Heereman, Frédéric

Joseph, Wout

Martens, Luc

Plets, David

Tanghe, Emmeric

English

Ghent University Academic Bibliography

Small-scale Fading and Delay in Conference Roomwith 802.11 Coverage ProblemsF. Heereman, W. Joseph, E. Tanghe, D. Plets and L. MartensDepartment of Information Technology, Ghent University/IBBTGaston Crommenlaan 8 bus 201, B-9050 Ghent, BelgiumEmail: frederic.heereman@intec.ugent.beAbstract—Small-scale fading and propagation delay have beencharacterized for the 2.4 GHz band in a specific conference roomwith 802.11 coverage problems and a more typical conferenceroom for comparison. The measurements were executed witha virtual SIMO (Single-Input Multiple-Output) system. Theimplications on the 802.11 performance have been investigated interms of received signal strength and intersymbol interference.A higher reverberation time was found in the room withcoverage problems, which causes a higher chance of intersymbolinterference.I. INTRODUCTIONIn literature, delay properties of indoor wireless channelsare mainly investigated for typical environments like offices,factories and colleges [1]. This paper is focused on a specificconference room where repeated coverage problems werereported with an 802.11 conference system. This system hasa SISO (Single-Input Single-Output) configuration. Accordingto the manufacturer, these problems only occur in this con-ference room and cannot be attributed to interference sourcesafter spectral analysis.In this paper, small-scale fading and propagation delay arecharacterized for the 2.4 GHz band in the conference roomwith coverage problems (room A) and in another more typicalconference room (room B) for comparison. Implications onthe performance of 802.11 systems are investigated in termsof reduced signal strength and intersymbol interference (ISI).II. MEASUREMENTSMeasurements were executed in room A and B with avirtual SIMO (Single-Input Multiple-Output) system. In thissetup, the Tx and Rx antenna, both broadband omnidirectionalElectro-Metrics antennas of type EM-6116, were connectedto a Rohde & Schwarz ZVR vector network analyzer, whichmeasured the scattering parameter S21 as a function of thefrequency. A coaxial cable with two amplifiers was used torealize the required Tx-Rx separation. The position of the Rxantenna, attached to a frame of BiSlides, was controlled by alaptop.The measurements were done in the frequency range 2.5– 3 GHz, which is out of the 2.4 GHz band to excludeinterference sources. In room A, 801 frequency points wereused, which allows to resolve power delay profiles for delaysup to 1.6µs (greater than the 802.11n Guard Interval of 800 ns[2]). This delay corresponds to a path length of about 475 m.A 23×23 Rx array was used, with a separation of 1.5 cm.This corresponds to 3.6 samples per λ/2 (where λ is thewavelength), and a total array dimension of 3λ. In room B,the following settings were used: 401 frequency points, a 4×4Rx array with a separation of 4 cm.In room A, the following positions of Tx and the Rx arraywere considered (Fig. 1): Tx at position 1 (usual positionof the access point during meetings) and Rx at position 2(chairman’s seat on the conference table) (case 1a) (and viceversa (case 1b)), and Tx at position 1 and Rx at position 3(other seat at the conference table) (case 2a) (and vice versa(case 2b)). At position 1, the height of the antenna (Tx orRx) was always 1.8 m, while at positions 2 and 3, the antennaheight was always 1.2 m. The distance between Tx and Rx forcase 1a-b is 8.9 m, for case 2a-b 6.9 m.Fig. 1. Plan of conference room A. The conference table is indicated by(3). Measurements were executed at Tx/Rx positions 1 – 3. The wall behindposition 2 (indicated as (1)) and the two dividing walls (indicated as (2))contain about 30 metal HVAC plates. The wall at the other side (indicated as(4)) consists of windows only.The wall of room A behind position 2 (indicated as (1) inFig. 