CDMA system description

Wireless communications can be traced back to 1898 when the first wireless telegram was produced. The history of wireless communication service can be traced back to the 1920s when police car wireless communication was first put in use in the Detroit Public Security System of the US. The wireless communication system put into real commercial services can be traced to the 1940s when Bell laboratories of the US conducted commercial mobile wireless communication systems tests and the 1960s when a new type of mobile telephone system called for modified mobile telephone services. However, as technologies were relatively underdeveloped in those days, mobile communications did not find extensive developments. Since the last 20 years, the large-scale integrated circuit and computer technologies have paved the way to the rapid development of the commercial applications of mobile communications.

In fact, the wireless mobile communication technologies have basically been developed based on exploring new mobile communication frequency bands, reasonable use of frequency resources and minimization, portability and multifunction of mobile stations. Ever since the “cellular” theory was put forward in the 1970s, cellular mobile communications have found extensive applications. Theoretically, the principle of a cellular system is the repeated use of wireless channels, namely frequency-division multiplexing. A service area is divided into abstract hexagonal cellular cells, and two non-adjacent cells can use the same frequency, with the sizes of cells depending on the user density. This greatly improves the frequency spectrum utilization, and thus effectively improves the system capacity. In addition, owing to the development of microelectronic technology, computer technology, communication network technology, signal coding technology and digital signal processing technology, mobile communications have made quite great progress in various aspects such as switching, signaling network mechanism and wireless modulation coding technology etc., and thus the cellular mobile communication system has come through changes from analog to digital, from FDMA to TDMA and CDMA, which represent the evolution from the first generation cellular mobile communication system to the third generation cellular mobile communication system. The following paragraphs will first make a simple retrospect of these three generations of cellular mobile communication systems, then describe related system technology principles and features, and, lastly, discuss the prospect of the third generation cellular mobile communication system.

1.1.1 History of wireless cellular mobile communications

1.1.1.1 First generation cellular mobile communication system

In the late 1970s, the first generation cellular mobile communication system characterized by frequency division multiple access (FDMA) and analog frequency module (FM) came into being, pioneering the commercialization of cellular mobile communication systems. The major modes in this phase include TACS of the UK, AMPS of the US and NMT of north Europe. This phase featured defects such as low frequency utilization, small system capacity, no united international standard, very complicated equipment, high cost, requirement of certain protection bands, no effective anti-interference and anti-attenuation measures, poor voice quality, low security etc., as well as limited number of subscribers and incapability of non-voice services and digital communication services. With the development of services, the firs generation cellular mobile communication system became unable to satisfy the market requirement. Further more, in the transmission system, the voice transmission was implemented in the analog mode, while signaling gateways adopted the digital mode, resulting in ineffective control of network management.

1.1.1.2 The second generation cellular mobile communication system

In mid 1980s, the second-generation cellular mobile communication system featuring TDMA, CDMA and digital modulation (QPSK, p/4-QPSK and GMSK) appeared. The major modes in this phase include GSM of Europe, DAMPS of the US and the CDMA system put forward by Qualcomm of the US. At that time, since some critical techniques in the CDMA system were not properly solved, the development of the CDMA technology was relatively slow. However, since the GSM system adopted the TDMA technology, which was mature at that time, the utilization of frequency spectrum was increased, and the shortcomings of the analog system were well solved. Therefore it gained wide support from telecom operators and equipment manufacturers of the world, and the globally united GSM system standard was made up. However, for the very reason that this kind system used the TDMA mode, the anti-interference and anti-attenuation capability of this kind of system was still unsatisfactory, certain protection time slots were required, and the system capacity was unable to meet the growing requirements of the users. Besides, the design of this kind of system is very complicated, the frequency utilization was not high, and the hard handoff mode was adopted for inter-cell handoff, which tended to cause call drops, and was unable to satisfy the users’ growing fast data transmission and broadband video multimedia service requirements.

