Ericsson Review: Non-line-of-sight microwave backhaul for small cells
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Ericsson Review: Non-line-of-sight microwave backhaul for small cells

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The evolution to denser radio-access networks with small cells in cluttered ...

The evolution to denser radio-access networks with small cells in cluttered
urban environments has introduced new challenges for microwave backhaul.
A direct line of sight does not always exist between nodes, and this creates a needfor near- and non-line-of-sight microwave backhaul.

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  • 1. The communications technology journal since 1924 2013 • 3Non-line-of-sight microwave backhaulfor small cellsFebruary 22, 2013
  • 2. Dispelling the NLOS myths2Non-line-of-sight microwavebackhaul for small cellsThe evolution to denser radio-access networks with small cells in clutteredurban environments has introduced new challenges for microwave backhaul.A direct line of sight does not always exist between nodes, and this creates a needfor near- and non-line-of-sight microwave backhaul. JONA S H A N S RY D, JONA S E D S TA M , BE NGT-E R I K OL S S ON A N D C H R I S T I NA L A R S S ON solution that is also effective for small- cell backhaul (see Figure 1). Network architects aim to dimen-Using non-line-of-sight (NLOS) Complementing the macro-cell layer by sion backhaul networks to supportpropagation is a proven approach adding small cells to the RAN introduc- peak cell-capacity3 – which today canwhen it comes to building es new challenges for backhaul. Small- reach 100Mbps and above. However, inRANs However, deploying cell outdoor sites tend to be mounted reality, there is a trade-off among cost,high-performance microwave 3-6m above ground level on street fix- capacity and coverage resulting in abackhaul in places where there tures and building facades, with an backhaul solution that, at a minimum,is no direct line of sight brings inter-site distance of 50-300m. As a large can support expected busy-hour trafficnew challenges for network number of small cells are necessary to with enough margin to account for sta-architects. The traditional belief support a superior and uniform user tistical variation and future growth: inin the telecom industry is that experience across the RAN2, small-cell practice around 50Mbps with availabil-sub-6GHz bands are required backhaul solutions need to be more cost- ity requirements typically relaxed to effective, scalable, and easy to install 99-99.9 percent. Such availability levelsto ensure performance for than traditional macro backhaul tech- require fade margins of the order of justsuch environments. This article nologies. Well-known backhaul tech- a few decibels for short-link distances.puts that belief to the test, nologies such as spectral-efficient LOS For small-cell backhaul simplicityproviding general principles, microwave, fiber and copper are being and licensing cost are important issues.key system parameters and tailored to meet this need. However, Light licensing or technology-neutralsimple engineering guidelines for owing to their position below roof block licensing are attractive alterna-deploying microwave backhaul height, a substantial number of small tives to other approaches such as linkusing frequency bands above cells in urban settings do not have access licensing, as they provide flexibility4.20GHz. Trials demonstrate that to a wired backhaul, or clear line of sight Using unlicensed frequency bands cansuch high-frequency systems to either a macro cell or a remote fiber be a tempting option, but may result incan outperform those using sub- backhaul point of presence. unpredictable interference and degrad­6GHz bands – even in locations The challenges posed by locations ed network performance. The riskwith no direct line of sight. without a clear line of sight are not new associated with unlicensed use of thePoint-to-point microwave is a cost-­ to microwave-backhaul engineers, 57-64GHz band is lower than that asso-efficient technology for flexible and who use several established methods ciated with the 5.8GHz band, owing torapid backhaul deployment in most to overcome them. In mountainous higher atmospheric attenuation, sparselocations. It is the dominant backhaul terrain, for example, passive reflectors initial deployment, and the possibilitymedium for mobile networks, and is and repeaters are sometimes deployed. of using compact antennas with nar-expected to maintain this position as However, this approach is less desir- row beams, which effectively reducesmobile