Development and Implementation of a Real Time High-Resolution Channel Sounder – IF Stage D. Ferreira * and R. F. S. Caldeirinha *† * Instituto de Telecomunicações (DL-IT), Polytechnic Institute of Leiria, Leiria, Portugal dferreira@mail.co.it.pt, rafael.caldeirinha@ipleiria.pt † University of Glamorgan, Faculty of Advanced Technology CF37 1DL Treforest, United Kingdom Abstract—This paper presents an intermediate frequency (IF) stage of a swept time delayed cross-correlation (STDCC) channel sounder. The proposed topology employs the real-time sliding correlation technique of pseudo-random (PN) sequences, enabling amplitude and Doppler spectrum measurements and analysis of multipath components. It makes use of recent technologies, specifically in the generation and transmission of PN sequences, which allows for an easy adjustment of the main specifications of the sounder. The system presented herein has been tested and validated by using a PN sequence at 1GHz chip rate, which allows for a time-delay resolution of consecutive multipath components of 2ns. The dynamic range at the input of the IF stage was measured to be better than 40dB. Keywords: Channel sounder, Sliding correlation, Multipath, Pseudo-random sequence, Power Delay Profile, Wideband. I. INTRODUCTION Over the past few years, the world has witnessed a fast and continuous evolution of the information technologies. Nowadays, a trend is becoming visible in the industry in making use of evermore higher frequencies in order to have the required bandwidth, so that new and enhanced telecommunication services can be accommodated. The use of such frequencies raises doubts regarding the propagation mechanisms of electromagnetic waves in various media. It is therefore required to undertake appropriate studies in order to characterise and model those propagation mechanisms. This paper presents the IF stage of a wideband channel sounder, implemented using the STDCC technique. This method allows for the characterisation of highly dynamic channels, and it can provide both amplitude and Doppler spectra measurements. Some authors, [1] to [4], have already implemented similar channel sounders that operate under this principle, although with different technical specifications between them. For example: in [1] the sounder is limited to a time-delay resolution for consecutive multipath components of 5ns; in [2] the system does not perform the correlation in real time; and in [3] and [4] the sounders are not able to measure Doppler spectrum. The topology proposed in this paper makes use of high-speed Digital to Analogue Converters (DAC) up to 1.25GHz, in order to generate, download and transmit the desired PN sequence, which in turns will modulate a highly stable phase lock loop (PLL) carrier. The modulated signal will then be transmitted through the radio channel. The novelty of this topology relies on the fact that one can have full control over the properties of the PN sequence transmitted, simply by uploading the desired vector into the DAC’s driver. II. SLIDING CORRELATION TECHNIQUE The STDCC technique, also known as sliding correlation, was first implemented by Cox in 1972 [5]. This technique is based on the auto-correlation properties of PN sequences. The PN sequences commonly used in STDCC sounders are of the type MLSR (Maximal Length Linear Shift Register) [1], [2]. By adjusting the receiver PN sequence clock slightly slower than the PN clock on the transmitter, the two sequences will “slide” against each other. When both sequences are perfectly aligned in time, the correlation output will have its peak value. Otherwise, when the sequences are not aligned, the correlation result will have its minimum value [6]. In a radio channel, different multipath components, which exhibit different propagation paths and delays, will maximally correlate at different instants in time [1]. Fig. 1 depicts the auto-correlation of a PN sequence. Although the channel multipath components typically arrive at the receiver with different time delays, in the order of nano seconds, the “sliding” property of these systems effectively spreads out in time those components at the output of the correlator [5]. The amount by which the correlation peaks are time dilated is described as sliding factor, denoted by (1), R T T f f f k - = (1) where T f is the transmitter PN chip frequency, and R f is the receiver PN chip frequency. Hence, when considering a transmitter PN sequence clocked at 100MHz and a receiver PN sequence clocked at 99.999MHz (which yields to a sliding factor of 100000), a 20ns correlation pulse ( c T 2 ) will be displayed at the output of the system as a 2ms pulse. This is also known as time dilation. According to [1] and [7], the theoretical dynamic range of a STDCC sounder can be specified as (2), ( ) PN R N D 10 log 20 = dB (2) where PN N is the PN sequence length in chips. This work was partially supported by Fundação para a Ciência e a Tecnologia (FCT), under pluriannual funding and research project PTDC/EEATEL/099973/2008–ADCOD.