Nevertheless, since the CDMA technology involves multiple critical technologies, it has many unique performances, which largely increases the system capacity (analyses show that its system capacity is ten times that of FDMA, and over four times that of TDMA), and it does not require protection bands and timeslots. The CDMA technology itself has provided the basis for the realization of soft handoff and software capacity. Further more, the frequency classification in the CDMA system has become relatively simpler, and its anti-interference and anti-attenuation capabilities are also better the former two ones. In a word, the overall performances of the CDMA cellular mobile communication system are all superior to those of all the other currently existing cellular mobile communication systems.

It is because CDMA has the all the above-mentioned merits, and it is more because Qualcomm has solved some of the critical technologies, that the CDMA has attracted extensive attention from the world’s telecom businesses, which makes all believe that CDMA is the most prospective communication technology in the future wireless technology development, thus making it an outstanding one among the digital cellular mobile communication systems. The development of CDMA has been a progressive process, and the commercial products on the current market are basically all based on the IS-95A narrow-band N-CDMA technology. It is presently the development direction of CDMA to realize low-cost, high-quality, inter-connective and inter-working, and IP-supporting and data-supporting services and wireless intelligent network (WIN) services, aiming at providing users with convenient and effective communication services, on the basis on the existing narrow-band N-CDMA. From the point of view of the communication technologies and people’s requirements, the future wireless communication world will be a broadband, comprehensive data and multimedia network. The broadband CDMA technology will be an import pillar supporting this network.

1.1.2  The third generation cellular mobile communication system

1.1.2.1 The drive for the development of the third generation cellular mobile communication system

The first generation cellular mobile communication system represented by AMPS and TACS has solved the people’s calling-while-moving problem, and greatly satisfied the users’ requirements. However, as the first generation mobile system had such problems as poor voice quality, low frequency spectrum utilization, poor security etc., it was soon replace by the digital second generation cellular mobile communication system represented by GSM and IS95. Compared with the first generation, the second generation cellular mobile communication system been greatly improved in aspects such as voice quality, frequency utilization, security and privacy, and has satisfied the people’s requirement within a period of time. Along with the development of mobile communication technologies and the growth of the scale of mobile communications, the shortcomings of the second generation cellular mobile communication system have been gradually uncovered.

1. Scanty Wireless Frequency Resource

The rapid growth of the number of mobile subscribers has caused the frequency resource of the second generation cellular mobile communication system to become relatively insufficient. The fastness of the mobile communication development has gone far beyond people’s expectation. Today, China has over 60 million mobile subscribers, and the number is growing at a speed of 10 to 20 million per year. It is believed that China will have 350 million mobile subscribers by the year 2010. As a result of system capacity expansion, cells of certain major cities have shrunk to less than 500 meters, and the system capacity can hardly be further increased by means of cell splitting. On the other hand, the small cell ranges are causing frequent handoffs and serious interference, which greatly lower the voice quality.

Low frequency utilization is another reason for the scanty frequency resource. Compared with the first generation mobile communication system, the second generation cellular mobile communication system that uses digital technology has greatly improved the frequency utilization. However, when compared with the third generation cellular mobile communication system that uses the CDMA technology as its kernel, its frequency utilization is still low.

2. Unable to Satisfy the Requirements of New Services

The second generation cellular mobile communication system adopts the voice-oriented design. To provide high-quality and high-efficiency voice services is the main objective of the second generation cellular mobile communication system. Along with the development of the Internet and e-business, data services will take the dominating position. In the future, multimedia services with the medium- and high-speed data services as the bearer will become the application most frequently used by the users, and, as second generation cellular mobile communication system with voice services as its main design objective can hardly provide high-speed data services, and therefore it is doomed to be replaced by the new generation.

