Comparison between DS-CDMA and modified Gegenbauer functions for a multiuser communication ultra-wideband system F. Elbahhar, A. Rivenq-Menhaj, J.M. Rouvaen, M. Heddebaut and T. Boukour Abstract: An innovative multiuser communication system based on the ultra-waveband (UWB) technique is studied. Two coding techniques are proposed: the first is based on direct-sequence code-division multiple access (DS-CDMA) associated with the UWB system. The second innovative technique uses orthogonal functions called modified Gegenbauer functions. To evaluate the performances of these techniques, bit error rate values are calculated and analysed using computer simulations. It is shown that Gegenbauer functions offer superior performance to the previously proposed DS/CDMA-UWB technique for multiuser communications system. 1 Introduction In the last few years there has been a rapidly growing interest in ultra-wideband (UWB) communication systems. These systems operate by running, as signalling waveforms, baseband pulses of very short duration, typically of the order of one nanosecond or less, rather than the traditional method using a sinusoidal carrier. The large bandwidth of an UWB system is dominated by its pulse shape and duration. This large system bandwidth relative to the information bandwidth allows UWB systems to operate with a low power spectral density. Such a low power spectral density implies that the UWB signal may be kept near or below the noise floor of hostile detection devices. According to the Federal Communication Commission (FCC) first Report and Order on UWB technology [1] , a UWB system is characterised by either a parameter Z greater than 0.2, or a 10 dB bandwidth B (10 dB) greater than 500 MHz, where Z ¼ 2ðf u  f l Þ f u þ f l ð1Þ B ð10 dBÞ ¼ f u  f l ð2Þ f u and f l being the high and low-10dB cut-off frequencies, respectively. Concerning UWB interference on narrowband systems at frequencies below 3.1 GHz, the FCC restricts UWB indoor communications to the frequency band between 3.1 and 10.6 GHz. UWB technology has many potential advantages. Indeed, UWB allows a high data rate, is almost insensitive to interference from other signals and doesn’t require a dedicated frequency band. Moreover, the spectral power density is very low, so that the other systems perceive it as background noise. However, UWB signals can be affected by frequency dispersion and distortion problems. UWB systems are less expensive than optical ones, but their performance will probably never exceed the optical systems advantages. Ultra wideband technology can be used for radar, localisation and communication applications. Various systems based on UWB have already been implemented [2] . While some examination of CDMA for UWB has been considered, it has mostly focused on time hopping and not direct sequence varieties. One main difficulty is to provide a continuous synchronisation. In this paper, two UWB systems, using different multiple- access techniques, are compared in terms of bit error rate (BER), in the presence of additive white gaussian noise (AWGN) in a multiuser situation. The first technique is the direct-sequence CDMA that is well known as a powerful multiple-access technique in the presence of strong narrow- band interference. DS-CDMA makes use of a periodic spreading sequence to separate users [3, 4] . DS-CDMA systems are very effcient when there are many users (47 users). The second technique uses original mathematical tools called modified Gegenbauer functions (MGF), derived from orthogonal polynomials. 2 First proposed system This system combines the power of both UWB and DS- CDMA techniques. In this application the DS-CDMA is not used for spread spectrum but for multiuser access. This system uses the traditional waveform for UWB commu- nication, i.e. the monocycle pulse [5, 6] , given as uðtÞ¼ t t exp  t t  2 ð3Þ with t the pulse duration. The receiver and transmitter block diagrams are depicted in Fig. 1. The transmitter is composed of the code generator and the modulator. To differentiate between various users, Gold codes (often used in CDMA techniques) are used, since they have good correlation characteristics and provide a large number of F. Elbahhar, A. Rivenq-Menhaj, J.M. Rouvaen and T. Boukour are with IEMN-DOAE, University of Valenciennes, Le Mont-Houy, 59313 Valenciennes, France M. Heddebaut is with INRETS-LEOST, 20 rue Elis! ee Reclus, 59650, Villeneuve-d’Ascq, France E-mail: rouvaen@univ-valenciennes.fr r IEE, 2005 IEE Proceedings online no. 20045236 doi:10.1049/ip-com:20045236 Paper first received 6th October 2004 and in final revised form 8th April 2005 IEE Proc.-Commun., Vol. 152, No. 6, December 2005 1021