Key technology of CDMA

Abstract: The key technology of CDMA is the base of 3G. In this paper, the key technologies in CDMA are discussed in eight aspects, such as multiple access technology, rake receiver, multiuser detection, power control, soft capacity, soft switching, address code selection and diversity technology, so that we can have a comprehensive understanding of the key technologies of CDMA.

1 Multiple Access Technology

Multiple access technology enables a large number of users to share common communication lines. There are basically three ways to implement multiple access for signal multiplexing, they use frequency, time or code-separated multiple access mode, that is commonly known as frequency Division multiple address (FDMA), TDMA (TDMA) and Code Division multiple address (CDMA) Three kinds of accessing methods. Figure 1 shows a simple concept of these three methods with a model.

FDMA is based on different frequency channels to achieve communication, TDMA is to communicate with different time slots, CDMA is a different code sequence to achieve communication.

1.1 Frequency Division Multiple Access

Frequency division, sometimes referred to as the channel, is to divide the entire distribution of the spectrum into a number of single radio channels (transmit and receive carrier pairs), each channel can transmit a voice or control information. Under the control of the system, any user can access any one of these channels. Analog cellular system is a typical example of FDMA structure, digital cellular system can also be used in FDMA, just do not use pure frequency points, such as GSM and CDMA systems used FDMA. (Computer science)

1.2 Hours Multiple Access

Time Division Multiple access is in a bandwidth of the wireless carrier, according to the Times (or known as time slot) divided into a number of channels, each user occupies a time slot, only in this specified time slot within the received (or sent) signal, it is called division multiple address. This multiple access method is used in the digital cellular system, and the GSM system is used in this way.

TDMA is a more complex structure, in the simplest case the single carrier frequency is divided into many different time slots, each of which transmits bursts of information. The key part of TDMA is the user part, each user is assigned to a time slot (allocated at the start of call), the user communicates with the base station synchronously, and the time slot is counted. When the time slot arrives, the mobile station initiates the receiving and demodulation circuit and decodes the burst information from the base station. Similarly, when a user wants to send a message, the information is first cached until his or her time slot arrives. After the time slot begins, the information is emitted at a doubling rate, and then the next burst of transmission is accumulated.

One of the variants of TDMA is to transmit and receive on a single frequency channel, called Time Division Duplex (TDD). The simplest structure is the use of two time slots, one to send a collection. When the base station is received when the mobile station is launched, the mobile station receives it alternately when the base station is launched. TDD has many advantages of TDMA structure: burst transmission, no need for antenna transceiver and so on. Its main advantage is that it can be launched and received on a single carrier, without the need of uplink and downlink two carrier frequencies, no frequency switching, thus can reduce the cost. The main disadvantage of TDD is that capacity requirements for large-scale systems are not met.

1.3 Code Division Multiple Access

CDMA is a multiple access method which is realized by different code sequences formed by spread spectrum technology. It does not separate the user's information from frequency and time like FDMA and TDMA, it can transmit multiple user's information simultaneously on a channel, that is to say, allow the user to interfere with each other. The key is that the information in the transmission before the special encoding, the encoded information after mixing will not lose the original information. How many code sequences are orthogonal, how many users can communicate on one carrier at the same time. Each transmitter has its own unique code (pseudo-random code), while the receiver also know to receive the code, using this code as a signal filter, the receiver can be from all other signals from the background to restore the original information code (this process is called the extension). CDMA can be divided into direct sequence spread spectrum (DS), FH (FH) and Hop Time (TH) According to the modulation method of obtaining bandwidth signal, as shown in Figure 2 below:

2 Rake Receiver

Rake receiver is also known as Multipath receiver, that is, mobile Taichung has multiple rake receivers, because of the multipath effect of wireless signal transmission, so the signal from the base station will pass through different paths to the mobile station, through different paths to the signal time of the mobile station is different, If two signals arrive at the mobile station at a time lag of more than one signal code element width, rake receiver can be successfully demodulation, the mobile station will each rake receiver received the signal to add the vector (that is, the signal to the mobile station at different times for different time delay to reach the same phase), Each receiver can receive a single multipath signal, so that the mobile station can handle several multipath components, to achieve the goal of multipath fading, improve the reception performance of Mobile station. The base station also uses the same principle to receive the signal of each mobile station, that is, it also uses multiple rake receivers. In addition, when the mobile platform for soft switching, it is because the use of different rake receivers to receive different base station signal to achieve.

