URSI-F JAPAN MEETING NO. 502

1

A Single-bounce Channel Model for Dense Urban
Street Microcell
Mir Ghoraishi
Jun-ichi Takada
Department of International Development Engineering
Tokyo Institute of Technology
Tokyo, 152-8550, Japan{mir, takada}@ap.ide.titech.ac.jp

A BSTRACTA channel model for line-of-sight (LoS) small street microcell in dense urban areas is proposed. The distribution of thescatterers in the channel seen from the receiver (Rx) basedon the physical phenomenas is obtained. The coefficients ofthe distribution function is derived by approximation to aset of urban street microcell measurement data. The powerazimuth spectrum (PAS) for the proposed model is comparedwith those for conventional elliptical model as well as withexperimental results obtained from measurement in streetswith three different widths. It is shown that the proposed modelin contrast to the conventional models produces results thatclosely agrees with the experimental results.
I. I NTRODUCTION
Characterization and modeling of the radio propagation
channel are essential for mobile and wireless systems design.
A detailed knowledge about mobile communication propagation channel leads to a more successful design of the communication system. Especially to design and evaluate the multiantenna systems understanding the spatial properties of thechannel is a prerequisite. An overview of the existing spatialchannel models is available at [1]. One of the most commonlyused directional channel models is based on a geometricdescription of the scattering process, which focuses on thedetailed internal construction or realization of the channel.
The model assumes a statistical distribution of scatterers in
the wireless link and channel properties are derived from thepositions of the scatterers by applying the fundamental lawsof propagation mechanism of electromagnetic waves. Singlebounce scattering geometric models are among the mostwidely used geometric models, where propagation between
the transmitter (Tx) and receiver (Rx) antennas is assumed totake place via single scattering from an intervening obstacle.
Usually the bistatic scattering cross section of the scatterer isassumed to be isotropic, but its density is assumed to vary fromlocation to location. Geometrical modeling of the propagation
channel has always been attractive for the researchers due to its
advantages. The very well known Jakes model is a geometricalchannel model itself [3]. In this model the scatterers are
considered to be uniformly distributed over a circular ring. The
model proposed in [4] is circular as well however the scatterers
are assumed to be uniformly distributed within a circular disk

Tetsuro Imai
R&D Center
NTT DoCoMo Inc.
Kanagawa, 239-8536, Japan
imai@mlab.yrp.nttdocomo.co.jp

while [5] proposes a circular gaussian scattering distributionof scatterers around mobile station. In the models suggested
in [6], [7] the scatterers are considered to be distributed overan elliptical disk. The major axis of the ellipse is assumed to
be along the base station to mobile axis in [6] while in [7]it has aligned to the street where mobile is located. All ofthese models only the local scattering cluster which is locatedaround the mobile station is taken into account. Therefore
these models are suitable for macrocell environments wherebase station antenna heights are relatively large and therefore
there is no signal scattering from locations near the basestation. As a result, for scenarios where scatterers are locatedaround base station as well as mobile station the model’s
performance degrades.
For the small cells with relatively low height antennas theelliptical model called geometrically based single bounce elliptical model (GBSBEM) proposed in [1], [8] is more attractive.
The model assumes a uniform distribution of the scatterers
within an ellipse where the base station and mobile are thefoci of the ellipse and therefore scattering near the base station
is as likely as near the mobile. Nonuniform distribution of thescatterers over the ellipse was suggested in [9] by dividing
the main ellipse into a number of elliptical subregions where
each subregion correspond to one rang of excess delay time.
The number of scatterers within each subregion is selectedfrom a Poisson random variable with its mean chosen fromthe measured power delay spectrum. The elliptical models
are particularly attractive for consideration of the locus of
the scatterers with the same delay. They have been widelyused in the literature and simulation of the microcell and evenmacrocell (e.g. in [10]).
Geometric channel models are well suited for simulations. The
advantage of these models is their simplicity for simulation.
The shape and size of the spatial scatterer density function
required to achieve a reliable simulation of the propagation
phenomenon however is subject to debate [11]. The maindrawback of the geometric channel models is that only a single
specular reflection is accounted for and neither scattering,
diffraction, nor multiple bounces are considered.
In particular, for a street microcell, the single-bounce assumption is rather restrictive as by no means the street width issufficient to match the ellipse of the maximum delay. To
overcome this shortage the concept of effective street width
was introduced in [12]. In a street microcell scenario, the base

URSI-F JAPAN MEETING NO. 502

Fig. 1. (a) The street microcell scenario and the definition of axes. (b) An
ellipse with Tx and Rx locations as its foci.

