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LTE femtocell density modelling
Michael Fitch
Chief of wireless research
Technology Services and Operations
BT Adastral Park, IP5 3RE
October 2014
© British Telecommunications plc
What is a femtocell ?
BRAS LTE EPC
Internet
BT broadband
Home
hub
LTE femto
cell
A femocell is a very small cell designed to cover a large house
with broadband - and to about 30m outside the house for voice and web browsing.
Typical transmit power is < 100mW at 2.6GHz.
DSLAM
Long Term Evolution Evolved Packet Core
Broadband Remote Access Server
Digital Subscriber Line Access Multiplexer
© British Telecommunications plc
Benefits of doing the modelling
• For BT, it gives guidelines on the conditions and confidence under which we can deploy femtocells,
• For the consumer, it gives confidence of the experience of voice and data
• Gives installation guidelines and shows the importance of having interference mitigation mechanisms in place
© British Telecommunications plc
The applications we used in the modelling
UE type Range from
femtocell
Location Max bit-rate
needed
Set-top
Box (STB)
Up to 8m Inside only 20 Mbit/s
Voice (VoLTE) Up to 30m Inside and
outside
LoS
25kbit/s
Tablet Up to 8m Inside only 4Mbit/s
Smart phone
(Googler)
Up to 8m Inside only 500kbit/s
Smart phone
Openzone / Hotspot
Up to 30m Outside only
LoS
500kbit/s
© British Telecommunications plc
The question • What is the maximum density of femtocells that:
– will support the applications
– and give adequate inside-to-outside coverage
– and give an improved user experience
© British Telecommunications plc
London Bayswater is a very dense area (10,000 premises in 1 km2), and was chosen as the area to use in the model
Number of BT
served premises
is 3062
© British Telecommunications plc
So let us set up a model…
• MATLAB was chosen for this task, because
– of the easy ability to run many times and collect statistics
– the structure of the coding using functions enables straightforward debugging and calibration
– the reading and writing of large files is uncomplicated
• The model is based on 3-D geometry and pathlosses
© British Telecommunications plc
Up to 30m
(LoS)
1
3
4
2 5
Up to 8m
(inside)
Arrangement of different UE
types
1. Set top box (inside)
2. Voice (inside and outside)
3. Tablet with HD (inside)
4. Googler (inside)
5. Hotspot (outside only)
© British Telecommunications plc
Up to 30m
(LoS)
1
3
4
2 5
Up to 8m
(inside)
Wanted paths are either
totally inside or inside-to-
outside
Wanted paths
1. Set top box (inside)
2. Voice (inside and outside)
3. Tablet with HD (inside)
4. Googler (inside)
5. Hotspot (outside only)
© British Telecommunications plc
Up to 30m
(LoS)
1
3
4
2 5
Up to 8m
(inside)
Unwanted inside
to outside interference
(nLoS and LoS)
Unwanted inside
to inside interference
(nLoS and LoS)
Interference from neighbouring
femtos is inside-to-inside or
inside-to-outside
Wanted paths
© British Telecommunications plc
Outline method of modelling outage probability • Femtocell locations are chosen at random from the test area, with a density
of 0.01 to 1,
• The horizontal position is dithered, otherwise all femtocells would be in the exact centre of the property,
• One UE of each type is placed around each femtocell, 1.5m from the floor
• For each UE the 3D matrix of pathlosses is calculated, and the UE is attached to the home one (STBs) or the best one (other UEs) unless the best one is full, then it drops back to second best etc. UEs are attached in priority order,
• For every UE, SINR is calculated, required resource blocks is calculated and the femtocell loadings are tracked, max load on any femto = 90%,
• An abstraction of the scheduler assumes that interference can be mitigated from the two nearest neighbours, until the load-demand product exceeds a threshold
© British Telecommunications plc
Model flow
Input a probability of a
broadband line having
a femtocell
Randomly place
cells Put one of each type of
UE around each cell
Start with highest
priority UE Calculate pathloss
to every femtocell and
sort
Lowest pathloss is
wanted femtocell.
Calculate wanted signal
Calculate unwanted
signal by summing
signals from other cells
Calculate wanted /
unwanted and look up
spectral efficiency
Calculate resource
blocks taken by UE
and keep track of loading
When loading > max
then UE cannot get
what it needs
Increment outage
count for that UE type
© British Telecommunications plc
Model flow
Input a probability of a
broadband line having
a femtocell
Randomly place
cells Put one of each type of
UE around each cell
Start with highest
priority UE Calculate pathloss
to every femtocell and
sort
Lowest pathloss is
wanted femtocell.
Calculate wanted signal
Calculate unwanted
signal by summing
signals from other cells
Calculate wanted /
unwanted and look up
spectral efficiency
Calculate resource
blocks taken by UE
and keep track of loading
When loading > max
then UE cannot get
what it needs
Increment outage
count for that UE type
Repeated for other UEs
© British Telecommunications plc
Model flow
Input a probability of a
broadband line having
a femtocell
Randomly place
cells Put one of each type of
UE around each cell
Start with highest
priority UE Calculate pathloss
to every femtocell and
sort
Lowest pathloss is
wanted femtocell.