1.1.2.2 Brief descriptions of the third generation cellular mobile communication system

The third generation cellular mobile communication system (3G) is also called IMT-2000, implying that the system’s working frequency band is 2000MHz, and its maximum service rate can be as high as 2 Mbit/s. Its technical basis is broadband W-CDMA, characterized mainly by multimedia and intelligent features. It can improve the multi-element transmission rate, and realize the general integration of ground cellular system, cordless system, cellular mobile communication system and satellite system - the real global services. It provides a unified platform for the combination and distribution of various services. Although the third generation cellular mobile communication system still has room for perfection, the general framework has been defined. It has the following tree major features:

Seamless global roaming.

High-speed transmission. High-speed mobile environment: 144kbit/s; walking low-speed mobile environment: 384kbit/s; Indoor static environment: 2Mbit/s;

Seamless service transfer. That is, interworking is available in fixed networks, mobile networks and satellite services.

The technology of 3G is the multimedia communication system that uses the IP technology as bearer to realize end-to-end IP and provide multiple serviced. Although the development of 3G and the formulation of its standard have been held up due to different technical, political and commercial interests, and there are as many as ten commercial standards for 3G have been put forward up to now, yet the basis for the transmission mode of all these standards is CDMA.

The following paragraphs will present a simple description of the 3G system structure.

1. System Vertical Layers

Bearer Layer

Located at the bottom of the structure is the bearer layer. The IP technology-centered bearer layer is responsible for the transmission and routing of all the data applied on the upper layer, including voice, data and video frequency etc. As the corner stone of the future third generation cellular mobile communication system, the IP protocol should have major progresses in various aspects such as security, efficiency, address space etc., should be able to provide end-to-end QOS guarantee, and should be able to use multiple transmission mechanisms, such as IP Over ATM, IP Over SDH and IP Over DWDM. High speed, high efficiency and flexibility will its main features.

Switching Layer

The second layer is the switching layer. In this layer contains multiple servers with concentrated functions, that is each server implements a certain specific function. For example, the CSCF call status control server is responsible for call establishment, maintenance and release, the RADIUS server performs subscriber identity authentication, the HSS (Home Subscriber Server) stores various subscription and location information of the subscribers and takes part in the mobility management, and the VOD server provides the VOD server. By coordinating with one another, these servers can provide some basic services. For example, by cooperation with other entities, the CSCF server can provide the basic voice service.

Application Layer

The highest layer is the application layer, which is equivalent to the SCP layer in an intelligent network. The functional entity of this layer work in coordination with various functional servers of the switching layer to control the connection flow of subscriber calls and quickly generate various new services to satisfy the users’ requirements.

2. System Lateral Layers

3G mobile Station

The 3G mobile stations should completely support the IP protocol and various applications on the IP protocol, such as Web browsing, VOD etc. It should become the center of the future personal office work and entertainment.

Full-IP Radio Access Network

The RAN system of 3G supports all-roundly the high-speed packet services, and can perform transparent transmission of IP data. RAN is also responsible for wireless resource management, including the distribution, maintenance and release of the subscriber resources, and implements the mobility management by coordinating with other entities.

Full-IP Core Network

The kernel network is responsible for the subscribers’ call control, multimedia data flow transmission, routing etc., so as to provide abundant multimedia services for the subscribers. The core network of 3G is connected with other networks through various media gateways. For example, it is connected with the PSTN via signaling and transmission gateways, with the Internet via PDSN, and with the traditional second generation networks through roaming gateways.

1.1.2.3 Process of evolution from 2G to 3G

As mentioned above, there are presently mainly two research and development directions, and the evolution from the IS-95A-based narrow-band N-CDMA system to 3G is shown in Fig. 1-1.

