Evolution Of Telecommunication Systems Computer Science Essay

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Wireless digital communications is an emerging field, which has seen enormous growth in the last several years. The huge uptake rate of mobile phone technology, Wireless Local Area Networks (WLAN) and the exponential growth of the Internet have resulted in an increased demand for new methods of obtaining high capacity wireless networks [1].

In 1990, a mobile telephone was still quite expensive, whereas today most teenagers have one, and they use it not only for calls but also for data transmission. More and more computers use wireless local area networks (WLANs), and audio and television broadcasting has become digital .Many of the above-mentioned communication systems make use of one of two sophisticated techniques that are known as Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA) [2] .

Most WLAN systems currently use the IEEE802.11b standard, which provides a maximum data rate of 11 Mbps [3]. Newer WLAN standards such as IEEE802.11a [4] and HiperLAN2 [5] are based on OFDM technology and provide a much higher data rate of 54 Mbps. However systems of the near future will require WLANs with data rates of greater than 100 Mbps, and so there is a need to further improve the spectral efficiency and data capacity of OFDM systems in WLAN applications.

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Most first generations systems were introduced in the mid 1980's, and can be characterized by the use of analog transmission techniques, and the use of simple multiple access techniques such as Frequency Division Multiple Access (FDMA). First generation telecommunications systems such as Advanced Mobile Phone Service (AMPS) only provided voice communications. They also suffered from a low user capacity, and security problems due to the simple radio interface used.

Second generation systems were introduced in the early 1990's, and all use digital technology. This provided an increase in the user capacity of around three times. This was achieved by compressing the voice waveforms before transmission.

Third generation systems are an extension on the complexity of second-generation systems and was begin roll out of services sometime after the year 2001. The capacity of third generation systems is expected to be over ten times original first generation systems. This was be achieved by using complex multiple access techniques such as CDMA, or an extension of TDMA, and by improving flexibility of services available. 

Table 1 and Table 2 show some of the major cellular mobile phone standards in North America and Europe.

Table 1 Major Mobile Standards in North America [6].

Cellular System

Year of Introduction

Transmission Type

Multiple Access Technique

Channel Bandwidth

System Generation

Advanced Mobile Phone System (AMPS)

1983

Analog

FDMA

30kHz

First

Narrowband AMPS (NAMPS)

1992

Analog

FDMA

10kHz

First

U.S. Digital Cellular (USDC)

1991

Digital

TDMA

30kHz

Second

U.S Narrowband Spread Spectrum(IS-95)

1993

Digital

CDMA

1.25MHz

Second

Wideband cdma One

2000

Digital

CDMA

-

Third

Table 2 Major Mobile Standards in Europe [6].

Cellular System

Year of Introduction

Transmission Type

Multiple Access Technique

Channel Bandwidth

System Generation

E-TACS

1985

Analog

FDMA

25kHz

First

NMT-900

1986

Analog

FDMA

12.5kHz

First

Global System for Mobile (GSM)

1990

Digital

TDMA

200kHz

Second

Universal Mobile Tele-communications System (UMTS)

>2000

Digital

CDMA/ TDMA

-

Third

1.2.2Third Generation Wireless System

Third generation (3G) mobile systems such as the Universal Mobile Telecommunications System (UMTS) and CDMA2000 are introduced in the 2001 [7]. These systems are striving to provide higher data rates than current 2G systems such as the Global System for Mobile communications (GSM) [8] and IS-95. 2G systems are mainly targeted at providing voice services, while 3G systems will shift to more data oriented services such as Internet access [9].

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3G systems use Wide-band Code Division Multiple Access (WCDMA) as the carrier modulation scheme. This modulation scheme has a high multipath tolerance, flexible data rate, and allows a greater cellular spectral efficiency than 2G systems. 3G systems will provide a significantly higher data rate (64 kbps - 2 Mbps) than 2G systems (9.6 - 14.4 kbps). The higher data rate of 3G systems will be able to support a wide range of applications including Internet access, voice communications and mobile videophones. In addition to this, a large number of new applications will emerge to utilize the permanent network connectivity, such as wireless appliances, notebooks with built in mobile phones, remote logging, wireless web cameras, car navigation systems, and so forth. In fact most of these applications will not be limited by the data rate provided by 3G systems, but by the cost of the service.