Calculate wanted signal
Calculate unwanted
signal by summing
signals from other cells
Calculate wanted /
unwanted and look up
spectral efficiency
Calculate resource
blocks taken by UE
and keep track of loading
When loading > max
then UE cannot get
what it needs
Increment outage
count for that UE type
Repeated for other UEs
Repeated for other probabilities of deployment to get required density
© British Telecommunications plc
System assumptions
Parameter Value Comments Femtocell transmit power
(maximum)
13dBm per transmit port
Femtocell transmit power
(minimum)
3dBm per transmit port No UEs in connected mode
Frequency 2600MHz FDD
UE receiver noise figure 7dB
Bandwidth 15MHz
Symbols per second per femtocell 12.6 Million This is 900 sub-carriers * 2000 slots per second * 7
symbols per slot
Transmit overheads 3/14 This is for reference and control signals
Maximum femtocell load 90% The percentage of resource blocks allocated / number
available
Minimum distance a UE can be from any femtocell 1m This limit is tighter than the use-case UEs and is imposed
by the path-loss models
Wall penetration loss (per external wall) 12dB mean, 2dB STD. Minimum loss capped at 2dB. This is for brick or block built housing. Across all building
types, the mean is still 12dB but the STD climbs to 9dB.
Indoor path-loss model ITU-R P.1238-7 + 5dB 5dB correction added as a result of field measurements,
caused by femtocell being low down, 500mm from floor.
Outdoor path-loss model ITU-R P.1411-4 Includes LoS and non-LoS components with 20m
transition region
Minimum required SNR or SNIR -4dB For a UE to be able to attach (based on 1/12 QPSK)
© British Telecommunications plc
With STB at 10Mbit/s
With STB at 20Mbit/s
Example results - outage probability
No STB
© British Telecommunications plc
Capacity estimate
• The spread of bit-rates across femtocells is commercially sensitive,
• Below are some broad results that we can share:
Density limit /km2
(femtocells)
Total system capacity /
Tbit/s
300 Max less 30%
900 Max (> 250)
1500 Max less 10%
2300 Max less 30%
No limit A tiny bit less again
So, from the point of view of outages, and of maximising
system capacity, a density of about 1000 per km2 is the
optimum
© British Telecommunications plc
Some guidelines from the modelling The maximum density is determined by the heaviest user, which in this case is the
set-top box,
STBs should be discouraged from being connected using LTE femtocells
A density of around 1000 per km2 may be a suitable target density
– It allows a good user experience with a low outage probability
– It gives the maximum system capacity
© British Telecommunications plc
The question – how well answered ? • Can we deploy LTE femtocells at a density that is high enough so we can send
one to every BT broadband customer: - almost
– while supporting the applications in the business case
• Yes except for STBs
– and give inside-to-outside coverage that is comparable to WiFi
• Yes
– and give an improved experience with voice and data
• For voice yes, for data maybe – for further work
– and give a reduction in customer complaints (propensity to call)
• Remains to be seen
– and cost the consumer no more, and cost BT as little as possible…
• Ongoing work
© British Telecommunications plc
The question – how well answered ? • What is the maximum density of femtocells that:
– will support the applications
– and give adequate inside-to-outside coverage
– and give an improved user experience
• About 1000 per km2
© British Telecommunications plc
A bit about the MATLAB implementation
• Probability based (no time functions)
– Although we are now developing this, a per-TTI simulation
– LTE toolbox not used in current model
• Biggest challenges
– Handling and sorting large files • Input files are .txt with around 6 million lines
• Fastest way of sorting into histogram is using integer division and direct allocation to kilometre squares
– Working through large matrix calculations • Up to 3000 * 3000 * 5 radio path-losses and summing interference
• Would do better by implementing a nearest neighbour function using tree structure, and then ignoring path-losses that are above a threshold
© British Telecommunications plc
Example code for sorting broadband lines into tiles
load 'bblocsx.txt'
load 'bblocsy.txt'
num = size(bblocsx);
for i = 1:num
xint(i) = uint32(round(bblocsx(i)));
yint(i) = uint32(round(bblocsy(i))); end
a = max(xint);
b = max(yint);
sq = zeros(a, b);
for i = 1:num
if xint(i)~= 0 && yint(i) ~= 0 % If the square km has a location
sq(xint(i), yint(i)) = sq(xint(i), yint(i)) + 1;
end
end
% m = 1;
% for i = 1:a
% for j = 1:b
% for k = 1:num
% if xint(k) == a && yint(k) == b
% sq(m) = sq(m) + 1;
% end
% end
% m = m + 1;
% end
% end
[a, b] = size(sq);
sqr = reshape(sq, a * b, 1)
[h, values] = hist(sqr(sqr~=0), 100);
c = cumsum(h);
x = sum(h);
Not
efficient!
Efficient!
© British Telecommunications plc
Many thanks for listening