Fig. 2. (a) A few uni-reflection paths. (b) The locus of the uni-reflection pathscattering points.

station and the mobile are not so far from each other. Moreover
the antenna heights are lower than the surrounding buildings.
In such a scenario the propagation channel is experiencing
sever multireflections. Considering the locus of the scatterers
with equal delays, as it is in the elliptical models, is not
sufficient. In fact, the prime importance in a propagation
channel is the scattering loss or path gain.
In this paper we introduce a scattering distribution function
based on major propagation mechanisms in the street microcell. In the proposed model the multi-reflections are taken
into account however we visualize them as single-bounce
scatterers. By this we can give the distribution of the scatterers
to develop the model, however this is not according to the definition of the geometrical modeling. Hence we call this modela pseudo-geometrical channel model. The physical descriptionof the model is described in section II and the scattering
distribution is obtained in III. In section IV the coefficientsfor the proposed model is given using measurement data. The
power-azimuth spectrum (PAS) from the proposed model iscompared to PAS of GBSBEM and the measurement data in
section V. Section VI is conclusion.
II. M ODEL D ESCRIPTION
The presumptions for the model are as following:a) Tx and Rx are in the same street, that is the scenario isline-of-sight (LoS).
b) Channel is two dimensional that is Tx, Rx and thescatterers are in the same plane. This is a regularassumption for most geometrical models.
c) Tx and Rx antenna heights are lower than surrounding
buildings.
d) A street surrounded by dense tall buildings with nobuilding irregularity is assumed. No crossing road and2Fig. 3. (a) Two tri-reflection paths. (b) The locus of the tri-reflection paths
single-bounce scattering points.

Fig. 4. (a) A bi-reflection path. (b) The equivalent single-bounce scattering
point.

no additional object in the environment is considered.
The schematic of such a scenario is shown in Fig.1. For
clarity an ellipse according to the conventional model has beensketched as well. Obviously the scattering point s2 can not
be inside the building zone and path p2 might be a multireflected wave, however to make the analysis simpler and toobtain a pseudo-geometrical distribution for the scatterers we
present the scatterers as single-bounce. By this assumption,
the scattering point for path p2 is identified at s2 located on
the cross point of the ellipse with the path length equal top2 and the radial line from the Rx with an azimuth equal tothe azimuth-of-arrival (AoA) of p2 . Next we observe that eventhough both s1 and s2 make equi-length paths and therefore
equal free space path losses, the actual path losses can be very
different. This is because p1 is a multi-reflected path but s1
is not. In fact, in small scenarios due to the small distance
between base station and the mobile, the dominant paths are
not so long. On the other hand because of smaller heightof the both link ends antennas the channel is experiencing
severe multi-scatterings. That is the scattering loss for eachpath is large. Therefore the dominant loss is these scenarios
is scattering loss.
Consequently the ellipse with Tx and Rx as its foci may not
be a good choice for the geometrical locus of the scatterers.
Instead, we are after the locus of the scattering points for thepaths with equal number of reflections. Fig. 2(a) illustrates a
few uni-reflection received paths and Fig. 2(b) is the locus ofthese paths scattering points. The locus of these scatterers are
two bands along two sidewalls of the street. It has to be notedthat the symmetry axis for the distribution of these points is
not the Tx-Rx axes but the x axis. On the other hand, Fig. 3(a)URSI-F JAPAN MEETING NO. 502