Fig. 1-1 Evolution from 2G to 3G

In Fig. 1-1, IS95-A integrates the IP protocol in the mobile phone, and it is not necessary to include the IP layer in the network’s packet transmission layer. As the result, the hardware is compatible with all the IP-based standard networks in the future. The data transmission rate of the IS95-A network is 14.4kbit/s; IS95-B increases the data transmission rate to 64kbit/s by upgrading the core network and wireless network, and makes CDMA a packet mode network by adding a data basis device through the base station controller; as the first phase of CDMA2000, 1XRTT doubles the voice capacity, and increases the data transmission rate to 144kbit/s, and it is estimated that the typical rate available for the subscribers is 130kbit/s; 1XEVDO can provide high-speed packet data service on a carrier frequency. If the subscribers require voice or any other real-time service, the 1XEVDO system will automatically returns to 1XRTT, and execute and complete that service, and this process is transparent to the subscribers; 1XEVDV is the second phase of CDMA2000, with its object being integrating the capability on the first phase to the same carrier frequency, while keeping the capability of transmitting packet data services on separated carrier frequency. This phase provides real-time, non-real-time, mixed real-time/non-real-time service modes, and a data transmission rate as high as 2Mbit/s.

1.2  Basic concepts of CDMA wireless transmission system

1.2.1 Wireless multiple access communication

As we all know, it is a primary issue that must be considered in any transmission system how to establish channel links among subscribers within the network in the radio wave coverage area in the environment of wireless communication. In fact, the essence of this question is a question of multiple address mobile communication. The wireless multiple access modes currently in use include: FDMA in analog systems, and TDMA and CDMA in digital systems. The theoretical basis for the realization of multiple access connections is the signal division technology. That is, suitable signal design is made at the transmitting end so that the signals sent from different stations are different; the receiving end has the signal identifying capability, and can choose the corresponding signal from mixed signals.

When multiple access mobile communication is established based on the difference of carrier frequencies of the transmission functions, the multiple access mode is called Frequency Division Multiple Access (FDMA); when multiple access mobile communication is established based on the difference of signal existence time, it is called Time Division Multiple Access (TDMA) mode; when the multiple access mobile communication is established based on the difference of transmission signal code forms, it is called Code Division Multiple Access (CDMA) mode. Fig. 1-2 gives a schematic diagram of the time domains and frequency domains of FDMA, TDMA and CDMA transmission processes.

Fig. 1-2 Schematic diagram of time domains and frequency domains of FDMA, TDMA and CDMA

1.2.2  Concept of CDMA

The so-called CDMA refers to such a technology that the transmitting end modulates the signals that it sends using mutually different and (quasi) orthogonal pseudo-random address codes, and the receiving end detects the corresponding signals by demodulating the mixed signals using the same pseudo-random address codes.

1.2.3 Concept of spread spectrum communication

A spread spectrum technology is adopted in CDMA transmission systems. The so-called spread spectrum technology refers to such a technology that the original signals are converted to transmission signals with much wider bandwidth the original, so as to achieve the anti-interference purpose of the communication system. Its mathematic model is the Shanon equation in the information theory. That is, under the condition of noise interference, the channel capacity is:

C = B log2 (1 + S / N)    

Where, B is the channel bandwidth, S is the average signal power, N is the average noise power, and C is the channel capacity.

From the above equation, we can see: when S/N decreases, the purpose of high quality communication can be achieved without reducing the system capacity, as long as the bandwidth B is increased.

1.2.4 Technical features of CDMA

Based on the above analysis, it can be deduced that CDMA has the following technical features:

1. Invisibility and security;

2. Strong anti-interference and anti-multi-path ability;

3. Realization of multiple access technology, increase of capacity and improvement of frequency reuse pattern;

4. Wide frequency band seizure, increased system complicity and high synchronization requirement.

1.2.5  Principle of CDMA transmission system

1.2.5.1 CDMA wireless transmission system structure

In CDMA communication systems, the pseudo random address codes are periodic code series with strong self-correlation but 0 or very small mutual correlation. Based on the different signal modulation modes, CDMA systems can be divided into DS-CDMA system and MC-CDMA system.