3 Multi-user detection

The CDMA receiver based on rake receiver treats other users ' signals as jamming signals, but the optimized receiver can detect all signals or subtract other signals from the specified signals.

When new users or sources of interference into the network, other users of the quality of service will decline, the network anti-interference ability, the more users can serve. Multi-channel interference jamming to a base station or mobile station is the sum of the interference in the small and inner area.

Multi-user detection (MUD), also known as joint detection and interference cancellation, provides a way to reduce the impact of multiple access disturbances and thus increase system capacity. At the same time, mud significantly reduces the near-far effect of CDMA systems. MUDs can relieve the system's need for power control.

4 Power Control

Because the CDMA system uses the same frequency for different users at the same time, so the CDMA system is a self jamming system, if the system adopts the spread spectrum code is not completely orthogonal (the actual system used in the address code is approximate orthogonal), thus causing interference between each other. In a CDMA system, each code-point channel is disturbed by other code-division channels, which is an inherent internal interference. Because each user distance base station is different from the base station to receive each user's signal strength is different, because of the interference between the signal, especially strong signal will cause a lot of weak signal interference, and even cause the system crash, so must adopt some way to control the transmission power of each user, It makes the signal strength of each user reach the base station basically consistent. The capacity of CDMA system is mainly limited by the mutual interference of the internal mobile station, so the signal to noise ratio of each mobile station reaches the minimum, and the system capacity will reach the maximum value.

CDMA power control is divided into: forward power control and reverse power control, and the reverse power control is divided into Open-loop and closed-loop power control.

Reverse Open loop power control

The reverse Open-loop power control is the change of the mobile station's receiving power in the cell, which adjusts the transmitting power of the mobile station rapidly. The aim is to try to make the signal from all mobile stations have the same nominal power when they reach the base station. Open-loop power control is to compensate for the change of mean path fading and shadow and turn, it must have a large dynamic range. IS95 Air interface stipulates that the dynamic range of Open-loop power control is -32db~+32db.

1) Just enter the access channel (closed-loop correction is not activated)

Average output power (dbm) =-Average input power (dbm) -73+NOM_PWR (db) + INIT_PWR (db)

Among them: Average power is relative to 1.23 MHz nominal CDMA channel bandwidth;

INIT_PWR is the adjustment required for the first access channel sequence;

The NOM_PWR is to compensate for the path loss caused by the correlation between the forward CDMA channel and the reverse CDMA channel.

2 Thereafter the trial sequence increases the transmit power (step to pwr_step) until an effect or sequence end is received. The output power level is:

Average output power (dbm) =-Average input power (dbm) -73+NOM_PWR (db) +INIT_PWR (db) +pwr_step (db)

3 The average output power of the mobile station becomes: Average output power (dbm) =-Average input power (dbm) -73+NOM_PWR (db) +INIT_PWR (db) +pwr_step (db) + when a power control bit is received after the reverse service channel begins to send The sum of all closed-loop power corrections (DB)

Where: The range of NOM_PWR is -8~7 DB, the nominal value is 0dB

The range of INIT_PWR is -16~15 DB with a nominal value of 0dB

The range of Pwr_step is 0~7 DB

Reverse Closed Loop power control

The purpose of closed-loop power control is to make the base station to correct the Open-loop power of the mobile station, so as to keep the optimal transmitting power of the mobile station. The power control bit is continuously sent, at a rate of 1.25ms per bit (i.e. 800bit/s). "0" bit indicates that the mobile station increases the average output power, "1" bit indicates that the mobile station decreases the average output power and the step is 1db/bit. The length of a power control bit is exactly equal to the length of two modulation symbols for the forward business Channel (i.e. 104.66us). Each power control bit replaces two consecutive forward-service channel modulation symbols, a technique commonly referred to as the symbolic extraction technique.

Forward power control

The base station periodically reduces the transmitting power of the transmitter to the mobile station, the mobile station measures the false frame rate, and when the error frame rate exceeds the predefined value, the mobile station requires the base station to increase its transmitting power by 1% and adjust each 15~20ms. The dynamic range of downlink Low speed control adjustment is ±6db. The report of Mobile station is divided into periodic report and Threshold report.