Advantages and problems of CDMA technology and handover

Because of the inherent characteristics of CDMA technology, it is very suitable for digital cellular mobile communication system. Its advantages mainly lie in the following aspects: 10.

1. voice activation technology

The statistical results show that people only spend 35% of their time talking, and the other 65% of their time listening to each other, pausing between sentences or waiting for others. In CDMA digital cellular mobile communication system, all users share the same wireless channel. When the user is not talking, the transmitter of the user does not emit or emits less power, and the interference of other users is correspondingly reduced. Therefore, in CDMA system, the corresponding coding technology is adopted, so that the power transmitted by the user transmitter can be adjusted according to the user's voice coding requirements. When the user speaks, the output rate of the speech encoder is high, and the average power transmitted by the transmitter is large; When the user is not talking, the output rate of the speech encoder is very low, and the average power emitted by the transmitter is very small, which is the voice activation technology. In cellular mobile communication system, using voice activation technology can reduce the interference between users by 65% on average. That is to say, when the system capacity is large, using voice activation technology can increase the system capacity by about 3 times, but when the system capacity is small, the added value of the system capacity will decrease. Among the three systems of frequency division multiple access, time division multiple access and code division multiple access, only code division multiple access can make full use of voice activation technology conveniently. If voice activation technology is used in frequency division multiple access and time division multiple access, the system capacity will be improved to a certain extent, but both of them must add more complex power control systems to realize dynamic channel allocation, which will inevitably lead to the increase of time delay and system complexity, but it is relatively simple to realize this function in CDMA system.

2. Sector division technology

Sector division technology is that the base station located in the center of the cell uses the directional characteristics of the antenna to divide the cell into different sectors, as shown in the following figure. Commonly used ways are

Clover-shaped wireless area composed of directional antennas, with circular coverage 120 (Figure (a))；); Triangular wireless cellular area composed of directional antennas with 60 sector coverage (Figure (b))；); The 120 sector wireless cellular area composed of directional antennas covers the 120 sector (Figure (c)).

In frequency division multiple access (FDMA) and time division multiple access (TDMA) systems, the use of sector antennas in each cell can only reduce interference, but not increase system capacity. In CDMA cellular mobile communication system, when a cellular cell is divided into three sectors by directional antenna with sector coverage of 120 (as shown in figure (c)), the average number of mobile users in each sector is one third of that of the cell, and the corresponding multiple access interference component among users is also reduced to about one third, so that the capacity of the system will increase by about three times (actually due to the existence of adjacent sectors).

3. High system capacity

Because the code division digital cellular mobile communication system can directly or indirectly improve the system capacity by adopting the above two methods and other technologies, the capacity of the code division system is several times higher than that of the analog FDMA system and the digital GSM system. Theoretical analysis shows that, under the same frequency bandwidth, the number of channels provided by each cell is about 20 times that of analog FDMA system and 10 times that of digital GSM system. For narrowband code division system, its system capacity has certain advantages.

It is 10 times higher than analog FDMA system and 3 times higher than digital GSM system. It can be seen that with the rapid development of mobile communication today, the number of mobile users is increasing rapidly and the frequency resources are becoming increasingly tense, so it is imperative to adopt code division digital cellular mobile communication system.

4. Soft capacity

In analog frequency division system and digital time division system, communication channels are divided by different frequency bands or time slots, and once the number of channels provided by each cell is fixed, it is difficult to change. When there is no free channel, the system will have a busy tone, and mobile users can't make or receive other users' calls. When mobile users switch, it is also easy to interrupt the call. In code division system, channel division is based on different code types, and the standard number of channels is based on a certain input-output signal-to-noise ratio. When a user is added to the system, the input-output signal-to-noise ratio of all users will be reduced, but there will be no phenomenon that people cannot talk because there is no channel. For example, in a sector with 40 standard channels, when the 4th1th user calls, the impact on all mobile users is that the input signal-to-noise ratio of the receiver drops10lg (4 1/40) = 0.1db, even if two more users communicate, there are three more than the standard. Its influence is that the input signal-to-noise ratio of all receivers decreases 10lg[(40+3)/40]=2.3dB, which increases the bit error rate of mobile user information data in this sector and reduces the call quality, but the three additional users will not be busy because of insufficient channels. This is undoubtedly very beneficial to solve the communication congestion problem during the peak communication period and improve the success rate of user switching.

