Assessing IEEE 802.11 and IEEE 802.16 as backhaul

Assessing IEEE 802.11 and IEEE 802.16 as
backhaul technologies for rural 3G femtocells in
rural areas of developing countries
Javier Simo-Reigadas, Eduardo Morgado, Esteban Municio, Ignacio Prieto-Egido
and Andres Martinez-Fernandez
Department of Signal Theory & Communications
Rey Juan Carlos University
Madrid, Spain
Email: [email protected]
Abstract—3G mobile networks are experiencing a great development in urban areas worldwide, and developing countries
are not an exception. However, rural sparsely populated areas
in developing regions usually don’t have any access to telephony
because operators cannot get enough revenues that justify the
investment required. In this sense, 3G small cells are becoming
a promising solution for inexpensive rural 3G coverage in
small villages, but the high cost of backhaul infrastructures
remains an obstacle. Although VSAT links are the most common
solution for rural 3G small cells, the FP7 TUCAN3G project
is considering multihop wireless networks using WiFi for LongDistances (WiLD) and/or WiMAX as an improved and lower
cost alternative for many scenarios. WiLD is considered due to
its low price and low power consumption, but several limitations
must be studied: the link capacity drops with the distance, the
performance varies enormously in real links, and the QoS support
is limited to level-2 traffic differentiation. WiMAX may offer a
parametric QoS support and a high capacity almost independent
with the distance, but power consumption and price may be
limiting factors. Both technologies are studied and compared
among them and with VSAT communication links in terms of
performance and cost. This paper analyzes and shows under what
conditions and limitations WiLD, WiMAX and VSAT can be used
for the backhaul of rural 3G femtocells.
I.
I NTRODUCTION
3G operators may provide coverage as far as revenues
compensate the CAPEX (capital expenditures, mainly related
to the deployment of infrastructures) and the OPEX (operation
expenditures, including maintenance, operation, licenses, etc.).
Consequently, many large rural areas in developing regions that
are sparsely populated lack of 3G coverage and often they do
not even have basic telephony. Hence, in the context of the
current trend towards ubiquitous communications worldwide,
the need of more affordable solutions both for 3G access and
for backhaul solutions becomes apparent.
Small cells, which were initially conceived for covering
holes in particular points of the urban area, are now becoming
an interesting solution for rural access networks [1]. Femtocells
are generally used inside buildings and are designed to be lowcost, consume low power, and be quite flexible and adaptative
so that a residential ADSL line may be used as the backhaul. A
3G femtocell, or a few of them, may be a well suited solution
for the access network in a small village, while its cost and
power consumption makes it much more affordable for an
operator than a common base station. However, a few small
cells spreaded out over a large region still present a critical
problem when it comes to find an adequate solution for the
backhaul that must connect those cells to the operator’s core
network.
Special backhaul solutions are needed for sparsely populated areas that are too far away from any well-connected
location as to use a one-hop terrestrial link for backhauling.
This implicitly leaves only two alternatives: multi-hop networks for backhaul, or satellite communications. While VSAT
systems have a high OPEX and its round-trip delay cannot
be reduced under 500 ms (if geostationary communications
satellites are used), terrestrial multi-hop wireless networks may
have an important CAPEX. This trade-off attracts the interest
on non-licensed low-cost technologies compliant with wireless
broadband standards IEEE 802.11 [2] (WiFi) and IEEE 802.162009 [3] (commonly known as WiMAX, although the WiMAX
Forum does not have a profile for the WirelessHUMAN
physical layer that describes the operation in non-licensed
bands).
This paper analyses the expected performance of WiFi,
WiFi-based TDMA solutions and WiMAX for long-distance
links and determines de conditions for these technologies to
be adequate as backhaul technologies for rural 3G femtocells.
II.
