1 A Measurement Based Spatial Correlation Analysis for MB-OFDM Ultra Wideband Transmissions Junsheng Liu (1) , Ben Allen (1) , Wasim Q. Malik (2) , David J.Edwards (2) (1) Centre for Telecommunications Research, King’s College London, London, UK junsheng.liu, ben.aLlen@kcl.ac.uk (2) Department of Engineering Science, University of Oxford, Parks Road, Oxford, UK wasim.malik, david.edwards@eng.ox.ac.uk Abstract Spatial diversity can be applied to Ultra Wideband (UWB) systems to achieve a higher bit rate. Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) breaks the UWB bandwidth into 13 sub-bands and the sub-bands into 128 narrow-band tones to achieve a high data rate from wide band systems. In this paper, the spatial correlation analysis is applied to an indoor light-of-sight(LoS) and non-light-of-sight(NLoS) UWB channel with one vertically polarized transmit antenna and two vertically polarized receive antennas. The analysis is supported by measurement results which confirms that the subcarriers in MB-OFDM system can be treated as narrow band signals, and an inter-sensor distance of 3cm is large enough to accomplish uncorrelated received signals for the given measurement environment. I. I NTRODUCTION Since the Federal Communications Commission (FCC) assigned a bandwidth from 3.1GHz to 10.6 GHz for UWB usage, UWB signaling has become a candidate for high data rate transmission over short ranges. Applications of UWB wireless have been proposed with data rates from hundreds of Mbps to several Gbps over a range of 1 to 10m and even tens of meters [1], with a trade-off between range and data rate. MB-OFDM is one of the two candidates proposed to the IEEE 802.15 task group 3a as a signaling scheme for UWB indoor transmission. MB-OFDM provides a variety of data rates from 53.3 Mbps to 480 Mbps [2]. Each MB-OFDM sub-band has 128 sub-carries and each sub-carrier occupies a bandwidth of 4.125MHz, which makes the channel appear much less frequency selective compared with the channel experienced by impulse radio UWB that occupies all of the available UWB spectrum. Diversity combining has been developed over several decades as a means of increasing the wireless communication capacity. The two key parameters determining the diversity gain are: the level of the mean power difference between branches; and the correlation between them. Similar mean powers and low correlation leads to a good diversity performance [3]. In wireless communications, diversity techniques have become an essential means of enhancing the capacity, one of which is spatial diversity. In the case of spatial diversity, the closer the receivers are located together, higher signal correlation and lower level of mean power difference are experienced by the receivers due to similar scattering environments. In this paper, a simple tranceiver system consisting of one transmitter and two receivers is considered in order to access the spatial diversity performance of a MB-OFDM system. Since similar received mean power levels are often achieved in practical spatial diversity application, only the correlation of the received signals is analysed here. Because of the reciprocal property of the channel, the result of the correlation analysis for the receive diversity presented in this paper also applies for the transmit diversity. The remainder of this paper is organized as follows. A brief introduction to MB-OFDM is given in section II. Details of the measurement environment are shown in section III, and the data analysis of the correlation coefficient as a function of frequency and space is given in section IV. Conclusions are given in section V. II. I NTRODUCTION TO MB-OFDM In MB-OFDM systems, the bandwidth, ranging from 3.1GHz to 10.6GHz, is divided into 13 sub-bands, with a bandwidth of 528MHz for each [2]. The 13 sub-bands are numbered from 1 to 13, with number 1 having the lowest center frequency and number 13 the highest center frequency. All the bands are organized into four groups. Sub-bands 1 to 3 belong to group A, 4 to 5 group B, 6 to 9 group C and 10 to 13 group C. Group A is intended for first-generation devices, whilst the other three groups are reserved for future use. For the proposed standard, an IFFT/FFT of size 128 points is used for OFDM signaling in each sub-band, which results in a sub-carrier frequency spacing of: Δ F = 528MHz/128 = 4.125MHz (1) Frequency hopping is achieved in MB-OFDM system to improve the performance by using using time frequency code [2], where different sub-bands are used in different time slots. In the first generation of MB-OFDM system, time frequency code is restricted among the 3 sub-bands in group A. A. MB-OFDM and frequency selective fading In the time domain, one of the main advantages of an OFDM system is the ability to avoid Inter-Symbol-Interference (ISI) without using an equalizer. The length of one OFDM symbol (not including the zero padding sequence) is equal to 1/Δ F . Δ F is made small enough comparing to the channel coherence bandwidth, which will be shown in section IV, so that the OFDM symbol length is much larger than the channel delay, thus ISI is negligible compared with the symbol length.