5. Soft handover

When a mobile user moves from one cell (or sector) to another, the mobile user moves from the jurisdiction of one base station to the jurisdiction of another base station. In order not to interrupt users' communication, the control system of communication network has to make a series of adjustments, including communication link conversion and location update. This process is called handover. Handover realizes channel switching between cells (or sectors) and ensures uninterrupted operation of the call being processed or in progress.

In analog FDMA system and digital TDMA system, mobile users need to find idle channels in another cell (or sector) when switching, and can only switch when there are idle channels in their own cell. At this time, the receiving and transmitting frequencies of the mobile station should be adjusted accordingly, which is called hard handover. This switching process is to cut off the original call path first, and then connect a new call link with the new base station. This switching mode of breaking before connecting is bound to cause a short communication interruption. In addition, due to the influence of communication environment, in the overlapping area of two cells, the signal strength received by the mobile station from two base stations sometimes changes alternately, resulting in a "ping-pong" effect in handover, and users will hear a "click", which will adversely affect communication. In addition, the switching time is also long.

In CDMA system, because all cells (or sectors) can use the same frequency, cells (or sectors) are distinguished by different code types. When a mobile user moves from one cell (or sector) to another cell (or sector), it is not necessary to switch the receiving and transmitting frequencies of the mobile station, but only to adjust the code sequence accordingly, which is called soft handover. The advantage of soft handover is that the new call is connected to the new base station first, and then the original call link is cut off. This switching mode of make-before-break will not appear "ping-pong" effect, and the switching time is also very short. In addition, because CDMA system has the advantage of "soft capacity", the handover success rate is much higher than that of analog FDMA system and digital TDMA system, especially in the peak communication period.

6. Unique forms of diversity

In CDMA system, broadband transmission has unique frequency diversity characteristics, that is, when the channel has frequency selection characteristics, it has little influence on information transmission in CDMA system.

CDMA system has the ability to separate multipath signals and can realize path diversity. Due to the complexity of the mobile communication environment and the constant movement of the mobile station, the received signal is often the superposition of multiple reflected waves, forming multipath fading. In analog FDMA system and digital TDMA system, in order to solve the adverse effects of multipath fading on communication, a series of measures have been taken, including increasing transmission power. In CDMA system, we can use its unique technology (such as RAKE receiving technology) to separate multipath signals.

This not only overcomes the adverse effects of multipath fading on communication, but also increases the power of receiving useful signals (or increases the power of transmitting signals). Due to this unique diversity form and other measures, the transmission power of CDMA system is relatively low.

In addition to this unique diversity form, CDMA system also adopts other diversity technologies, such as spatial diversity and time diversity, which greatly improves the performance of CDMA system.

7. System with narrow band (analog system) * * *

When the code division system and narrowband system (such as analog FDMA system) work in the same frequency band, because CDMA system adopts broadband transmission mode, the transmission power is low, and the average interference signal power within the bandwidth of each narrowband system is very small. Especially for wideband CDMA system, its influence on narrowband system can be ignored, and the influence of narrowband system on CDMA system can be equivalent to "human interference". Due to the unique anti-interference ability of CDMA system, this interference is reduced to a minimum.

The existence of this interference only reduces the capacity of CDMA system, but it does not hinder the normal work of CDMA system. The wider the bandwidth of CDMA system, the smaller the influence between the two systems, and vice versa. This makes it possible for CDMA system and analog narrowband system to coexist in dual mode, and also makes it possible for analog mobile communication system to make a smooth transition to digital mobile communication system.

8. Good confidentiality.

The system of code division digital mobile communication system itself determines that it has good security ability. First of all, CDMA digital mobile communication system must adopt spread spectrum technology, so that the frequency spectrum of its transmitted signal is very wide, so that the transmitted signal is completely hidden in noise and interference, and it is not easy to be found and received, thus realizing secure communication. Secondly, in the communication process, mobile users use different address codes, and only users with the same code type and phase can receive the corresponding transmission data at the receiving end, which is a background noise for irrelevant users, so CDMA system can prevent intentional or unintentional theft and has good security performance.