W I F I AND W I MAX: CHARACTERISTICS THAT ARE
RELEVANT FOR THEIR USE AS BACKHAUL OF 3G
FEMTOCELLS IN RURAL AREAS
A. WiFi for long distances
WiFi was conceived for local and metropolitan area networks operating in 2.4 GHz or 5 GHz non-licensed bands. The
medium access control protocol used is CSMA/CA, which is
not well suited to long-distance communications. Nonetheless,
some researchers [4], [5] have demonstrated that WiFi may
also be used for long-distance links in rural areas, as far as
line of sight may be assured, and [5] additionally proposed
adjustments to CSMA/CA for optimal performance are also
valid for long distances. WiFi can be used normally with 20
MHz channels, but 5, 10 and 40 MHz channels are also permitted. Additionally, spatial diversity with MIMO (Multiple Input
Multiple Output) can be used, and many commercial outdoor
WiFi systems are prepared with dual antennas for MIMO
2x2. Other interesting properties offered by WiFi are frame
aggregation, which permits to exchange big bundles containing
many frames with a single header, and traffic differentiation,
which permits to prioritize some traffic classes over others. The
standard includes several alternatives for the PHY (physical
layer), being HT (the High Throughput OFDM PHY) the one
with best performance in any case. Depending on the received
signal strength, different modulation and coding schemas may
be used permitting different physical bitrates. Athough WiFi
looks like a promising solution, there is not any previous
systematic analysis aiming to know the real capacity of a
long-distance WiFi link depending on the distance, the channel
quality and the real impact of the different features mentioned
above.
Some researchers such as [6], [7] and some industrial
products such as [8], [9] have proposed WiFi-based solutions
for outdoor long-distance links in which the CSMA/CA MAC
protocol is replaced by a simple TDMA protocol that avoids
contention and inefficiencies in the use of the channel. [11]
demonstrated that, under similar conditions, this kind of solutions tend to be better than common WiFi for very long
distances, while WiFi can be tweaked to be better for short
and medium distances.
No matter what MAC protocol is used, any communications solution using WiFi hardware uses a standard 802.11
PHY and can beneficiate from the spatial diversity in the HT
PHY, which raises the question whether these benefits apply
to long links with line of sight or not. There is a flexible
trade-off between diversity gain and multiplexing gain [13].
In order to benefit from any of them, the multiple data spatial
data streams transmitted in paralel must be highly independent.
[14], [15] show that it is theoretically possible to achieve longdistance high-rank LOS MIMO channels, at the cost of an
excessive size of towers to ensure the necessary separation
between antennas. Nonetheless, cross-polarization permits to
obtain two orthogonal channels [16], [17], [18].
B. WiMAX for long distances
WiMAX standards [3] were conceived for fixed metropolitan area networks, though the solution was also valid and
promising for rural broadband networks. The WirelessHUMAN PHY in the standard describes the operation in the 5
GHz non-licensed band. WiMAX uses OFDM at the PHY
layer and TDMA (Time Division Multiple Access) with TDD
(Time Division Duplexing) for contention-free operation. Different traffic classes are recognized and can receive differentiated QoS (Quality of Service) guarantees. In WirelessHUMAN, channels can be as wide as 10 MHz. The capacity
of a WiMAX link depends on the modulation and coding
schema (BPSK1/2 - 64QAM3/4), the guard interval in OFDM
symbols (1/4 - 1/32), the frame size (2.5 - 20 ms), etc. A
stable WiMAX link may a raw capacity between 1.6 Mbps and
33 Mbps with 20 ms frames and this does not depend much
on the distance except for the link budget, which determines
what modulation and coding schema can be used. Regarding
the use of spatial diversity, the same considerations previously
explained for WiFi can also be extended to WiMAX.
Even though 802.11 and 802.16 systems for operation
in non-licensed bands are inexpensive themselves, both may
require expensive infrastructures for long-distance links in
flat rural areas due to the high towers required in order to
guarantee the line of sight. This is particularly important in
non-licensed bands, where the maximum transmission power
is strictly limited (see [5]). This may imply a high capital
expenditure (CAPEX) for some scenarios, while these costs
may be substantially lower in landscapes in which towers may
be placed in naturally elevated points. On the other hand,
the OPEX is extremely low, which may result in an overall
possitive balance for WiFi and WiMAX in most real scenarios
compared to other alternatives such as VSAT communications.
Regarding the use of non-licensed bands, which are more prone
to interferences than licensed bands, their exploitation for a
carrier-grade service cannot be neglected in unpopulated rural
areas that lack of RF emisions, specially if directive antennas
are used.