The demand for use of the radio spectrum is very high, with terrestrial mobile phone systems being just one of many applications vying for suitable bandwidth. These applications require the system to operate reliably in non-line-of-sight environments with a propagation distance of 0.5 - 30 km, and at velocities up to 100 km/hr or higher. This operating environment limits the maximum RF frequency to 5 GHz, as operating above this frequency results in excessive channel path loss, and excessive Doppler spread at high velocity. This limits the spectrum available for mobile applications, making the value of the radio spectrum extremely high.

1.2.3 Fourth Generation System and Beyond

Research has just recently begun on the development of fourth generation (4G) mobile communication systems. The commercial rollout of these systems is likely to begin around 2008 - 2012, and will replace 3rd generation technology. Few of the aims of 4G networks have yet been published, however it is likely that they will be to extend the capabilities of 3G networks, allowing a greater range of applications, and improved universal access. Ultimately 4G networks should encompass broadband wireless services, such as High Definition Television (HDTV) (4 - 20 Mbps) and computer network applications (1 - 100 Mbps). This will allow 4G networks to replace many of the functions of WLAN systems. However, to cover this application, cost of service must be reduced significantly from 3G networks. The spectral efficiency of 3G networks is too low to support high data rate services at low cost. As a consequence one of the main focuses of 4G systems will be to significantly improve the spectral efficiency.

A significant improvement in spectral efficiency will be required in order for 4G systems to provide true broadband access. This will only be achieved by significant advances in multiple aspects of cellular network systems, such as network structure, network management, smart antennas, RF modulation, user allocation, and general resource allocation [9].

Figure (1.1), Current and future mobile systems. The general trend will be to provide higher data rates and greater mobility [9].

1.3 The Multipath Effects and Diversity Techniques

In a radio link, the signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles. This gives rise to multiple transmission paths at the receiver. Figure (1.2) shows some of the possible ways in which multipath signals can occur.

Figure (1.2) Multipath Signals [6].

The effects of channels are many Rayleigh fading, Frequency Selective Fading, Delay Spread, Attenuation and if in move Doppler Shift.

The solving of the effects (not all) by Diversity Techniques, The method commonly employed to overcome the degradation in performance due to fading is the use of diversity. The goal of diversity is to reduce the fading effect by supplying the receiver with multiple replicas of the signal that have passed through a multipath channel. If the signal bandwidth is sufficiently wide, as in CDMA communication, a receiver can resolve the multipath components and combine the multipath copies in an advantageous manner. For spread spectrum CDMA, a modern implementation of multipath diversity involves the use of a Rake receiver. [10, 11]

Polynomial cancellation coding (PCC) is diversity technique that makes OFDM much less sensitive to frequency errors and more tolerant to multipath with large delay spreads [12]. An OFDM system with PCC is called a polynomial cancellation coded OFDM (PCC-OFDM).

Accurate frequency domain representation of OFDM subcarriers are presented and the relationship of the spectral roll off and frequency is given for the power spectrum in the case of both OFDM and PCC-OFDM. In addition, the effect of PCC on the time domain signal is discussed.

1.4 Literature Survey

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This survey can be classified into three fields: one dealing with OFDM structure (with new transform) by PCC and the second for multi-user CDMA terminals and the third concern with the use of Space Time Block Code (STBC).

OFDM Literature Survey

The concept of using parallel data transmission by means of frequency division multiplexing (FDM) was published in mid of 60s [13]. Some early development can be traced back in the 50s. A U.S. patent was filled and issued in January 1970. The idea was to use parallel data streams and FDM with overlapping subchannels to avoid the use of high speed equalization and to combat multipath distortion as well as to fully use the available bandwidth. The initial applications were in the military communications. In 1971, Weinstein and Ebert [14] applied the DFT to parallel data transmission system as a part of the modulation demodulation process. In addition to eliminating the banks of subcarrier oscillators and coherent demodulators required by FDM, a completely digital implementation could be built around special purpose hardware performing the FFT. Recent advances in VLSI technology enable making of high speed chips that can perform large size FFT at affordable price.

In the 1980's [13], OFDM has been studied for high speed modems, digital mobile communications and high density recording. One of the systems used a pilot tone for stabilizing carrier and clock frequency control and trellis coding was implemented. Various fast modems were developed for telephone networks.

In 1990's, OFDM has been exploited for wide band communications over mobile radio FM channels, HDSL, ADSL,VHDSL, DAB and HDTV terrestrial broadcasting.

S.A.Karim [15], studied details of the OFDM technique and presented its features and characteristics over a wireless communication channel. A model for the OFDM transceiver is proposed, also the proposed system has been implemented on TMS320C6711 using cyclic prefix and zero pad approach.