In a DS-CDMA system, i.e. the so-called direct spread code division multiple assess system, specific spread spectrum codes are used at the transmitting end to perform time domain spread spectrum processing to the original signals, and the same spread codes are used at the receiving end for the signal demodulation to obtain finally the required useful signals. In a MC-CDMA system, i.e. the so-called multi-carrier code division multiple assess system, specific spread spectrum codes are used at the transmitting end to perform frequency domain spread spectrum processing to the original signals, and the same method is used at the receiving end for the signal demodulation to obtain finally the required useful signals. Since the MC-CDMA system works in frequency domain, the fast Fourier transformation (FFT) technology must be employed at the transmitting end, while inversed Fourier transformation (IFFT) technology must be used at the receiving end. 

In a commercial CDMA cellular mobile communication system, CDMA is mainly combined with the direct spreading technology to form the DS-CDMA system. The system’s transmission in both forward and backward directions is sketched in Fig. 1-3.

Fig. 1-3  Sketch of forward and backward transmission and receiving in CDMA transmission system

1.2.5.2 Communication Standards for CDMA Wireless Transmission System

The main standards used in the CDM process are as follows:

1. Either in forward or backward direction, the signals have to be pre-coded first, and corresponding decoding processing is to be performed in the respective receiving process;

2. Frequency division duplex (FDD) mode is adopted as the transmission mode;

3. Qualcom variable rate code-excited linear prediction (Q-CELP) mode is used for voice coding;

4. The convolution coding and block interleaving combination mode is adopted for channel error correction;

5. QPSK is adopted for forward modulation, and p/4-QPSK is adopted for backward modulation;

6. The spread spectrum signal rate is 1.2288Mbit/s;

7. Frequency bands: 824-849MHz (backward channels/BS receiving), 869-894MHz (forward channels/BS transmission);

8. Carrier separation: 1.25MHz.

1.2.6 Critical technologies in CDMA wireless transmission system

Several new technologies are used in the CDMA wireless transmission system to improve the system’s safe and stable operation, and thereby the system’s service quality has been largely enhanced. The following paragraphs will present a brief introduction of the major critical technologies.

1.2.6.1 Voice coding technology

The CDMA wireless transmission system adopts Q-CELP variable rate vocoder technology. The purpose is to lower the data transmission rate as much as possible while keeping the communication quality at a certain level. Q-CELP mainly uses code table vector quantification differential signals, and then generates a variable output data rate based on the voice activation level. Generally speaking, for a typical two-party call, the average output data rate is almost twice, or more than twice, lower than the maximum data rate.

The implementation process is briefly described as follows: The input voice signals are sampled at 8kHz first, then they are divided into many 20ms-long frames to generate sub-frames - parameter frames containing three types of parameters (linear prediction code filter, tone parameter and code table parameter). The three types of parameters are constantly updated, and the updated parameters are transmitted to the receiving end according to a certain frame structure. Of these parameters, the linear prediction code filter parameter is updated once per 20ms (one frame) under any data rate, while the tone parameter and code table parameter change with the selected data rate. The implementation sketch is shown in Fig. 1-4.

Fig. 1-4  Q-CELP variable rate vocoder block diagram

1.2.6.2 Voice activation technology

Generally, the mobile subscriber voice activation unremittance probability is 35%. In the CDMA transmission system, making use of this feature, when all subscribers share the same wireless channel and at the instance when there is no information transmission among the subscribers, the vocoder output rate controller transmitting power is reduced or stops transmission, thus the system capacity is increased by nearly 3 times.

1.2.6.3 Synchronization technology 

In the CDMA transmission system, the importance of synchronization lies in the system’s full application of orthogonality of spread spectrum codes. It is due to the introduction of synchronization technology, the signals of various channels are orthogonal to one another rather than introducing interference (in fact, synchronization error may introduce some interference, but with a very small level). The realization of synchronous CDMA includes three processes: synchronization detection, synchronization establishment and synchronization holding.

1.2.6.5 Power control technology

In the CDMA transmission system, the condition for the separation of the signals of different mobile stations using the CDMA method is that the powers of the received signals of various channels are basically the same, and the method to ensure the same power of various signals is to control the transmitting power of the base stations and mobile stations. The power control technologies include forward power control technology and backward power control technology. The backward power control technology can be further divided into mobile station-involved backward loop control technology and mobile station and base station jointly involved closed loop and outer loop control technology. No matter forward power control technology or backward power control technology, this rule must be followed: power decrease should be fast and power increase should relative slow.

