In future wireless networks, a significant number of users will be vehicular. One promising solution to improve the capacity for these vehicular users is to employ moving relays or car base stations. The system forms cell inside the vehicle and then uses rooftop antenna for backhauling to overcome the vehicular penetration loss. In this paper, we develop a model for aggregate interference distribution generated from moving/parked cars to indoor users in order to study whether indoor femto-cells can coexist on the same spectrum with vehicular communications. Since spectrum authorization for vehicular communications is open at moment, we consider two spectrum sharing scenarios (i) communication from mounted antennas on the roof of the vehicles to the infrastructure network utilizes same spectrum with indoor femto-cells (ii) in-vehicle communication utilizes same spectrum with indoor femto-cells while vehicular to infrastructure (V2I) communication is allocated at different spectrum. Based on our findings we suggest that V2I and indoor femto-cells should be allocated at different spectrum. The reason being that mounted roof-top antennas facing the indoor cells generate unacceptable interference levels. On the other hand, in-vehicle communication and indoor cells can share the spectrum thanks to the vehicle body isolation and the lower transmit power levels that can be used inside the vehicle.

1. 3rd IFAC Symposium on TA
Modeling the interference generated from
car base stations towards indoor femto-cells
Byungjin Cho*, Konstantinos Koufos, Kalle Ruttik, Riku Jäntti
Department of Communications and Networking
Aalto University, School of Electrical Engineering
byungjin.cho@aalto.fi
http://arxiv.org/abs/1505.06855

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Introduction
Vehicle users directly connecting to existing cellular network (BS)
suffer from weak link between user and BS due to penetration loss
demand for higher capacity due to increasing traffic
Employing moving relays/car base stations
with one antenna outside vehicular for backhaul link communication
with another antenna inside vehicular for in-vehicular communication
Considering spectrum sharing between indoor femto-cell and vehicular
communications
backhaul link communication utilizes same spectrum with indoor
femto-cells
in-vehicular communication utilizes same spectrum with indoor femto-cells
while backhaul link uses different spectrum

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System model
Spectrum coexisting issues between the moving networks and indoor
femto cells
Need to model aggregate interference distribution, outage probability and
SIR distribution at the femto-cell
Street layout following Manhattan grid
Interference is modeled at the worst case
femto-cell facing a cross road
Distribution of vehicle along a street is
modeled by a Poisson point process
Rayleigh fading
dual-slope path loss model considering
contributions from LOS and NLOS cases

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Modeling Interference Distribution
For the non-singular path loss model, the total interference from all
vertical streets in a Manhattan grid is
LIΦ(z)
(s) = exp −2 · λ
∞
k=0
sz
βk
arctan
sz
βk
λ is PPP density of cars along a street
z = Pt · C · Lwall · η denotes the combination of the following
parameters, transmit power, attenuation constant, Wall
attenuation and car isolation
1
βk
corresponds to NLOS propagation from the crossing in
the k-th vertical street toward the victim building(femto-cell)

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Approximating Interference Distribution
No closed-form expression for the inverse LT motivates the use of
approximations for the aggregate interference
LT of interference with the singular path
loss model resembles the Characteristic
function of Levy distribution
LIΦ(z)
(s)=e
−πλ 1+ζ( α
2
)D
− α
2
√
sz
Fitted inverse Gamma is a good approx
particularly in upper tail, determining the
accuracy of approx. in lower tail of the
SIR distribution
(a) Excluding k = 0-th street, (b)
Including k = 0-th street
Contribution from street not facing the
femto-cell is negligible

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Outage probability
With Nakagami-m fading channel for propagation indoor, LT plays a
central role in quantifying the outage probability (OP) in femto-cell
P(SIR ≤ γ) =
1 − m−1
i=0
(−ξ)i
i!
di
dξi LIΦ(z)
(ξ)
where ξ = m·γ
Prx
- γ is SIR target
- Prx is mean wanted signal level at the
femto-cell
- m is the number of Nakagami paths
Impact of car isolation η on OP,
as η ↑, OP ↓
For mounted roof-top antenna,
η = 0dB. For in-car BS, η > 0dB.
Spectrum sharing for indoor femto-cells and in-car BS seems feasible

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SIR Distribution
Useful to approximate the SIR distribution by some known function so
as to assess its mean and higher moments in a low complex manner
SIR is product of two independent
Gamma RVs, SIR = S · I−1
Φ
- S ∼ Gamma(m, θ)
- I−1
Φ ∼ Gamma(a, 1/b)
CDF of the SIR distribution can be
expressed in terms of the Meijer G
function P S
IΦ
≤ γ =
G 2 1
1 3
1
m,a,0 γn
Γ(m)Γ(a)
.
High computation of OP for high m in
Nakagami fading
Approx. SIR is useful for estimating not
only OP but also its moments
(a) η = 0 dB, (b) η = 20 dB

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Conclusion
We developed a model for aggregate interference distribution generated
from cars to indoor users
- in order to study whether indoor femto-cells can coexist on the same
spectrum with vehicular communications
In dense urban cities vehicular communication may use same spectrum
with indoor femto-cells
- Better quality of experience for end user in car would come at the cost of
higher interference generated indoors
We suggest that V2I and indoor femto-cells should be allocated at
different spectrum.
- Mounted antennas on the top of the vehicles generate unacceptable
interference levels.
- In-car BS can share the same spectrum with indoor femto-cells thanks to
the vehicle body isolation

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Thank you
byungjin.cho@aalto.fi
http://arxiv.org/abs/1505.06855
originally published in IFAC-PapersOnline, DOI:
10.3182/20131111-3-KR-2043.00030, 2013