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The effect of multiple access interference in UMTS.

Introduction

The rapid worldwide growth in cellular telephone subscribers has demonstrated conclusively that wireless communication is a robust, viable voice and data transport mechanism. The wide spread success of cellular has led to the development of newer wireless systems and standards for many other types of telecommunication traffic besides mobile voice telephone calls [1]. The first generation cellular systems relied exclusively on analog frequency modulation techniques. The Advanced Mobile Phone System (AMPS) is the most popular first generation systems. Other first generation systems are Narrow band AMPS (NAMPS), Total Access Cellular Systems (TACS), Nordic Mobile Telephone system (NMT-900). All the first generation systems employ FDMA with each channel assigned to a unique frequency band within a cluster of cells.

The second generation standards use digital modulation formats, TDMA/FDD and CDMA/FDD multiple access techniques. The most popular second generation systems are United States digital cellular (USDC) standards IS-54 and IS-136, Global System for Mobile communications (GSM), Pacific Digital Cellular (PDC) and CDMA One [1].

The third generation cellular systems are being designed to support wide band services like high speed internet access, video and high quality image transmission with the same quality as the fixed networks. The primary requirements of the next generation cellular systems are

* Most efficient usage of the available radio spectrum.

* Back ward compatibility with pre-existing networks and flexible introduction of new services and technology.

* Support of multimedia services

* Support of high data rates

* Support of general Quality of Service (QoS).

3G-WCDMA (UMTS)

The Universal Mobile Telecommunications System (UMTS) is a visionary air interface standard. UMTS was submitted by European Telecommunication Standards Institute (ETSI) to International Telecommunication union's(ITU) International mobile telecommunications for the year 2000(IMT-2000) body in 1998 for consideration as a world standard [4]. The UMTS or WCDMA assures backward compatibility with the second generation GSM, IS-136, PDC TDMA technologies as well as all 2.5G TDMA technologies [1]. One of the most promising approaches to 3G is to combine a wide band CDMA (WCDMA) air interface with the fixed network of GSM.

Several organizations trying to merge their various WCDMA proposals are Japan's Association of Radio Industry and Business (ARIB), Alliance for Telecommunications Industry Solutions (ATIS), European Telecommunications Standards Institute (ETSI) through its Special Mobile Group (SMG). All these schemes try to take the advantage of WCDMA radio techniques without ignoring the numerous advantages of the already existing GSM networks.

The standard that has emerged is based on ETSIs universal mobile telecommunication system (UMTS) and is commonly known as UMTS terrestrial radio access (UTRA). The access scheme for UTRA is DS_CDMA. The information is spread over a band of approximately 5MHz. This wide band width has given rise to the name wide band CDMA or WCDMA [5]. A UMTS network may use either UTRA/FDD, UTRA/TDD. In FDD mode the uplink and down link transmissions employ two separated frequency bands for this duplex method [2]. A pair of frequency bands with specified separation is assigned for a connection.

WCDMA Key Features and Technical Characteristics

The WCDMA radio interface has the following key operational features.

* Support of high data rate transmission

* High service flexibility: support of multiple parallel variable rate services on each connection.

* Both Frequency Division Duplex (FDD) and Time Division Duplex (TDD).

* Built in support for future compatibility and coverage enhancing technologies like adaptive antennas, advanced receiver structures and transmitter diversity.

* Support of inter frequency hand over and hand over to other systems, including hand over to GSM.

* Efficient packet access.

The Effect of Propagation Channel

The channel effect on the quality of signal is a problem in transmission that should be considered in a communication system. The multi path profiles of the channels are shown in following tables, and contain a series of specular components with the relative delays and amplitudes indicated.

Case I: Fading Profile

This channel is considered as Indoor channel. In case I fading profile, there is a direct path and two reflected paths and the vehicle speed is 3km/h. In this case, one of the paths is of very high delay. All paths are of equal power level [3].

Case II: Fading Channel Profile

Table II illustrates the second fading channel profile that is considered in this paper. This channel represents a mobile traveling at speed of 120km/h. This channel is considered as vehicular channel or outdoor channel [3].

Multiple Access Interference

The fundamental problem of spread spectrum in CDMA is that each user causes multiple access interference (MAI) affecting all other users [2]. The correlation properties of the spreading code sequence may degrade the performance of the CDMA system by increasing the level of MAI. The average probability of bit error is given by the following equation [1].

Pe = Q [1/[square root of (K - 1/N + No/2Eb]

For large number of users the BER is limited by MAI than by thermal noise. The simplest way to despread a CDMA signal is to apply the RAKE principle and correlate the received signal with the known spreading sequence. The result will be good as long as the influence of other users can be neglected [3]. In practice, however the RAKE receiver does not take into account the existence of this interference i.e. the MAI. The signal quality can be improved when the knowledge of the cross correlation between the spreading codes is taken in to account instead of assuming that the correlations are zero. This leads to a more complex class of DS-CDMA receivers, based on multi user detection principle also referred to as joint detection or interference cancellation.