1) and the two dividing walls (indicated as (2) in Fig. 1)contain about 30 metal HVAC (Heating, Ventilation, and AirConditioning) plates (dimension 1 m by 1.5 m). The wall at theother side (indicated as (4) in Fig. 1) consists of windows only.The ceiling, which looks like a part of an ellipsoid, containsa metal wire mesh, with a minimal separation of about 1 cm.The dimensions of the room are 12 m × 53 m and the ceilinghas a maximal height of 13 m.Conference room B is cylinder-shaped with about 30 mdiameter and a height of about 7 m. Tx was positioned 1 mfrom the wall at a height of 2 m and the Rx array was setat two different positions (case 3a-b) near the middle of theroom, at a height of 1.2 m. The Tx-Rx separation is 18.4 m forcase 3a and 15.3 m for case 3b. For all measurements (roomsA and B), there was a line-of-sight condition.III. SMALL-SCALE FADINGThe performance of the wireless system can be re-duced by small-scale fading. The ratio between PR [dBm](the power leaving the Rx terminals) and PT [dBm] (thepower entering the Tx terminals) is given by the mea-sured scattering parameter S21: PR − PT = 20 log(|S21|),where | · | is the absolute value.An estimation of the minimum required |S21|2 value canbe made as follows. Taking into account the gains of themeasurement Rx and Tx antenna (1 dBi), the Rx gain of arealistic wireless system (4 dBi), and a maximum allowedEIRP (Equivalent Isotropically Radiated Power) of 20 dBm,we estimate the received power of the wireless system as|S21|2 + 22 dBm. The sensitivity of the 802.11n referencereceiver is -64 dBm for 65 Mbps (Packet Error Rate < 10%)[2]. Hence, we estimate that there is good coverage when|S21|2 > -86 dB.The cumulative distribution function (CDF) of the measured|S21|2 samples is determined for the different cases (Fig. 2).Each CDF is based on all Rx positions (of the array) and allfrequency points. The probability that |S21|2 < -86 dB is ofthe order of 10-5 – 10-4 for cases 1a-b and 2a-b, and of theorder of 10-4 – 10-3 for case 3a-b. These are low probabilities,but the position of deep fades (drops of |S21|2) can be easilyinfluenced by little changes in the environment (e.g. people).−110 −100 −90 −80 −70 −60 −50 −40 −3010−610−410−2100|S21|² [dB] Probability  case 1acase 1bcase 2acase 2bcase 3acase 3bFig. 2. CDF of |S21|2 for cases 1a-b and 2a-b (room A) and case 3a-b(room B).To compare the degree of small-scale fading in the CDFsof |S21|2 for the different cases, we should take into ac-count that for higher Tx-Rx separation, the |S21|2 values arelower. Therefore, we plot the CDF of |S21|2 − (|S21|2)av(see Fig. 3), where (|S21|2)av is the average of the (linear-scaled) |S21|2 samples. Note that for low probability, the CDFsin Fig. 2 and 3 are less accurate due to the finite number ofsamples. The degree of small-scale fading is higher for case1a than for case 2a (horizontal shift of the CDF tail of 8 dB ata probability of 10-3, see Fig. 3) and is also higher for case 1bthan for case 2b (horizontal shift of the CDF tail of 4 dB at aprobability of 10-3). The degree of small-scale fading of cases3a-b seems to lie between case 1a and case 2a. The differencein degree of small-scale fading between case 1a-b (position 2involved) and case 2a-b (position 3 involved) can be explainedby the fact that position 2 is closer to and more surroundedby metal HVAC plates than position 3. Note that in room Amore coverage problems were reported at position 2 than atother seats at the conference table (e.g. position 3). This isin agreement with the higher degree of small-scale fading forcases 1a-b (position 2 involved) than for cases 2a-b (position3 involved).−70 −60 −50 −40 −30 −20 −10 0 1010−610−410−2100|S21|² − (|S21|²)av [dB]Probability  case 1acase 1bcase 2acase 2bcase 3acase 3bFig. 3. CDF of |S21|2 − (|S21|2)av for cases 1a-b and 2a-b (room A) andcase 3a-b (room B).IV. DELAYIn this section, the propagation delay is investigated with thefocus on intersymbol interference (ISI). We assume a GuardInterval (GI) of 800 ns [3]. Based on the SIMO measurements,Averaged Power Delay Profiles (APDP) were made for allcases [4] (see Fig. 4). The resolution is 2 ns for all APDPs.The diffuse part of the APDP [dB] can be expressed asAPDP = C0 − 10 log(e)(τ − τ0)/τr, (1)where τr is the reverberation time (i.e. time over which theAPDP is reduced by the exponential factor e) and τ0 is thedelay of the first arriving path [5]. Thus, by definition, C0 is thepower of the extrapolated diffuse part of the APDP, evaluatedat τ0. Since for all cases, there is a line-of-sight condition, τ0is d/c, where d is the distance between Tx and Rx and c thespeed of light. The parameters τr and C0 are determined fromthe APDPs (Table I). We see that τr is a factor 2 higher inroom A (134 – 138 ns) than in room B (55 – 59 ns), whileC0 is 5 dB higher in room