9. Low transmission power and long service life of mobile battery.

Because in the code division digital mobile communication system, many unique technologies can be used to improve the performance of the system, and the required transmission power is greatly reduced, which is very beneficial to the reduction of battery volume and the increase of service life, and also to the reduction of the overall volume and cost of the mobile station.

10. Simple frequency allocation and management

In analog frequency division multiple access and digital time division multiple access mobile communication systems, frequency allocation and management is a more complicated technology, and dynamic frequency allocation is more complicated. In the code division digital mobile communication system, all mobile users can use only one frequency without dynamic allocation, and its frequency allocation and management are very simple.

The above are the main advantages of code division digital mobile communication system, but at the same time it also has difficulties that people need to overcome. In CDMA digital mobile communication system, the prominent problem is the near-far effect. The so-called near-far effect refers to the interference of users close to the receiver to distant users.

In the CDMA digital mobile communication system, because all users in the same cell use the same frequency, * * * enjoys a wireless channel. Due to path fading, signals transmitted by a mobile station near a base station may completely drown signals transmitted by a mobile station far away (for example, at the edge of a cell). If effective measures are not taken, the base station will not be able to receive the signal transmitted by the remote mobile station normally. In analog frequency division multiple access and digital time division multiple access mobile communication systems, because each channel uses different frequencies or time slots, there is a corresponding protection bandwidth or protection time between channels, and the problem of near-far effect is not very prominent.

At present, there are two main measures to solve this problem in CDMA system: the first is signal processing method, which cancels the strong signals one by one at the receiving end until the required signals can be demodulated, but it can not be used in practice because of the large amount of calculation and the current equipment running speed; When the mobile station is close to the base station, its transmission power decreases, and when it is far away, its transmission power increases, thus ensuring that the signal power of each mobile station received by the base station is equal, eliminating the influence of near-far effect and making the system in the best working state. Power control technology has been applied in practice and is one of the most critical technologies in CDMA digital mobile communication system. Power control technology is very complex, and its control range and accuracy directly affect the performance of the whole system. If the deviation is too large, not only the system capacity will drop rapidly, but also the communication quality will drop sharply.

The network structure of code division digital cellular mobile communication network is shown in the following figure.

It is an abstract plan, but it will actually change with the distribution of functional entities in various physical units. The roles and functions of each part are as follows:

1. Mobile station

It includes mobile phones, car platforms and so on. , which is the equipment for the user to terminate the wireless channel; Through the air wireless interface Um, users can access network services.

2. Base station

Located in a certain place, it is the general name of all wireless devices serving one or several cells. It is a device that communicates with mobile stations under the control of mobile switching center (MSC) in a certain wireless coverage area.

3. Mobile switching center

It is a functional entity for controlling and switching mobile stations located in its service area, and it is also an automatic connection device for user service with other MSC or other public switching networks.

4. Home Location Register (HLR)

Is a location register to which a user identity is assigned for recording purposes. The registered content is the user's information (such as ESN, DN, IMSI(MSI), service item information, current location, approval validity period, etc.). ).

5. Visitor Location Register (VLR)

It is a location register used by MSC to retrieve information. For example, processing the call information sent to or from the visiting user-user number, and providing users with the services and other parameters of local users.

6. Equipment Identification Registration (EIR)

Assign the user equipment identity to its register for recording purposes; Used to identify, monitor and lock mobile devices.

7. Certification Center

It is a functional entity that manages authentication information related to mobile stations.

8. Information Center

Is an entity that stores and forwards short messages.

9. Short Message Entity (SME)

Is an entity that synthesizes and decomposes short messages. Sometimes HLR, VLR, EIR and AC are located in MSC, and SMC is located in MSC, HLR or MC.

Code division digital cellular mobile communication network is not a simple extension of public switched telephone network (PSTN), but a parallel service network with PSPDN and ISDN. Due to the large-scale movement of mobile users, the network should be relatively independent in management.

The communication capacity of a communication system can be measured by different characterization methods. For a point-to-point communication system, the communication capacity of the system can be measured by channel efficiency, that is, the maximum number of channels that can be provided within a given frequency bandwidth. Generally speaking, the more channels can be provided in a given frequency bandwidth, the greater the communication capacity of the system. In the cellular mobile communication system, there are many methods to measure the system capacity, such as the number of available channels per cell (ch/cell) and the availability per MHz of each cell.

