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Nix, AR

Sarris, I

English

The authors present an investigation on the capacity of Line-of-Sight (LoS) optimised and conventional (narrow-spaced) Multiple-Input Multiple-Output (MIMO) communication systems in outdoor environments. Under a LoS scenario, the channel is normally rank deficient due to the linear dependence of the LoS rays’ phases on the receive elements. To overcome this problem, specifically designed antenna arrays can be employed, where the antenna elements are positioned to preserve the orthogonality of the LoS signal at the receiver; hence maximising the channel rank. A previously derived maximum capacity criterion is employed in the design of the proposed LoS-optimised MIMO system. The performance of this system is then assessed by means of a raytracing simulation and compared with that of a conventional MIMO system. It is shown that the proposed system exhibits a significant capacity enhancement (45.3 %) in the deployment area. Moreover, the capacity was found to be higher than that predicted by the i.i.d. Rayleigh fading model due to non-fading nature of the LoS signal

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A line-of-sight optimised MIMO architecture for outdoor environments

Ioannis Sarris

Andrew Nix

Crossref

A Line-of-Sight Optimised MIMO Architecture for Outdoor Environments

Andrew R. Nix

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Sarris, I., & Nix, AR. (2006). A line-of-sight optimised MIMOarchitecture for outdoor environments. In IEEE 64th VehicularTechnology Conference, 2006 (VTC-2006 Fall), Montreal (pp. 1 - 5).Institute of Electrical and Electronics Engineers (IEEE).https://doi.org/10.1109/VTCF.2006.53Peer reviewed versionLink to published version (if available):10.1109/VTCF.2006.53Link to publication record on the Bristol Research PortalPDF-documentUniversity of Bristol – Bristol Research PortalGeneral rightsThis document is made available in accordance with publisher policies. Please cite only thepublished version using the reference above. Full terms of use are available:http://www.bristol.ac.uk/red/research-policy/pure/user-guides/brp-terms/Ioannis Sarris ioannis.sarris@vil.mee.ite.comAndrew R. Nix andy.nix@bristol.ac.ukA Line-of-Sight Optimised MIMO Architecture for Outdoor EnvironmentsPresentation Outline• MIMO notation• Performance of MIMO in LoS• Rank-one and full-rank LoS response• Maximum capacity criterion• Measurement procedure• Results• ConclusionsSystem Model and MIMO CapacityThe input-output relationship  in an nt x nr MIMO system is represented mathematically by:y = Hx + ny - received signal vectorH - channel response matrixx - transmitted signal vectorn - noise at the receiverThe capacity of such a system is given by:Channel Response Matrix (LoS)=LoSH =NLoSHNLoSLoS KKK HHH11    1   +++=• In LoS conditions the channel response matrix is commonly modelled as:• In systems with inter-element spacing of the order of a wavelength the capacity reduces with increasing KRank-One vs Full-Rank LoS Channel• It is possible to achieve a full-rank LoS channel response by increasing the inter-element spacing• The capacity then increases with increasing K=LOSH =LOSHRank-one Full-RankMaximum Capacity Criterion• It is trivial to show that for a static MIMO channel the capacity is maximised whenIHH tH n=This equation gives the required inter-element spacing to achieve the maximum capacity in a MIMO system in free-space as a function of the T-R distance (D) and the orientation of the arrays (θ,ω)Using the above formula and assuming a free-space channel a simplified maximum capacity criterion for Uniform Linear Arrays was previously derived:Previous Work and Motivation• The performance of LoS-optimised antenna array structures has been previously investigated using MIMO measurements in an anechoic chamber an office environment and a home environment• Here we present an investigation on the performance of a distributed MIMO architecture suitable for an outdoor deployment corresponding to a hotspot scenario• The channel response is modelled using a MIMO-capable ray-tracing software• The operating frequency for the simulation was chosen to be 5.2 GHzScenario 1 (narrow-spaced arrays)• Lamppost-level (h=10m) access-points (AP) are employed• Each AP consists of two omni-directional antenna elements spaced by 20 cm and downtilted by 45 degrees facing the middle of the road• At the mobile terminal (MT) a 2-element ULA with 4 cm spacing is used• At each point in the route the MT communicates with the nearest APMobile Terminal4 cmCapacity Results (Scenario 1)Scenario 2 (LoS-optimised arrays)• A LoS-optimised architecture was employed where the AP elements where distributed at 10 m intervals along the deployment area• At each point in the route the two successive elements that are nearest to the MT are used for communication• At the MT the same array as in Scenario 1 is usedMobile Terminal4 cmCapacity Results (Scenario 2)Results (Fixed SNR = 20 dB)CmaxCiidCminFixed SNR vs Measured SNR• The results presented correspond to a system with normalised channel response (fixed SNR = 20 dB)• This assumption corresponds to a system employing optimal power control• In reality the SNR varies considerably especially between LoS/NLoS positions• To investigate the effect of the path loss on the capacity the capacity was also calculated for a system with fixed transmit power (variable SNR)Results (Fixed Tx Power – Variable SNR)Conclusions• LoS-optimised arrays can be used to overcome the problem of excessive correlation in environments with strong LoS signal such a LoS hotspot scenario• Capacity improvements of the order of 45.3% were observed in the deployment area• Even though the array geometry influences the capacity very considerably, when designing practical systems the SNR needs also to be taken into account• Future work needs to focus on the verification of the presented findings using MIMO measurements and in identifying potential implementation issues

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A line-of-sight optimised MIMO architecture for outdoor environments

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