ANTENNA SYSTEMS AND PROPAGATION FOR FUTURE WIRELESS COMMUNICATIONS Exploiting multiple-input multiple-output in the personal sphere D. Neirynck, C. Williams, A. Nix and M. Beach Abstract: An analysis of the propagation characteristics of communication links in the personal sphere is presented. The results of two wideband measurement campaigns in the 5.2 GHz band involving on-body multiple-input multiple-output (MIMO) arrays are reported. In the first measure- ment, transmission takes place between on-body antennas and a uniform linear array, positioned at close range and with line-of-sight (LOS) propagation conditions. Despite the LOS operation, MIMO is shown to offer a significant increase in the information theoretic capacity of the system when com- pared with a conventional single-input single-output (SISO) antenna system. Although this appears counter-intuitive to the well-known fact that uncorrelated scattering results in high capacity, two previously less well-known mechanisms are highlighted to explain this result. This analysis reveals the potential use of MIMO in personal area networks. The second measurement campaign uses two on-body MIMO arrays to focus on the subject of body area network propagation. Analysis of the SISO characteristics of the links highlights the influence of the user on the channel. Body sha- dowing and user motion are shown to lead to multiple rapid changes in the channel characteristics. Again, MIMO is shown to be able to offer performance enhancement. In the measured channels, polarisation diversity is shown to outperform spatial diversity. 1 Introduction The potential of multiple-input multiple-output (MIMO) antenna systems to increase the capacity of a wireless link is well known [1]. Its use in cellular and WLAN appli- cations has been extensively studied [2–4]. Through theor- etical analysis and measurements, fading correlation [5, 6] and power imbalances [3] were shown to have a negative influence on MIMO performance. In the personal sphere, the concept of personal area net- works (PAN) has driven the development of technologies such as Bluetooth and IEEE 802.15.3. Whereas Bluetooth [7] was originally intended as a cheap cable replacement technology, newer standards are being developed with ever- increasing data rates in mind [8, 9]. To date, the use of MIMO for PAN applications has not been considered. It is generally assumed that the high probability of a line of sight (LOS), and the subsequent correlation between the subchannels, prevents MIMO from offering throughput benefits in a PAN environment. This scepticism towards MIMO for PAN applications is reflected in the lack of avail- able literature in this area. To the authors’ knowledge, only the IST-MAGNET project has published a description of a MIMO measurement campaign using PAN devices [10]. However, the reported analysis is almost entirely restricted to single-input single-output (SISO) channel characteristics. In this paper, two novel measurement campaigns in the personal sphere are described. The results highlight the influence of the user on the channel characteristics and dispel the common view that MIMO based antenna signal processing has no benefit in PAN applications. In the first measurement campaign, it is observed that the MIMO theoretic information capacity of an LOS link over a 1 m distance exceeds that of the independent, identically distributed (IID) Rayleigh fading reference channel. This result is explained using a combination of previously less- known and more recent theoretical knowledge. A new sen- sitivity analysis is also performed to explain how the MIMO results still apply despite a 10 dB power imbalance due to body mounting of the antennas. The second measurement campaign focuses on communic- ation between two on-body locations. The term body area network (BAN) is used to distinguish this case from scenarios where only one of the devices is carried by the user. In these measurements, MIMO also offers a significant capacity increase compared with SISO. Despite the short range, LOS is far less common than expected. Moreover, misalign- ment of directional antennas, which are necessary to reduce the amount of radiation exposed to the user, means that the LOS component is often not dominant. Next generation systems will be far more user-centric. Therefore it can be expected that PAN will be a core element of 4G architectures. The results presented in this paper imply that MIMO should be considered as a candidate for next-generation PAN air interfaces. The presence of the multiple antennas could be exploited to mirror the high capacity of the bearer networks or, in combination with space-time coding, to save transmit power or to reduce the probability of link failure due to body shadowing. The remainder of this paper is organised as follows. Section 2 describes the first measurement campaign. Subsections contain the results and analysis of the SISO and MIMO channel characterisation. Section 3 respects # The Institution of Engineering and Technology 2007 doi:10.1049/iet-map:20060210 Paper first received 30th August 2006 and in revised form 28th January 2007 The authors are with Centre for Communications Research, University of Bristol, Merchant Venturers Building, Woodland Road, BS8 1UB, Bristol, UK D. Neirynck is currently with the Radio Communications Group, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, The Netherlands and IMEC-NL at the Holst Centre, PO Box 8550, 5605KN Eindhoven, The Netherlands E-mail: m.a.beach@bristol.ac.uk IET Microw. Antennas Propag., 2007, 1, (6), pp. 1170–1176 1170