B than in room A. The higher τrin room A can be explained by the vicinity of many metalstructures (HVAC plates, wire mesh in the ceiling).The signal transfer (voltage over Tx terminals to voltageover Rx terminals) has a power reduction of |S21|2, measuredat the carrier frequency (assuming that |S21|2 does not varymuch over the bandwidth of the signal). Paths which arrive0 0.2 0.4 0.6 0.8 1 1.2 1.4x 10−610−1410−1210−1010−810−610−4delay [s]normalized power  case 1acase 1bcase 2acase 2bcase 3acase 3bFig. 4. Averaged power delay profiles for cases 1a-b and 2a-b (room A) andcase 3a-b (room B).TABLE IPARAMETERS OF THE DIFFUSE PART OF THE APDPS FOR THE DIFFERENTCASES: REVERBERATION TIME τr AND C0 . POWER REDUCTION PGI (DUETO PATHS ARRIVING AT RX LATER THAN GI + τ0) (THEORETICALESTIMATION) IS COMPARED WITH (|S21|2)av .τr C0 PGI (|S21|2)av PGI - (|S21|2)av[ns] [dB] [dB] [dB] [dB]case 1a 135 -83.0 -56.7 -45.0 -11.7case 1b 134 -82.2 -56.0 -43.9 -12.1case 2a 138 -82.6 -55.4 -45.1 -10.3case 2b 137 -81.5 -54.8 -43.0 -11.8case 3a 58.5 -77.0 -91.6 -53.7 -37.9case 3b 54.9 -77.0 -96.0 -52.8 -43.2at Rx with a delay higher than τ0 + GI can cause ISI.The contribution of these paths can be worst-case estimatedtheoretically by a power reduction PGI of:PGI = 4 exp(−GI/τr)C0Mτ2resτ2r , (2)where τres [s] is the resolution of the APDP, and C0 and PGIare in linear scale. M is a margin due to constructive interfer-ence between paths with different angle of arrival. M has beendetermined experimentally as follows. Based on the powerdelay profiles of all Rx positions, the maximum power isobtained as a function of the delay (Fig. 5). M is the differencebetween this maximum power and the averaged power delayprofile. In this way, we find that the margin M is 9.5 ± 0.5 dBfor all cases. The power reduction PGI should be muchlower than |S21|2 to exclude ISI [3]. Based on the ex-perimentally determined parameters τr, C0 and M, we cal-culate PGI with (2). Here, a resolution τres of 3.3 ns isconsidered (see further). These PGI values should be com-pared with (|S21|2)av: PGI − (|S21|2)av = -12.1 to -10.3 dBfor room A and -43.2 to -37.9 dB for room B. This is causedby the higher τr (factor 2 higher) for room A. Note that small-scale fading has been cancelled out in (|S21|2)av . If we takeinto account small-scale fading, fades higher than about 10 dBand 40 dB (relative to (|S21|2)av) can theoretically cause ISIfor rooms A and B, respectively. This indicates that there is agreater chance of intersymbol interference in room A than inroom B.0 0.2 0.4 0.6 0.8 1 1.2 1.4x 10−610−1410−1210−1010−810−610−4delay [s]normalized power  Maximum of all PDPsAveraged PDPFig. 5. Maximum of all power delay profiles (of all Rx positions) andaveraged power delay profile (APDP) for case 1b (room A). The vertical shiftcorresponds to the margin M: M = 9.5 ± 0.5 dB.Equation (2) is a theoretical worst-case estimation of PGI.However, it is possible to determine PGI as a function offrequency f per Rx array position fully experimentally. This isdone by reconstructing the transfer function (voltage over Txterminals to voltage over Rx terminals) corresponding to thepaths which arrive at Rx with a delay lower than τ0 + GI. Thisreconstruction is done from the part of the impulse reponse(of which the absolute value gives the Power Delay Profiles(PDP)) with a delay < τ0 + GI. Thus, this transfer function isS21(f)− S21, G I(f) for paths arriving with a delay < τ0 + GIat Rx, where S21, G I(f) is the transfer function correspondingto the paths which arrive with a delay > τ0 + GI at Rx. PGIis then |S21, G I(f)|2. |S21(f) − S21, G I(f)|2 should be muchhigher than |S21, G I(f)|2 to exclude ISI.The reconstruction of the transfer function S21(f) −S21, GI(f) has been done for the frequency range 2.65 GHz– 2.85 GHz. To minimize the influence of the window, usedfor the Fourier transformation in the calculation of the impulseresponse, we calculate for each measured frequency point animpulse response to reconstruct the transfer function aroundthat frequency point. These PDPs are based on a lowerfrequency span (i.e. 300 MHz). Consequently, they have a bithigher resolution (i.e. 3.3 ns), but have the same maximumdelay, over which the PDP can be resolved. This maximumdelay has to be higher than GI (0.8µs). This maximum delayis 1.6µs for cases 1a-b and 2a-b, and 0.8µs for cases 3a-b.Therefore, we can do