broadband evolves; with micro- able for cost-sensitive small-cell back- interference.wave technology that is capable of pro- haul, as it increases the number of sites. Providing coverage in locations with-viding backhaul capacity of the order of In urban areas, daisy chaining is often out a clear line of sight is a familiar partseveral gigabits-per-second1. used to reach sites in tricky locations – a of the daily life of mobile-broadband and Wi-Fi networks. However, maybe because such locations are common- BOX A  Terms and abbreviations place, a number of widespread myths FDD frequency division NLOS non-line-of-sight and misunderstandings surrounding duplexing OFDM orthogonal frequency NLOS microwave backhaul exist – for LOS line-of-sight division multiplexing example, that NLOS microwave back- MIMO multiple-input, RAN radio-access network haul needs sub-6GHz frequencies, wide- multiple-output TDD time division duplexing beam antennas and OFDM-based radioE R I C S S O N R E V I E W • F EB RUA RY 2 2, 2013
  • 3. 3technologies to meet coverage andcapacity requirements. Despite this, a FIGURE 1  Microwave backhaul scenarios for small-cell deploymentnumber of studies on NLOS transmis-sion using frequency bands above 6GHz,for example, have been carried out for Daisy chain Penetration Reflection Diffractionfixed wireless access5 and for mobileaccess6. Coldrey et al. showed that itis realistic to reach 90 percent of thesites in a small-cell backhaul deploy-ment with a throughput greater than100Mbps using a paired 50MHz chan-nel at 24GHz7.NLOS principles FiberAs illustrated in Figure 1, all NLOS prop-agation scenarios make use of one ormore of the following effects: diffraction; reflection; and penetration. and application of these three propaga- actually increase as 20log(f) when car-All waves change when they encounter tion effects, giving network engineers rier frequency is increased for a fixedan obstacle. When an ­ lectromagnetic e simple rules to estimate performance antenna size. This relationship indi-wave hits the edge of a building, dif- for any scenario. cates the advantage of using higher fre-fraction occurs – a phenomenon often quencies for applications where a smalldescribed as the bending of the signal. System properties antenna form factor is of importanceIn reality, the energy of the wave is scat- A simplified NLOS link budget can be – as is the case for small-cell backhaul.tered in the plane perpendicular to the obtained by adding an NLOS path atten- To determine the importance ofedge of the building. The energy loss – uation term (ΔLNLOS) to the traditional NLOS-system properties, Ericsson car-which can be considerable – is propor- LOS link budget, as shown in Equation 1. ried out measurement tests on two com-tional to both the sharpness of the bend mercially available microwave backhauland the frequency of the wave8. Equation 1 systems in different frequency bands Reflection, and in particular random PRX = PTX + GTX + GRX - 92 - 20log(d) - 20log(f) - LF - ΔLNLOS (described in Table 1). The first systemmultipath reflection, is a phenomenon used the unlicensed 5.8GHz band withthat is essential for mobile broadband Here, PRX and PTX are the received and a typical link configuration for applica-using wide-beam antennas. Single-path transmitted power (dBm – ratio of pow- tions in this band. The air interface usedreflection using narrow-beam antennas er in decibels to 1 milliwatt); GTX and up to 64QAM modulation in a 40MHz-is, however, more difficult to engineer GRX are antenna gain (in decibels isotro- wide TDD channel with a 2x2 MIMOowing to the need to find an object that pic – dBi) for the transmitter and receiv- (cross-polarized) configuration pro-can provide the necessary angle of inci- er respectively; d is the link distance viding full duplex peak throughput ofdence to propagate as desired. (in kilometers); f is the frequency (in 100Mbps (200Mbps aggregate). The sec- Penetration occurs when radio waves gigahertz); LF is any fading loss (in deci- ond system, a MINI-LINK PT2010, usedpass through an object that completely bels); and ΔLNLOS is the additional loss a typical configuration for the licensedor partially blocks the line of sight. It is (in decibels) resulting from the deploy- 28GHz band, based on FDD, 56MHza common belief that path loss result- ment of NLOS-propagation effects. Not channel spacing and single-carriering from penetration is highly depen- shown in this equation is the theoreti- technology with up to 512QAM modu-dent on frequency, which in turn rules cal frequency dependency of the anten- lation, providing full duplex through-out