Measure the number of channels (ch/cell/MHz), Irish number per cell (Erl/cell), users per square kilometer (users /km) and calls per hour per square kilometer (calls h/km). These characterization methods measure the capacity of the system from different angles. They are related and can be converted to each other under certain conditions. Considering that channel allocation involves frequency reuse and co-frequency interference, it is generally believed that the reason for limiting the capacity of CDMA digital cellular mobile communication system by using the number of available channels per cell (ch/cell) or per cell per MHz is that there is multiple access interference in the system, that is, mutual interference between mobile users who communicate at the same time. In a certain cell, if there are n users communicating at the same time, the system must be able to provide n or more (logical) channels. The greater the number n of users communicating at the same time, the stronger the multiple access interference. The maximum value of n is the system capacity, that is, the maximum number of users communicating at the same time in the cell when the ratio of the required signal power to the interference power is greater than or equal to a certain threshold value.

Firstly, consider the capacity of general code division communication system (that is, temporarily ignore the characteristics of cellular mobile communication system). If n users communicate at the same time, each user's signal will be interfered by other N- 1 users' signals. Suppose the power of the system

The control is ideal, that is, all N signals arriving at the receiver have the same intensity, then theoretical analysis shows that the system capacity at this time is

Where w is the effective spectrum width occupied by CDMA system; Rb is the rate of information data; Eb is one bit energy of information data; N0 is the power spectral density of interference (noise) (interference power per hertz); W/Rb is the spread spectrum gain of CDMA system. When the spectrum width w occupied by CDMA system is constant, it increases with the decrease of information rate Rb. Eb/N0 is the ratio of bit energy to noise density, which depends on the system's requirements for bit error rate or speech quality, and is related to the modulation mode and coding scheme of the system.

For example, the effective spectrum width occupied by the N-CDMA system is W= 1.2288MHz, and the speech coding rate Rb = 8.6 kbit/s. If the ratio of bit energy to noise density Eb/N0=7dB, then N = 29.5 If Eb/N0=6dB, then N=37.

The results show that the smaller the ratio of bit energy to noise density Eb/N0, the larger the system capacity. But in the above results, the characteristics of CDMA cellular system are not considered, and the system capacity formula should be modified according to its characteristics.

1. Using voice activation technology to improve system capacity

The statistical results show that the active period (duty ratio) of the dialogue is d=0.35. In other words, during the call, people only talk for an average of 35% of the time.

Another 65% of the time is in a state of listening to the other person, pausing between sentences or waiting. In CDMA digital cellular mobile communication system, all users enjoy the same wireless channel. If voice activation technology is adopted, users in communication will only transmit signals when there is voice, and the transmitter of users will stop transmitting power when there is no voice, then when any user's voice stops, the interference of other users will be reduced by 65% on average, so the system capacity can be increased to 1/d = 2.86 times. Therefore, the calculation formula of CDMA digital cellular mobile communication system becomes

Where d is the speech duty ratio (d=0.35).

2. Use sector partition to improve system capacity.

In CDMA cellular mobile communication system, when a cell is divided into three sectors by directional antenna with sector coverage of 120, the number of mobile users in each sector is one third of that of the cell, and the corresponding multiple access among users.

The interference component is also reduced to about one-third of the original, so the capacity of the system will increase by about three times (in fact, due to the overlapping coverage areas of adjacent antennas, it can generally be increased to about G=2.55 times). Therefore, the calculation formula of CDMA digital cellular mobile communication system becomes

Where g is the sector allocation coefficient (G=2.55).

3. The influence of adjacent cell interference on system capacity.

According to the characteristics of CDMA cellular mobile communication system, in CDMA cellular mobile communication system, all users share the same wireless channel, that is, base stations and mobile stations in multiple cells work on the same frequency. Therefore, the mobile station in any cell will be interfered by the base station in the adjacent cell, and the base station in any cell will also be interfered by the mobile station in the adjacent cell. The existence of these interferences will inevitably affect the capacity of the system. In which the mobile stations of any cell pair

The total interference amount of the base station (reverse channel) in the adjacent cell is different from that of the base station in any cell to the mobile station (forward channel) in the adjacent cell, and the influence on the system capacity is also different, which will be briefly explained below.