the reconstruction only for cases 1a-band 2a-b.We investigate for cases 1a and 2a whether |S21(f) −S21, G I(f)|2 can be lower than |S21, G I(f)|2, for the 23×23Rx positions and all frequency points in the range 2.65 GHz -2.85 GHz (namely 321 frequency points). In this case, symbolerrors due to ISI can be expected. For case 1a, this occurs 7times (out of 169,809 samples). Here, the minimal (worst)ratio |S21(f) − S21, G I(f)|2 - |S21, G I(f)|2 is -8.5 dB. Thissituation is illustrated in Fig. 6, where |S21(f) − S21, G I(f)|2and |S21, G I(f)|2 are shown (in a small frequency range of10 MHz). For case 2a, this occurs 8 times. Here, the minimal(worst) ratio |S21(f) − S21, G I(f)|2 - |S21, G I(f)|2 is even -21.0 dB (Fig. 7). This shows that when (small-scale) fadesare deep enough, the contribution of the paths arriving witha delay > τ0 + GI can exceed the contribution of the pathsarriving with a delay < τ0 + GI, which results in ISI. FromFigs. 6 and 7, we see that fades of 30 - 40 dB (compared to(|S21|2)av level) can cause intersymbol interference. This isin agreement with the theoretical estimation in Table I (mostright column, case 1a and 2a), which says that intersymbolinterference can occur for fades deeper than about 10 dB.Fig. 6. Power reduction |S21(f)− S21,GI(f)|2, corresponding to the pathswhich arrive at Rx with a delay < τ0 + GI: solid line. Power reduction|S21,GI(f)|2, corresponding to the paths which arrive at Rx with a delay >GI + τ0: dashed line. This is for case 1a (room A). The markers indicate thecalculated points.Fig. 7. Power reduction |S21(f)− S21,GI(f)|2, corresponding to the pathswhich arrive at Rx with a delay < τ0 + GI: solid line. Power reduction|S21,GI(f)|2, corresponding to the paths which arrive at Rx with a delay >GI + τ0: dashed line. This is for case 2a (room A). The markers indicate thecalculated points.V. CONCLUSIONSSmall-scale fading and propagation delay have been char-acterized for the 2.4 GHz band in a specific conference room(room A) with 802.11 coverage problems and a more typicalconference room (room B) for comparison. The measurementswere executed with a virtual SIMO system. The implicationson the 802.11 performance have been investigated in terms ofreceived signal strength and intersymbol interference.At the chairman’s seat in room A (where most coverageproblems were reported), an 8 dB higher degree of small-scalefading was found compared to another seat on the conferencetable. Power delay profiles revealed that the reverberation timeτr in room A is a factor 2 higher than in room B. Thesefindings could be explained by the vicinity of many metalstructures in room A, especially near the chairman’s seat.We showed that the propagation properties of room A canresult in coverage problems due to a reduced received signalstrength as well as intersymbol interference. Due to the higherτr in room A, intersymbol interference can theoretically occurfor (small-scale) fades higher than 10 dB and 40 dB in roomsA and B, respectively.VI. ACKNOWLEDGMENTThis work was supported by the OMUS (Optimizing Mul-timedia Service Delivery) project, co-funded by the IBBT(Interdisciplinary institute for BroadBand Technology), a re-search institute founded by the Flemish Government in 2004,and the involved companies and institutions. W. Joseph is apostdoctoral fellow of the FWO-Flanders.REFERENCES[1] M. Awad, K. Wong, and Z. bin Li, “An integrated overview of theopen literature’s empirical data on the indoor radiowave channel’s delayproperties,” IEEE Transactions on Antennas and Propagation, vol. 56,no. 5, pp. 1451–1468, 2008.[2] IEEE Std 802.11nTM-2009 amendment 5 to part 11: Wireless LANMedium Access Control (MAC) and Physical Layer (PHY) specifications:enhancements for higher throughput, IEEE Std., Oct 2009.[3] Wikipedia. [Online]. Available: en.wikipedia.org/wiki/Guard\ interval[4] E. Tanghe, W. Joseph, J. De Bruyne, L. Verloock, and L. Martens, “Theindustrial indoor channel: statistical analysis of the power delay profile,”International Journal of Electronics and Communications, vol. 64, no. 9,pp. 806–812, 2010.[5] J. Andersen, J. Nielsen, G. Pedersen, G. Bauch, and M. Herdin, “Roomelectromagnetics,” IEEE Antennas and Propagation Magazine, vol. 49,no. 2, pp. 27–33, 2007.

Small-scale fading and delay in conference room with 802.11 coverage problems

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