the use of this effect at higher fre- na gain, which for a fixed antenna size put of 400Mbps (800Mbps aggregate).quencies. However, studies have shown will increase as 20log(f) and as a conse- To adjust the throughput based on thethat in reality path loss due to penetra- quence, the received signal – PRX – will quality of the received signal, bothtion is only slightly dependent on fre-quency, and that in fact it is the type andthickness of the object itself that creates Table 1: Test system specificationsthe impact on throughput9, 10. For exam- SYSTEM TECHNOLOGY CHANNEL ANTENNA OUTPUT PEAKple, thin, non-metallic objects – such as SPACING GAIN POWER THROUGHPUTsparse foliage (as shown in Figure 1) – 5.8GHz TDD/OFDM 40MHz 17dBi 19dBm 100Mbpsadd a relatively small path loss, even for 64QAMhigh frequencies. Deployment guidelines can be 28GHz FDD/single carrier 56MHz 38dBi 19dBm 400Mbpsdefined given a correct understanding 512QAM E R I C S S O N R E V I E W • F EB RUA RY 2 2, 2013
  • 4. Dispelling the NLOS myths4 advantages of using higher frequencies FIGURE 2  Link margin as a function of throughput and distance are clear: with comparable antenna siz- es, the link margin is about 20dB higher Link margin (dB) at a peak rate of 400Mbps for the 28GHz 80 system compared with the 5.8GHz sys- tem at a peak rate of 100Mbps. 28GHz 70 5.8GHz Measurements Diffraction 60 It is commonly believed that the dif- 90Mbps fraction losses occurring at frequencies 50 185Mbps above 6GHz are prohibitively high, and consequently, deploying a system using 280Mbps 10Mbps this effect for NLOS propagation at such 40 frequencies is not feasible. However, 60Mbps 400Mbps even if the absolute loss can be relative- 30 ly high, 40dB and 34dB for the 28GHz 80Mbps and 5.8GHz systems respectively (with 100Mbps a diffraction angle of 30 degrees), the 20 relative difference is only 6dB8 – much less than the difference in gain for com- 10 parable antenna sizes even when tak- 50 100 150 200 250 300 350 400 450 500 ing into account the higher free-space Link distance (m) loss for the 28GHz system (see Figure 2). Figures  3A and 3B show the set- up and measured results of a scenario designed to test diffraction. A first radio systems used adaptive modulation. Here, the margin is defined as the was positioned on the roof of an officePhysical antenna sizes were similar, but difference between received power building (marked in Figure 3A with adue to the frequency dependency of the (according to Equation 1) and the receiv- white circle). A second radio was mount-antenna gain and the parabolic type er threshold for a particular modula- ed on a mobile lift, placed 11m behindused in the 28GHz system, it offered a tion level (throughput) – in line of sight a 13m-high parking garage. The effectgain of 38dBi while the flat antenna of conditions without fading (Lf = 0). If on the signal power received by the sec-the 5.8GHz system reached 17dBi. ΔLNLOS caused by any NLOS effect can ond radio was measured by lowering the Link margin versus throughput and be predicted, the curves in Figure 2 can mobile lift. Figure 3B shows the mea-hop distance is shown in Figure  2. be used to estimate throughput. The sured received signal-power versus dis- tance below the line of sight for both test systems, as well as the theoretical FIGURE 3A  Test site for NLOS backhaul – diffraction received power calculated using the ide- al knife-edge model8. Both radios trans- mitted 19dBm output power, but due to the 21dBi lower antenna gain for the 5.8GHz system, the received signal for this radio was 20dB weaker after NLOS propagation than the 28GHz system. The measured results compare well against the results based on the theoreti- cal model, although an offset of a couple of decibels is experienced by the 28GHz system – a small deviation that is expect- ed due to the simplicity of the model. 0m To summarize, diffraction losses 15 can be estimated using the knife-edge ­ odel8. However, due to the model’s m simplicity, losses calculated by it are slightly underestimated. This can be © 2013 BLOM © 2013 Microsoft Corporation compensated for in the planning pro- cess by simply adding a few extra deci- bels to the loss margin.E R I C S S O N R E V I E W • F EB RUA RY 2 2, 2013