(1) Forward channel (from base station to mobile station)

In a cell, the base station continuously sends signals to all mobile stations in communication. The mobile station receives the signals it needs and the signals sent by the base station to all other mobile stations, which will interfere with the signals it needs. When the system uses forward power

In the control technology, due to the path propagation loss, the interference of the mobile station near the base station is greater than that of the remote mobile station, but the interference of the neighboring base station is less; The mobile station located at the edge of the cell is less interfered by the signal transmitted by the cell base station than the mobile station near the cell, but more interfered by the neighboring cell base stations. The most unfavorable position of the mobile station is at the junction of three cells, as shown in the following figure.

Assuming that the number of users communicating simultaneously in each cell is n, that is, the base station in each cell sends signals to n users at the same time,

When a mobile user moves from one cell (or sector) to another, the mobile user moves from the jurisdiction of one base station to the jurisdiction of another base station. In order not to interrupt communication, the control system of communication network must make a series of adjustments, including position update and communication link conversion. This process is called handover.

Handover realizes the channel switching between the cell (or sector) and the channel, and ensures the uninterrupted operation of the call being processed or in progress. Handover is caused by radio broadcasting, service allocation, operation and maintenance activation, equipment failure and other reasons. For example:

(1) The mobile station moves to the cell boundary, and the signal strength is low to a certain extent;

(2) When the mobile station enters the gap (shadow area) of signal strength in the cell, the signal deteriorates to some extent;

(3) When the mobile switching center finds that some cells are too crowded and others are idle, it can order some mobile stations in the crowded cells to switch in advance to adjust the load of each cell, and so on.

The basic requirements for handover are:

(1) high handover success rate;

(2) reducing unnecessary switching in the system;

(3) using the optimized handover algorithm to control the traffic of each cell;

(4) Fast switching speed and short switching time;

(5) It has little influence on speech quality.

There are two types of handover in CDMA system, namely hard handover and soft handover.

Hard handoff refers to the handoff of a mobile station between different channels, which requires the mobile station to change the transceiver frequency, that is, cut off the original transceiver frequency first, and then search and use a new channel.

Hard handover will cause a short interruption of the call, and when the handover time is longer (more than 200ms), it will affect the user's call.

Soft handover refers to the handover of mobile stations in the same CDMA channel. Soft handover does not require the mobile station to change the transmitting and receiving frequencies, but only needs to adjust the phase of the pseudo-random code. There are multiple RAKE receivers in the mobile station of CDMA system, which can receive signals from multiple base stations simultaneously. When handover is required, the mobile station will not only communicate with the original service.

In addition to maintaining the call link, the serving base station also established a call link with the new base station. The call link with the original base station is not cut off until the signal received by the mobile station from the original base station is below the threshold. This soft handoff ensures that the call will not be interrupted. The so-called soft handover also includes softer handover. Soft handover refers to the handover between different sectors in the same cell. At the boundary of the two sectors, the base station and the mobile station can simultaneously transmit signals in the two sectors through diversity technology.

In the soft handover process, because the mobile station has multiple RAKE receivers, when the mobile station starts to establish communication with the target base station, the communication with the original serving base station will not be interrupted, and at this time, the mobile station establishes call links with two base stations at the same time. When the signal strength of the original serving base station is low to the threshold, the communication with the original serving base station is cut off. Because the mobile station does not change the transmission and reception frequencies in soft handover, soft handover can only be performed between cells (or sectors) with the same CDMA channel.

Soft handoff is a unique and important concept in CDMA system. In CDMA cellular mobile communication system, all cells with the same CDMA channel use the same frequency, and mobile stations do not need to reallocate frequencies or time slots when moving between cells, which makes soft handover possible.

In CDMA system, each mobile station usually has more than three RAKE receivers, that is, each mobile station has multiple demodulators, which allows the mobile station to keep communication with two or more cells at the same time.

When communicating with base station A, the mobile station continuously monitors the pilot signal strength of neighboring cells, and immediately reports to the system when the strength of any pilot signal (such as base station B) exceeds a predetermined threshold. The system commands the base station B to establish communication with the mobile station and start soft handover. At this time, the mobile station receives communication signals from two base stations at the same time, and the two signals are closely combined to reinforce each other.

On the reverse link, the mobile switching center determines which base station has the stronger received signal according to the signal strength received by the base station, and then selects it.

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