In 2001, B.G.Negash and H.Nikookar [16] published their paper that deals with unconventional implementation of OFDM by changing its traditional bases function by some wavelet bases functions and deduce that wavelet based MCM transmission reduces the ICI power significantly and ISI slightly.

In 2004, H.Zhang et al [17] present their research that deals also with DWT-OFDM.

In 2005, G.R.Hassan [18] presents a comparative study between Wavelet based OFDM and conventional OFDM with different types of channels and modulation techniques.

PCC-OFDM Literature Survey

PCC-OFDM is a coding method to reduce the high sensitivity of OFDM to frequency errors caused by differences between transmitter and receiver local oscillators.

In 1996, Zhao and Häggman [19] have described a method of reducing sensitivity to frequency errors, which they call self-ICI cancellation.

In 1999, J.Armstrong [19] presents a new higher order ICI cancellation schemes.

Zhao and Häggman [20] introduce a work concentrates on improving Carrier to Interference power Ratio (CIR) and proved theoretically and by simulations, they also showed that their proposed scheme works well in a multipath radio channel with Doppler frequency spread and under the condition of the same bandwidth efficiency and larger frequency offsets, their proposed model performs much better than standard OFDM system.

A.S.Alaraimi and T.Hashimoto [21] analyzed ICI due to time variation of the channel and proposed an approximation for the average signal to interference plus noise power ratio (SINR).

1.4.3 Multi-user CDMA terminals

M.JunttiSu (1997) in [22] presented ,over a very interesting Ph.D. research, a deep analysis of multiuser demodulation algorithms for centralized receivers applied to asynchronous direct-sequence (DS) spread-spectrum code-division multiple-access (CDMA) systems in frequency-selective fading channels. The approximation of ideal infinite memory-length (IIR) linear multiuser detectors by finite memory-length (FIR) detectors was studied. In addition to this, the author demonstrated, aided with numerical examples, the fact that moderate memory-lengths of the FIR detectors are sufficient to achieve the performance of the ideal IIR detectors even under severe near-far conditions. Multiuser demodulation in relatively fast fading channels was investigated. The optimal maximum likelihood sequence detection receiver and suboptimal receivers were considered. The parallel interference cancellation (PIC) receiver was evaluated. Also, multiuser receivers for dynamic CDMA systems were studied. Algorithms for ideal linear detector computation were derived and their complexity was examined.

P. Schniter and C. R. Johnson (1998) in [23] considered blind estimation of linear chip-spaced receivers for the demodulation of a particular short code DS-CDMA mobile user under multipath propagation and in the absence of timing information. A family of schemes for blind acquisition and equalization was proposed. Also a specific algorithm that uses the second and fourth-order moments of a pre-whitened chip rate received signal was elaborate and found to be a near-far resistant initialization procedure for the Constant Modulus Algorithm (CMA) to DS-CDMA.

Abed Mansour [24], provided a powerful simulation tool that serves to describe, test and analyze the performance of some wireless multiple access protocols namely carrier sense multiple access with collision avoidance (CSMA/CA) and code division multiple access (CDMA) techniques together with spread spectrum technologies.

1.4.4 Space Time Block Code

Space-Time Codes (STC) were first introduced by Tarokh et al. from AT&T research labs [25] in 1998 as a novel mean of providing transmit diversity for the multiple-antenna fading channel.

In 1998, Alamouti [26] proposed a simple transmit diversity scheme, which improves the signal quality at the receiver on one side of the link by simple processing across two transmit antennas at the opposite end.

In 1996 and 1999, Foschini and Telatar proved in [27] and [28] that communication systems with multiple antennas have a much higher capacity than single-antenna systems. They showed that the capacity improvement is almost linear in the number of transmit antennas or the number of receive antennas, whichever is smaller. This result indicated the superiority of multiple antenna systems and ignited great interest in this area. In few years, much work has been done generalizing and improving their results.

1.5 Aim of the Work

Many points are considered as a target for this work as given below:

To eliminate the need for guard interval this decreases the transmitted data bit rate.

To apply multi-user OFDM by combining the OFDM with CDMA leading to obtain a new system called MC-CDMA.

To compare between the two systems OFDM, STBC-OFDM, PCC-OFDM and MC-CDMA characteristic and performance in different types of the channels.

To compare between the two systems MC-CDMA with STBC and PCC with STBC characteristic and performance in different types of the channels.