What is CDMA

Cellular services are now being used every day by millions of people worldwide. The number of customers requiring such services is increasing exponentially, and there is a demand for integration of a variety of multimedia services. The range of services includes short messaging, voice, data, and video. Consequently, the bit rate required for the services varies widely from just 1.2 kbps for paging up to several Mbps for video transmission. Furthermore, supporting such a wide range of data rates with flexible mobility management increases network complexity dramatically.

The CDMA is a digital modulation and radio access system that employs signature codes (rather than time slots or frequency bands) to arrange simultaneous and continuous access to a radio network by multiple users. Contribution to the radio channel interference in mobile communications arises from multiple user access, multipath radio propagation, adjacent channel radiation and radio jamming.

The spread spectrum system’s performance is relatively immune to radio interference. Cell sectorisation and voice activity used in CDMA radio schemes provide additional capacity compared to FDMA and TDMA. However, CDMA still has a few drawbacks, the main one being that capacity (number of active users at any instant of time) is limited by the access interference. Furthermore, Near-far effect requires an accurate and fast power control scheme. The first cellular CDMA radio system has been constructed in conformity with IS-95 specifications and is now known commercially as cdmaOne.

There is increasing demand for data traffic over mobile radio. The mobile radio industry has to evolve the current radio infrastructures to accommodate the expected data traffic with the efficient provision of high-speed voice traffic. The General Packet Radio Service (GPRS) is being introduced to efficiently support high-rate data over GSM. GPRS signalling and data do not travel through GSM network. The GPRS operation is supported by new protocols and new network nodes: Serving GPRS support node (SGSN) and Gateway GPRS support node (GGSN). One prominent protocol used to tunnel data through IP backbone network is the GPRS tunnel protocol (GTP). GPRS obtains user profile data using location register database of GSM network. GPRS supports quality of service and peak data rate of up to 171.2 kbps with GPRS using all 8 timeslots at the same time. GPRS uses the same modulation as that used in GSM, that is Gaussian Minimum Shift Keying (GMSK) with 4 coding schemes. GPRS packetises the user data and transports it over 1 to 8 radio channel timeslots using IP backbone network.

The Enhanced Data Rates for GSM Evolution (EDGE) employs an Enhanced GPRS (EGPRS) to support data rate up to 384 kbps through optimised modulation. EGPRS support 2 modulation schemes, namely GMSK with 4 coding schemes and 8-PSK with 5 coding schemes. Unlike GPRS where header and data are encoded together, headers are encoded separately in EGPRS.

The IS-95 CDMA system is a narrow band radio system. Bandwidth is limited to 1.25 MHz and a chip rate of 1.2288 Mcps. The system is intended to provide voice and low bit rate data service using circuit-switching techniques. Data rate varies from 1.2 kbps to 9.6 kbps. Forward (base station to mobile) and reverse (mobile to base station) link structures are different and each is capable of distinctive capacity. Forward transmission is coherent and synchronous while the reverse link is asynchronous. The 'chanellisation' in each link is achieved by using 64- chip orthogonal codes, including provision for pilot, synchronisation, paging, and network access. Consequently, the number of active users able to simultaneously access the network is limited by the level of interference, service provisions and the number of 'channels' available. In IS-95B, an active mobile always has a fundamental code channel at 9.6 kbps and when high data rate is required, the base station assign the mobile up to 7 supplementary code channels. Thus peak data rate is up to 76.8 kbps. Data rate is controlled at the base station and conveyed to mobile through the supplementary channel assignment message.

The Wideband CDMA (W-CDMA) system is the major standard in the next-generation Global Mobile Telecommunications standard suite IMT-2000. The W-CDMA supports high data rate transmission, typically 384 kbps for wide area coverage and 2 Mbps for local coverage for multimedia services. Thus W-CDMA is capable of offering the transmission of voice, text, data, picture (still image) and video over a single platform. However, in addition to the drawbacks arising from the mobile environment and multiple access interference, high bit rate transmission causes Inter-symbol interference (ISI) to occur. The ISI therefore has to be taken into account during transmission. The W-CDMA has 2 versions: frequency division duplex (FDD) and time division duplex (TDD).

The FDD version of W-CDMA will operate in either of the following paired bands:

Uplink: 1920 - 1980 MHz Downlink: 2110 - 2170 MHz

Uplink: 1850 - 1010 MHz Downlink: 1930 - 1990 MHz

The 3GPP architecture of the Universal Mobile Telecommunications System (UMTS) is composed of IP-based core network (CN) connected to the user equipment through UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN consists of a set of radio network subsystem comprising a radio controller and one or more node base station. The network controller is responsible for the handover decisions that require signalling to the user equipment. Each subsystem is responsible for the resources of its set of cells and each node B has one or more cells.

3GPP Release 2000 on the architecture for an all IP mobile networks proposed 2 reference architectures. The 1^st option is based on packet technologies and IP telephony for simultaneous real time and non real time wireless mobile services. The 2^nd option support the IP based services and also support the release 99 circuit switched terminals.

Multi-carrier modulation (MCM) is a data transmission technique where several subcarriers are employed to transport the user’s data stream signal. Originally this technique was implemented using a bank of analogue Nyquist filters which provide a set of continuous-time orthogonal basis functions. Today using very fast and cost effective digital signal processors, multi-carrier modulation can be implemented using discrete Fourier transform (DFT) as the set of orthogonal subcarriers. This makes the technique very attractive.

Multi-Carrier Modulation (MCM) improves system capacity by making transmission more robust to frequency selective fading and enhances user spectral efficiency. The main drawbacks are:

• Difficult subcarriers' synchronisation in fading transmissions.
• Sensitivity to frequency offset is more pronounced than for a single carrier.
• Sensitivity to non-linear amplification (peak factor problem).

To gain the advantages of both schemes (CDMA & MCM), a combination known as multi-carrier CDMA ( MC-CDMA ) was proposed in 1993 taking after both CDMA & MCM schemes.

An MC-CDMA transmitter spreads the original data stream in the frequency domain over different subcarriers using a given spreading code. In this system the subcarriers convey the same information at one time. The MC-CDMA offers better frequency diversity to combat frequency selective fading.

Figure 1. MC-CDMA transmitter

Figure 2. Spectrum of MC-CDMA signal

The simplicity of the multi-carrier system is an important aspect in a cellular system especially for the down link receiver (mobile station). The modulation-demodulation is done by IDFT - DFT. A wavelet-based system can be used instead of DFT for the multi-carrier modulation. Wavelet transform has a property of time-frequency multi resolution. By choosing the right wavelet function and scaling function, the system can achieve the optimum resolution according to need.

Digital communication systems can be viewed as general transmultiplexer systems, which consist of synthesis part and analysis part. The element, which plays an important role in characterisation of the system, is the filter set used in both synthesis and analysis parts. The time-frequency properties of these filters, i.e. time spread and frequency spread, will determine the type of communication systems (TDMA, FDMA, CDMA, OFDM, MC-CDMA, MC-DS-CDMA).

Consequently, the key decision is how to design and optimise this set of filters according to their applications. One of the optimisation results for multi-carrier systems is to use one of perfect reconstruction quadrature mirror filter (PR-QMF) types which is called discrete wavelet multi tone (DWMT). Using this DWMT system for MC-CDMA cellular system, yields the following advantages:

• lower interchannel interference
• more robust against multipath fading
• more robust against narrow band interference or jamming signal

From the above advantages, it can be expected that this wavelet-based MC-CDMA system will provide higher spectral efficiency and more capacity.

Figure 3. A PSD Comparison of WB-MC-CDMA and FB-MC-CDMA

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