shows two tri-reflection paths, and in Fig. 3(b) the locus of
the single-bounce equivalent scattering points of these paths
has been illustrated. It indicates that the locus is two bandsof 3Ws apart parallel to the street walls. It is also clear thatthe symmetry axis for the locus is again the x axis. A similar
description can be given for all odd-number-reflection paths
and it can be observed that the axes of symmetry is always x
axis.
Figure 4 illustrates a bi-reflection path. Obviously the singlebounce scattering point for this path lies somewhere between
uni- and tri-reflection path scattering point locus. The singlebounce scattering point for bi-reflection path can be obtainedby first identifying sl the last scattering point of the path using
its AoA and then drawing the ellipse with foci of Tx and sl
and the major axes of the path length up to the sl . The singlebounce scattering point will then be obtained using this ellipse
and the AoA.
The previously described geometrical locus of the scatterers
indicates that the scattering loss is linearly increasing along
the y axis in both directions. This is because as the scattering
point is considered deeper into the building zone the pathhas experienced a larger number of scatterings. On the other
hand, there is no scatterings inside the street in ideal case as
it is assumed that there is no scatterer. In practice however,scatterings happens due to existence of the objects in this area.
Therefore we expect to have similar trend but in a lower ratein the street as well. That is the scattering loss increases along
the x axis as scatterer moves further from Tx or Rx.
III. S CATTERING D ISTRIBUTION
The experimental results from measurements in the dense
urban street microcell shows that the scattering distribution has
an elliptical shape but is different from conventional ellipse
because most scatterers are distributed along the street. Thisagrees with the description presented in section II which indicates that the scattering distribution diminishes more rapidly
along the y axis compared to those along x axis. An ellipse
with a focus larger than the focus of the conventional ellipse
is proposed here as the shape of the scatterer distribution. The
foci of this ellipse are on the x axes. The scattering distribution
in the street Ps and inside the building zones Pb are defined
as following and shown in Fig. 5:
√√22
Ps = e− αx (x−fn ) + αx (x+fn ) +Cx
√ 2
√ 2
2
2
Pb = e− αy y +αx (x−fn ) + αy y +αx (x+fn ) +Cy
where fn is the focus parameter, αx and αy are loss coefficients along x and y axes respectively and Cx and Cy are
constants.
IV. D ISTRIBUTION C OEFFICIENTSIn this section the results from a set of measurements in thedense urban areas are used to obtain the parameters of the scattering distribution. The measurement has been accomplished
for a small microcell LoS scenario [13]. Both the Tx and Rx
antennas were installed at equal height of 3 meters from theground level. The measurements were performed in 3 streets3

Fig. 5. The distribution function of the scatterers.

with different widths of 26, 18 and 10 m. The received data
was recorded for two different position of Rx in the streetof 26 m wide and for 3 different positions in each of theother streets. The Tx to Rx distance was always 60 m fixed.
The average height of the surrounding buildings is 30 m. The
details of the measurement campaign is explained in [13].
The coefficients for the scattering distribution for each environment can be acquired from the directional measurement
data. Fig. 6 shows the measured received power along x axis.
In this figure the Tx position for both measurement data is at
100 m and of Rx on 160 m. The solid line in this figure is
the following approximation:
L = L0 + αL (lp − lLoS ) + βL
In this equation L is the path loss, L0 is the free space pathloss, lp is the path length, lLoS is the LoS distance and αL andβL are constants related to αx and Cx . A similar process isperformed for the y axis. Fig. 7 shows the measured received
power for the point on the y axis and the solid line is theapproximation:
L = L0 + αW (lp − lLoS ) + βW
where αW and βW are constants related to αy and Cy . With
simple mathematical techniques the values of αx , αy , Cx , Cyand fn can be obtained from these approximations.
V. P OWER A ZIMUTH S PECTRUM
To evaluate the goodness of the proposed scattering distribution a directional parameter of the model shall be comparedto the experimental results. For this purpose, the PAS acquired
from measurement data for each of the streets was compared tothe one calculated from proposed scatterer distribution. Fig. 8
illustrates this for the street of 26 m wide. For comparison the
PAS obtained from GBSBEM model is included as well. The
azimuth of 0◦ is the Tx direction and increases clockwise. The
LoS component is not included in the models, therefore at theazimuth values around 0◦ and 360◦ the measurement data andthe PAS obtained from models are different. For the non-LoS
components, the proposed model obviously matches better
than the conventional elliptical model with the measurementURSI-F JAPAN MEETING NO. 502

40Measurement 1
Measurement 2
GBSBEM
Proposed−10

Power [dB]

Normalized Received Power [dB]

0

−20

−30
Measurement 1
Measurement 2
Approximation

−40

−50
0

50

100
150
Distance [m]

200

−40

0

250

Fig. 6. The measured received power along x axis and the approximation,
street width=26 m.