Simulation Result

To evaluate the Bit Error Rate (BER) at the uplink of a WCDMA system the simulator is designed according to the ETSI standards [6, 7]. The data transmission is done on frame by frame basis over a speculative time varying multi path channel. At the front end of the receiver additive white Gaussian noise is added. The simulator developed can be modified to incorporate adaptive antennas. Simulation is carried out for 100 iterations for different values of signal to noise ratio. The values of Eb/No are taken as 0,2,6,10,12 and 16db.Spreading factor is set to 32 and 5 samples per chip are taken. It is assumed that MAI signals do not pass through multipath fading channel but they pass through AWGN channel only. The average bit error rate is calculated by taking the ratio of the sum of symbols that are transmitted with error to the sum of symbols that are transmitted. The simulation results for WCDMA Uplink system for different fading profiles [8] are shown in figures (a), (b), (c) and (d). The assumption is that perfect power control is maintained. Fig (a) shows the average bit error rate versus Eb/No at the WCDMA uplink system for Indoor channel. Fig (b) shows average bit error rate Vs Eb/No when MAI signals are assumed to pass through multi path fading channel. From figures (a) and (b) it is observed that there is an increase in bit error rate if the number of interferers increases and if there is Multiple access Interference (MAI). Hence system becomes interference limited as the number of interferers increase.

[FIGURE A OMITTED]

[FIGURE B OMITTED]

Simulation is carried out for out door channel by taking the simulation parameters as for Indoor channel. Fig (c) and (d) show how the Multiple access interference (MAI) effects the system performance when the number of interferers increases. The MAI degrades severely the performance of a CDMA system. Simulation is carried out by generating MAI in a structured way instead of assuming it as AWGN.

[FIGURE C OMITTED]

[FIGURE D OMITTED]

Figures (E) and (F) show the BER Vs number of interferers. In this case the signal to noise ratio is fixed at 12db.The simulation is carried out for both the channels (Indoor and Out door) by assuming MAI signals passing through AWGN channel only.

[FIGURE E OMITTED]

[FIGURE G OMITTED]

Figures (G) and (H) show average BER Vs Number of interferers for both the channels by assuming MAI signals pass through multi path fading channel.

It is observed from above a figure that as the system load reaches 50%, the BER approaches to 10% which is obviously unacceptable. However by using adaptive antennas the BER can be pushed back to an acceptable limit.

[FIGURE F OMITTED]

[FIGURE H OMITTED]

Conclusion

According to the physical layer specifications of the IMT-2000 WCDMA system, the simulator is developed. The signal transmission is done on frame by frame basis and the Multiple access interference (MAI) corrupts the transmitted signal. The signal is further corrupted by AWGN at the front end of the receiver and RAKE diversity combining is employed at the receiver. The bit error rate is computed for different number of interferers. We investigated the average bit error rate at the receiver for different time varying channels. It is observed that the system is interference limited as the number of users or interferers increases. By employing adaptive antennas the system performance can be improved. The simulator developed can be used to implement adaptive antennas.

References

[1] T. S. Rappaport, Wireless Communication Principle & Practice, Prentice hall, 1998.

[2] UMTS Javier Sanchez and Mamadou Thioune

[3] Andrew Richardson WCDMA Design Hand book, Cambridge university press2005.

[4] P.Nicopolitidis, M.S. Obaidat, G.I. Papadimitriou, A.S. Pomportsis wireless networks John Wiley &Sons,2003.

[5] H Holma and A Toskala, WCDMA for UMTS, England, John Wiley & sons ltd 2000.

[6] 3GPP technical "specification, spreading and modulation (FDD, release 5)" TS 25.213,v5.3.0, 2003-03.

[7] 3GPP technical report, Deployment aspects (release 5), TR 25. 943, V5.1.0, 2002-06.

Santhi Ch.Rani (1), Venkata P. Subbaiah (2) and Chennakesava K. Reddy (3)

(1) DMS SVH College of Engineering, Dept of ECE, Machilipatnam, Krishna (Dt), A.P., India-521 002 E-mail: Santhirani_ece@yahoo.com

(2) Amritasai institute of Science and Technology, Dept of ECE Paritala, Krishna (Dt), A.P., India.

(3) Jawaharlal Nehru Technological University College of Engineering Dept of ECE, Jagityala, A.P., India
Table 1: Indoor channel fading profile

Relative Delay (ns)   Average Power
                          (db)

0                           0
976                         0
20000                       0

Table 2: Vehicular channel fading profile

Relative Delay (ns)    Average
                      Power(db)

0                         0
260                      -3
521                      -6
781                      -9
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Title Annotation:universal mobile telecommunications system
Author:Ch.Rani, Santhi; Subbaiah, Venkata P.; Reddy, Chennakesava K.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:Dec 1, 2008
Words:1718
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