  • 5. 5The 28GHz system can sustain fullthroughput at much deeper NLOS FIGURE 3B  Throughput and received power – diffractionthan the 5.8GHz system, which is to be Throughput (Mbps) Received power (dBm)expected as it has a higher link mar-gin. Full throughput – 400Mbps – was 700 -10achieved at 28GHz up to 6m below the Received power 28GHzline of sight, equivalent to a 30-degree Received power 5.8GHzdiffraction angle, while the 5.8GHz sys- Theoretical power 28GHz 600 -20tem dropped to under 50Mbps at 3m Theoretical power 5.8GHzbelow the line of sight. The link mar- Throughput 28GHzgin is the single most important system Throughput 5.8GHz 500 -30parameter for NLOS propagation and, asexpected, the 28GHz system performsin reality better in a diffraction scenar- 400 -40io than a 5.8GHz system with compara-ble antenna size.Reflection 300 -50The performance characteristics of the5.8GHz and 28GHz systems were mea-sured in a single-reflection scenario in 200 -60an area dominated by metal and brickfacades – shown in Figure  4A. Thefirst radio was located on the roof of the 100 -70office building (marked with a whitecircle), 18m above ground level; and thesecond on the wall of the same build- 0 -80 –2 0 2 4 6 8 10ing, 5m above ground, facing the street Distance below line of sight (m)canyon. The brick facade of the buildingon the other side of the street from thesecond radio was used as the reflecting The 28GHz system shows a stable at severe multipath fading, result inobject, resulting in a total path length of throughput of 400Mbps, while the a graceful degradation of throughputabout 100m. The reflection loss will vary throughput for the 5.8GHz system, with – as illustrated. However, the narrowwith the angle of incidence, which in a much wider antenna beam, dropped a ­ ntenna lobe at 28GHz, in combina-this case was approximately 15 degrees, from 100Mbps to below 70Mbps. These tion with the advanced equalizer of theresulting in a ΔLNLOS of 24dB for the variations are to be expected owing high-­ erformance MINI-LINK radio, p28GHz system and 16dB for the 5.8GHz to the fact that the wider beam expe- effectively suppresses any multipathsystem – figures that are in line with ear- riences a stronger multipath. OFDM degradation, enabling the use of a sin-lier studies11. Reflection loss is strongly is an effective mitigation technolo- gle-carrier QAM technology for NLOSdependent on the material of the reflect- gy that combats fading, which will, conditions – even up to 512QAM anding object, and for comparison purpos- 56MHz channel bandwidths.es ΔLNLOS for a neighboring metal facadewas measured to be about 5dB for both FIGURE 4A  Test site for NLOS backhaul – reflectionsystems with similar angle of incidence. To summarize, it is possible to cover ­areas that are difficult to reach usingmultiple reflections in principle.However, taking advantage of morethan two reflections is in practice prob-lematic – due to limited link margins 100mand the difficulty of finding suitablyaligned reflection surfaces. ΔLNLOS pre-dictions for a single-facade reflectionin the measured area can be expect-ed to vary between 5dB and 25dB at28GHz and between 5dB and 20dB at5.8GHz. The throughput for both sys- © 2012 TerraItaly © 2013 Microsoft Corporationtems measured over 16 hours is shown Pictometry Bird’sEye © 2012 Pictometry International Corp.in Figure 4B. E R I C S S O N R E V I E W • F EB RUA RY 2 2, 2013
  • 6. Dispelling the NLOS myths6 up to more than 28dB. A complementa- FIGURE 4B  Throughput over time – single reflection ry experiment showing similar excess Throughput (Mbps) path loss was carried out at 5.8GHz. 28GHz 5.8GHz To summarize, contrary to popular belief, a 28GHz system can be used with 420 excellent performance results using 400 the effect of NLOS penetration through sparse greenery. 380 Deployment guidelines So far, this article has covered the key 120 system properties for NLOS propaga- 100 tion – diffraction, reflection and pene- tration – dispelling the myth that these 80 effects can be used only with sub-6GHz 60 frequencies. The next step is to apply the theory and the test results to an ­ ctual a 0 2 4 6 8 10 12 14 16 deployment scenario for microwave Time (hours) backhaul. Table 2 shows the indicative through- put for each NLOS scenario, using thePenetration shown in Figure 5A. The circle and tri- measured loss from the examples aboveAs with the case for NLOS reflection, angle symbols indicate where the radio together with the graphs in Figure 2.the path loss resulting from penetra- beams exit the foliage. A trial site, shown in Figure 6, wastion is highly dependent on the mate- Measurements were carried out selected to measure the coverage for anrial of the object blocking the line of under rainy and windy weather con- NLOS backhaul deployment ­ cenario. ssight. The performance of both test ditions, resulting in variations of the Four- to six-story office buildings withsystems was measured in a scenario NLOS path attenuation, as shown in the a mixture of