180
Azimuth [°]

270

360

0
Normalized Received Power [dB]

Normalized Received Power [dB]90

Fig. 8. The PAS from measurements, GBSBEM and proposed model, street
width=26 m.

0

−20

−40
Measurement 1
Measurement 2
Approximation

−60
0

−20

50

100
Distance [m]

150

200

Fig. 7. The measured received power along y axis and the approximation,
street width=26 m.

data. For the azimuth values less than 90◦ and more than 270◦
the proposed model’s PAS matches well to the measurement
data. However on the azimuth values between 90◦ and 270◦that is for the minus values of x the performance of the
proposed model shows minor degradation. The same trend
was observed for the other two streets as well. The reason isinvestigated by a precise look at the Fig. 6. It can be observedthat for the distances less than 100 m that is close to Tx but
on the opposite side of the Rx, the linear approximation does
not match precisely to the measurement data. This is most
likely the effect of a crossing street at this locations, however
to confirm the reason for this phenomenon measurements inmore streets are necessary. To improve the model different
coefficients for the negative and positive parts of the x axis
were estimated. The result is shown in Fig. 9. The PAS of the
proposed model after this modification is illustrated in Fig. 10
and the improvement is clear.
Same process was performed for other two streets and the PAS

−10

−20

−30
Measurement 1
Measurement 2
Approximation

−40

−50
0

50

100
150
Distance [m]

200

250

Fig. 9. The measured received power along x axis and the modified approximation, street width=26 m.

obtained from two models and from experiments are shown in
Fig. 11 and Fig. 11. The PAS obtained from the model after
modification is obviously matching the measurement results
better than conventional elliptical model. Table I shows thevalues for three streets. It is observed that the coefficients forthe scattering distribution are dependent on the street width.
The dependency of the coefficients to other parameters likeantenna height, base station to mobile distance, surrounding
building height, etc. could not be analyzed due to lack ofappropriate measurement data.
VI. S UMMARY
A channel model according to the physical mechanisms
in the street microcell was proposed. The expression for the
scattering distribution was provided and the necessary parameters for the distribution estimated from the measurements
data. The PAS predicted by the model were compared to thoseof conventional elliptical model and experimental results. The

URSI-F JAPAN MEETING NO. 502

50Received Power [dB]

Received Power [dB]

0

−20

−40
Measurement 1
Measurement 2
GBSBEM
Proposed
−60
0

90

180
Azimuth [°]

270

360

Fig. 10. The PAS from measurements, GBSBEM and modified model, street
width=26 m.

−20

Measurement 1
Measurement 2
Measurement 3
GBSBEM
Proposed

−40

−60
0

90

180
Azimuth [°]

270

360

Fig. 12. The PAS from measurements, GBSBEM and modified model, street
width=10 m.
TABLE IL OSS C OEFFICIENTS

Measurement 1
Measurement 2
Measurement 3
GBSBEM
Proposed

Received Power [dB]0

Street Width [m]
26
18
10

αx

αy

1.0 × 10−3

1.0 × 10−4
2.0 × 10−4
2.6 × 10−4

2.1 × 10−3
3.0 × 10−3

−20

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in outdoor propagation environment,” Proceedings of the IEEE VehicularTechnology Conference (VTC’03 Fall), Vol. 1, pp. 104-108, Oct. 2003.

−40

0

90

180
Azimuth [°]

270

360

Fig. 11. The PAS from measurements, GBSBEM and modified model, street
width=18 m.

PAS of the proposed model matches to the experimental results
better than the conventional models.
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