. If multiple mobile stations attempt a link on the access channel at precisely the same moment, a collision occurs. If the base station does not acknowledge the access attempt, the mobile station will wait a random time and try again.
On making contact, the base station assigns a traffic channel, whereupon basic information is exchanged, including the mobile station’s serial number. At this point, the conversation mode is started. As a mobile station moves from one cell to the next, another cell’s pilot signal will be detected that is strong enough for it to use. The mobile station will then request a “soft handoff,” during which it is actually receiving both signals via different correlative elements in the receiver circuitry.
Eventually, the signal from the first cell will diminish, and the mobile station will request from the second cell that the soft handoff be terminated. Abase station does not hand off the call to another base station until it detects acceptable signal strength.
This soft handoff technique is a significant improvement over the handoff procedure used in analog FM cellular systems, where the communication link with the old cell site is momentarily disconnected before the link to the new site is established. For a short time, the mobile station is not connected to either cell site, during which the subscriber hears background noise or nothing at all.
Sometimes the mobile stations Ping-Pong between two cell sites as the links are handed back and forth between the approaching and retreating cell sites. Other times, the calls are simply dropped. Because a mobile station in the CDMA system has more than one modulator, it can communicate with multiple cells simultaneously to implement the soft handoff.
At the end of a call placed over the CDMA system, the channel will be freed and may be reused. When the mobile station is turned off, it will generate a power-down registration signal that tells the system that it is no longer available for incoming calls.
Voice Detection and Encoding
With voice activity detection, the transmitter is activated only when the user is speaking. This reduces interference levels—and, consequently, the amount of bandwidth consumed—when the user is not speaking. Through interference averaging, the capacity of the system is increased. This allows systems to be designed for the average rather than the worst interference case.
However, the IS-95 CDMAstandard requires that no interfering signal be received that is significantly stronger than the desired signal, since it would then jam the weaker signal. This has been called the “near-far problem” and means that high cell capacity does not necessarily translate into high overall system capacity.
The speech coder used in CDMA operates at a variable rate. When the subscriber is talking, the speech coder operates at the full rate; when the subscriber is not talking, the speech coder operates at only one-eighth the full rate. Two intermediate rates are also defined to capture the transitions and eliminate the effect of sudden rate changes. Since the variable-rate operation of the speech coder reduces the average bit rate of the conversations, system capacity is increased.
Increased privacy is inherent in CDMA technology . CDMA phone calls will be secure from the casual eavesdropper because, unlike a conversation carried over an analog system, a simple radio receiver will not be able to pick out individual digital conversations from the overall RF radiation in a frequency band.
ACDMA call starts with a standard rate of 9.6 kbps. This is then spread to a transmitted rate of about 1.25 Mbps. “Spreading” means that digital 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 the original rate of 9.6 kbps. Because of the wide bandwidth of a spread-spectrum signal, it is very difficult to identify individual conversations for eavesdropping. 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 analog technologies, the power of the signal is concentrated in a narrower band, which makes it easier to detect with a radio receiver tuned to that set of frequencies. The use of wideband spread-spectrum signals also offers more protection against cloning, an illegal practice whereby a mobile phone’s electronic serial number is taken over the air and programmed into another phone. All calls made from a cloned phone are “free” because they are billed to the original subscriber.
CDMA systems rely on strict control of power at the mobile station to overcome the so-called near-far problem. If the signal from a near mobile station is received at the cell site receiver with too much power, the cell site receiver will become overloaded and prevent it from picking up the signals from mobile stations located farther away.
The goal of CDMA is to have the signals of all mobile stations arrive at the base station with exactly the same power level. The closer the mobile station is to the cell site receiver, the lower is the power necessary for transmission; the farther away the mobile station, the greater is the power necessary for transmission. Two forms of adaptive power control are employed in CDMA systems: open loop and closed loop.
Open-loop power control is based on the similarity of loss in the forward and reverse paths. The received power at the mobile station is used as a reference. If it is low, the mobile station is assumed to be far from the base station and transmits with high power. If it is high, the mobile station is assumed to be near the base station and transmits with low power. The sum of the two power levels is a constant.
Closed-loop power control is used to force the power from the mobile station to deviate from the open-loop setting. This is achieved by an active feedback system from the base station to the mobile station. Power control bits are sent every 1.25 millisecond (ms) to direct the mobile station to increase or decrease its transmitted power by 1 decibel (dB). Lack of power control to at least this accuracy greatly reduces the capacity of CDMA systems.
With these adaptive power-control techniques, the mobile station transmits only enough power to maintain a link. This results in an average power requirement that is much lower than that for analog systems, which do not usually employ such techniques. CDMA’s lower power requirement translates into smaller, lightweight, longer-life batteries—approximately 5 hours of talk time and over 2 days of standby time—and makes possible smaller, lower-cost hand-held computers and hybrid computer-communications devices. CDMAphones can easily weigh in at less than 8 ounces.
Among the various forms of diversity is that of spatial diversity, which is employed in CDMA, as well as in other multiple access techniques, including FDMA and TDMA. Spatial diversity helps to maintain the signal during the call handoff process when a user moves from one cell to the next. This process entails antennas in two different cell sites maintaining links with one mobile station.
The mobile station has multiple correlative receiver elements that are assigned to each incoming signal and can add these. CDMAuses at least four of these correlators: three that can be assigned to the link and one that searches for alternate paths. The cell sites send the received data, along with a quality index, to the MTSO, where a choice is made regarding the better of the two signals.
Not all these features are unique to CDMA; some can be exploited by TDMA-based systems as well, such as spatial diversity and power control. These already exist in all TDMA standards today, while soft handoff is implemented in the European Digital Enhanced Cordless Telecommunications (DECT) standard, which is based on TDMA.