brick, glass and metalshown in Figures 5A and 5B. The send- received signal spectra for the 28GHz facades dominate the trial area. The hubing and receiving radios were located radio link in Figure 5B. Under LOS con- node was placed 13m above ground on150m apart, with one tall sparse tree ditions the amplitude spectrum enve- the corner of a parking garage at theand a shorter, denser tree blocking the lope reached -50dBm. Consequently, south end of the trial area. By using theline of sight. The radio placed on the the excess path loss for the single-tree measured loss in the diffraction, reflec-mobile lift was positioned to measure (sparse foliage) scenario varied between tion and penetration from the tests as athe radio beam first after penetration 0 and 6dB when measured for 5 min- rule of thumb, an indicative through-of the sparse foliage and then lowered utes. In the double-tree (dense foliage) put for each NLOS scenario has beento measure the more dense foliage, as case excess path loss varied from 8dB taken from Figure 2 and summarized in Table 2. FIGURE 5A  Test site for NLOS backhaul – penetration The colored areas in Figure 6 show the line of sight conditions for the trial site: the green areas show where pure LOS exists; the yellow areas indicate the use = sparse foliage of single-path reflection; the blue areas = dense foliage indicate diffraction; and the red areas show where double reflection is need- ed. Areas without color indicate either that no throughput is expected or that they are outside the region defined for measurement. Measurements were made within the region delineated by the dashed white lines. Referring to Table 2, it is expected that the 5.8GHz system will meet small-cell backhaul requirements (>50Mbps throughput) within a 250m radius of the hub; and the 28GHz system should provide more than 100Mbps full duplex throughput up to 500m from the hub. To test the actual performance, a receiver nodeE R I C S S O N R E V I E W • F EB RUA RY 2 2, 2013
  • 7. 7was placed 3m above ground measuringfull duplex throughput along the main FIGURE 5B  Channel amplitude response – penetrationstreet canyon and in the neighboring Sparse foliage Dense foliagestreets. On account of the wide antenna Received power (dBm) Received power (dBm)lobe of the 5.8GHz system, realignment –40 –40was not needed for the hub antenna for Maximum power over 5 minutes Maximum power over 5 minutesmeasurement purposes. For the 28GHz Minimum power over 5 minutes Minimum power over 5 minutes –50 –50system, realignment of the narrowantenna beam was needed at each mea- –60 –60surement point – a fairly simple proce- ~6dB fluctuation >20dB fluctuationdure even under NLOS conditions. –70 –70 The actual values observed at eachmeasurement point exceeded or –80 –80matched the predicted performance lev- 29.32 29.34 29.36 29.38 29.40 29.42 29.32 29.34 29.36 29.38 29.40 29.42els in Table 2. Due to the lack of correct- Frequency (GHz) Frequency (GHz)ly aligned reflection surfaces, providing Resolution bandwidth: 50kHzbackhaul coverage using the double-reflection technique (the red areas ofthe trial area in Figure 6) was only pos- Table 2: Indicative bitrate performance for different NLOS key scenariossible for a limited set of measurements. LOS SINGLE DOUBLE DIFFRACTION* PENETRATION***Multipath propagation, including REFLECTION REFLECTIONthe reflection effects created by vehi- 0-100m 100Mbps 100Mbps 10Mbps** 80Mbps 100Mbpscles moving along the street canyon,was significant for the 5.8GHz system, 5.8GHz 100-250m 100Mbps 80Mbps <10Mbps** 60Mbps 100Mbpsbut resulted only in slightly reduced 250-500m 100Mbps 60Mbps <10Mbps** 10Mbps** 80Mbpsthroughput in some of the more diffi- 0-100m 400Mbps 400Mbps 280Mbps** 400Mbps 400Mbpscult scenarios for the 28GHz system. 28GHz 100-250m 400Mbps 400Mbps 185Mbps** 400Mbps 400MbpsSummary 250-500m 400Mbps 400Mbps 185Mbps** 280Mbps 400MbpsIn traditional LOS solutions, high sys- *30-degree diffraction angle; **not recommended for small-cell backhaul; ***sparse foliage or similartem gain is used to support targeted linkdistance and mitigate fading caused byrain. For short-distance solutions, this FIGURE 6  NLOS backhaul trial areagain may be used to compensate for LegendNLOS propagation losses instead. Sub-6GHz frequency bands are proven for line-of-sighttraditional NLOS usage, and as shown single reflectionin this article, using these bands is a 500mviable solution for small-cell backhaul. diffractionHowever, contrary to common belief,but in line with theory, MINI-LINK double reflectionmicrowave backhaul in bands above20GHz will outperform sub-6GHz sys-tems under most NLOS conditions. The key system parameter enablingthe use of high-frequency bands is themuch higher antenna gain for the same 250mantenna size. With just a few simpleengineering guidelines, it is possible toplan NLOS backhaul deployments thatprovide high network performance.And so, in the vast amount of dedicat-ed spectrum available above 20GHz, 100mmicrowave backhaul is not only capa-ble of providing fiber-like multi-gigabitcapacity, but also supports high perfor-mance backhaul for small cells, even inlocations where there is no direct line © 2013 BLOM © 2013 Microsoft Corporationof sight. E R I C S S O N R E V I E W • F EB RUA RY 2 2, 2013