1.2.6.6 Soft handoff technology

In the CDMA transmission system, soft handoff technology refers to the inter-cell handoff using “connecting the new cell before disconnect the original one” mode, and it may occur in the following three cases: between different sectors with the same BTS, between different BTSs within the same BSC, and between different BSCs within the same MSC.

1.2.6.8 Diversity technologies 

In order to thoroughly eliminate the signal attenuation phenomenon caused by multi-path attenuation, the CDMA transmission system has introduced the diversity technologies. In the CDMA transmission system, three types of diversity technologies have been introduced: time diversity, frequency diversity and space diversity.

1.2.6.9 Multi-access technology 

1. Walsh Codes

Differentiating forward channels: In the CDMA system, each forward code division channel uses 64-level Walsh functions of the bit rate of 1.2288Mbit/s for spectrum spreading, so that the forward code division channels are mutually orthogonal.

2. PN Codes

2¹⁵-1 short code: To differentiate base stations;

2⁴²-1 Long code: To differentiate mobile stations backward, and used for scrambling forward.

In the CDMA system, two m series are used, one is 242-1 (r=42) long and the other is 2¹⁵-1 (r=15) long. In forward channels, the m series with length of 2⁴²-1 are used to scramble the service channels, and the m series with the length of 2¹⁵-1 are used for orthogonal modulation of the forward channels. Different base stations use m series with different phases for modulation, with the minimum phase difference being 64 bits. Thus, there can be up to 512 phases available.

In backward channels, the series with the length of 2⁴²-1 are used for direct spectrum spreading, with each subscriber allocated with phase of one m series. Calculated by the users’ ESN, these m series phases are randomly distributed and non-repeated, and these users’ backward channels are basically orthogonal to one another. The PN code with the length of 2¹⁵-1 is also used for orthogonal modulation of backward service channels. However, as it is not necessary to differentiate the base stations on backward channels, the m series of the same phase is used for all mobile stations, with its phase offset being 0.

1.2.6.10 RAKE receiver

The forward channel receiver (mobile station) in the CDMA transmission system is equipped with three correlators and one searching correlator. The signals modulated by QPSK are sent these three correlators, which implement the separation and reception of the signals of these three paths. The searching correlator is used to give the time delay values t1, t2 and t3 of the related address codes, and then the receiving system performs comparison between the delay data and the code elements to determine the path to be received and the correct sampling and judgment of the weight circuit, and finally obtain end maximum output signal signal-to-noise ratio. In the backward channel receiver (within base station), the signal processing mode is basically the same with that in the forward channel receiver, but with a n additional space diversity receiving circuit.

1.2.6.11 Network and control technologies

The important effect of mobile communication cannot be brought into full play until a huge network is built up. Therefore, the network and control technologies appear vitally important, and that is why the modern digital mobile communication technology includes not only the latest development of wireless and wired communications, but also the computer control technologies and network technologies. Similarly, the CDMA system system’s many supreme features are realized by means of the extremely complicated but flexible and reliable network and control technologies in the system.

The initial control is implemented on the wireless interface (i.e. the Uｍ interface between the mobile station and the base station) through the pilot channel, synchronizing channels and paging channel in the forward channels, and the access channel in the backward channels. After the establishment of communication, the control is implemented only by means of the signaling service multiplexed in the service channel between the forward channels and backward channels (such as inter-cell handoff, power control technology etc.).

In addition, complex interface, signaling, network, maintenance and management (OMC) and control technologies exist between a base station’s BTS and BSC (Abis interface), between a BSC and MSC, and between BSCs in the same MSC. Especially, the interface, signaling, network and control technologies at the MSC are the most complicated. This is the very reason that effective control must be exercised on the network in order for the safe operation of the network.