  • 8. Jonas Edstam Jonas Hansryd eferencesR joined Ericsson in 1995 joined Ericsson1. Ericsson, 2011, Ericsson Review, Microwave capacity Research in 2008 and is and is currently head of evolution, available at: http://www.ericsson.com/res/ currently managing the technology strategies at docs/review/Microwave-Capacity-Evolution.pdf microwave high-speed Product Line Microwave2. Ericsson, 2012, White Paper, It all comes back to backhaul, and electronics group. He holds a Ph.D. and Mobile Backhaul. He is an expert in available at: http://www.ericsson.com/res/docs/ in electrical engineering from the microwave radio-transmission whitepapers/WP-Heterogeneous-Networks-Backhaul.pdf Chalmers University of Technology, networks focusing on the strategic3. NGMN Alliance, June 2012, White Paper, Small Cell Gothenburg, Sweden and was a visiting evolution of packet-based mobile Backhaul Requirements, available at: http://www.ngmn. researcher at Cornell University, backhaul and RAN. He holds a Ph.D. in org/uploads/media/NGMN_Whitepaper_Small_Cell_ applied solid-state physics from Ithaca, US, from 2003-2004. Backhaul_Requirements.pdf Chalmers University of Technology,4. Electronic Communications Committee (ECC), 2012, Report Gothenburg, Sweden. 173, Fixed service in Europe – current use and future trends post, available at: http://www.erodocdb.dk/Docs/doc98/ official/pdf/ECCRep173.PDF5. Seidel, S.Y.; Arnold, H.W.; 1995, Propagation measurements Bengt-Erik Olsson Christina Larsson at 28 GHz to investigate the performance of local multipoint distribution service (LMDS), available at: http://ieeexplore. joined Ericsson joined Ericsson ieee.org/xpl/articleDetails.jsp?arnumber=502029 Research in 2007 to work Research in 2010.6. Rappaport, T.S.; Yijun Qiao; Tamir, J.I.; Murdock, J.N.; on ultra-high-speed Her current focus area optical communication is microwave backhaul Ben-Dor, E.; 2012, Cellular broadband millimeter wave systems. Recently he switched interest solutions. She holds a Ph.D. in propagation and angle of arrival for adaptive beam steering to wireless-technology research, and is electrical engineering from Chalmers systems (invited paper), available at: http://ieeexplore.ieee. currently working on NLOS backhaul University of Technology, Gothenburg, org/xpl/articleDetails.jsp?arnumber=6175397 applications for microwave links. He Sweden, and was a post-doctoral7. Coldrey, M.; Koorapaty, H.; Berg, J.-E.; Ghebretensaé, Z.; holds a Ph.D. in optoelectronics from researcher at the University of Hansryd, J.; Derneryd, A.; Falahati, S.; 2012, Small-cell Chalmers University of Technology, St. Andrews, St. Andrews, UK, from wireless backhauling: a non-line-of-sight approach for point- Gothenburg, Sweden. 2004-2006. to-point microwave links, available at: http://ieeexplore. ieee.org/xpl/articleDetails.jsp?arnumber=63992868. ITU, 2012, Recommendation ITU-R P.526, Propagation by diffraction, available at: http://www.itu.int/  cknowledgements A rec/R-REC-P.526-12-201202-I The authors gratefully acknowledge the colleagues who have contributed to9. Anderson, C.R.; Rappaport, T.S.; 2004, In-building this article: Jan-Erik Berg, Mikael Coldrey, Anders Derneryd, Ulrika Engström, wideband partition loss measurements at 2.5 and 60 GHz, Sorour Falahati, Fredrik Harrysson, Mikael Höök, Björn Johannisson, available at: http://ieeexplore.ieee.org/xpl/articleDetails. Lars Manholm and Git Sellin jsp?arnumber=129664310. Okamoto, H.; Kitao, K.; Ichitsubo, S.; 2009, Outdoor-to- Indoor Propagation Loss Prediction in 800-MHz to 8-GHz Band for an Urban Area, available at: http://ieeexplore.ieee. org/xpl/articleDetails.jsp?arnumber=455526611. Dillard, C.L.; Gallagher, T.M.; Bostian, C.W.; Sweeney, D.G.; 2003, 28GHz scattering by brick and limestone walls, available at: http://ieeexplore.ieee.org/xpl/articleDetails. jsp?arnumber=1220086Telefonaktiebolaget LM EricssonSE-164 83 Stockholm, Sweden 284 23-3190 | UenPhone: + 46 10 719 0000 ISSN 0014-0171Fax: +46 8 522 915 99 © Ericsson AB 2013