Hence, at a first glance, WiFi, WiMAX and WiFi-based
TDMA solutions might be adequate technologies for low-cost
rural networks that may be used as backhaul for rural small
cells.
III.
R EQUIREMENTS FOR THE BACKHAUL OF RURAL 3G
FEMTOCELLS
A rural 3G access network based on femtocells use to
include the following elements, represented in Figure 1:
•
Femtocells in rural villages, often called Home Node
B (HNB), which provide final voice and data connectivity to terminals.
•
An HNB controller that physically resides in the
operator’s core network, usually far from the rural
area.
•
Some kind of backhaul solution that presents enough
capacity and limited delay, jitter and packet-loss for
an acceptable communication between the femtos and
the operator’s network in which the HNB controller
resides.
Hence, the traffic supported by the backhaul in both directions contains three main aggregated traffic clases: voice realtime traffic, high priority signalling traffic exchanged among
the HNB and the controller, and data. Of course, a more
complex classification is possible because ’data’ may include
traffic of very different natures, but this three classes vision is
general enough for the scope of this paper.
The backhaul must be able to assure ressources in excess
for a high quality transport for all the traffic or, alternately
it must be able to prioritize voice traffic and signalling over
more intensive data traffic and to limit the delay, jitter and
packet-loss as required for each traffic class.
In order to obtain a first approach to the capacity required
for the backhaul of rural femtocells in developing regions, a
prospective study has been done based on usage data obtained
by the incumbent operator in Peru. The data has been obtained
and provided by Telefonica del Peru. This study has shown
that:
•
For voice traffic, approximately 50% of the population
become users in the long term, generating a traffic
intensity of 15 mEr/user in the busy hour.
Regarding the delay and the packet-loss, ITU recommendations suggest certain end-to-end values for telephony. 150 ms is
a maximum limit for the one-way delay, and 2% is a maximum
packet-loss. The jitter is recommended not to go over 30 ms.
However, the backhaul has a share in those end-to-end values.
Based on the estimations of 3G femtocell manufacturers, a
maximum of 60 ms of delay can be produced in the backhaul.
It is difficult to establish a limit for the packet-loss and for the
jitter. In this paper a maximum of 1% packet-loss and 12 ms
jitter are considered acceptable.
In the case of a backhaul made up with a multihop network,
these performance limits must be respected end-to-end for
every path. This means that the maximum per-link delay
depends on the number of hops for the longest path.
IV.
M EASURING THE PERFORMANCE OF THE
TECHNOLOGIES BEING ASSESSED
Fig. 1: HNB-based 3G rural access network interconnected to the
operator’s core network.
•
•
For data traffic, the potential market is approximately
5% of the population. A peak throughput per user
of 3 Mbps must be assured for a good quality of
experience. The minimum capacity per user is 15 kbps
(DL) + 5 kbps (UL).
Signalling between the HNB and the HNB controller
is calculated as a 20% of the traffic generated by voice
and data.
The voice traffic may be modelled as to apply the ErlangB model to calculate the number of channels needed, with a
blocking probability lower than 2%. The throughput generated
by voice channel depends on several factors such as the codec
and the number of simultaneous calls. The AMR voice codec at
12.2 kbps is assumed and circuit switch multiplexing is applied
in the uplink. Besides de payload, each packet also contains
several headers for all the layers of the protocol stack: RTP,
UDP, IPSEC and IP. Based on this considerations, the linklayer throughput is around 22 kbps in the uplink and 60 kbps
in the downlink per phone call. A small village with only 500
inhabitants would generate a voice traffic throughput of 736
kbps in the busy hour, and a ’large’ village 3936 kbps in the
busy hour.
With the previous assumptions, it can be estimated that
the required capacity for the backhaul of a 3G femtocell in a
small village with less than 500 inhabitants must be at least
4.5 Mbps, and this requirement grows up slightly over 10
Mbps for a population of 5000 inhabitants. In fact, the data
analyzed come from areas where 3G coverage already exists,
in which the average income of the population is higher than
in remote areas. Therefore, the projection overestimates the
expected demand.
In order to obtain complehensive performance results for
IEEE 802.11, IEEE 802.16, and 802.11-based TDMA solutions, it is necessary to combine several methodologies.
WiMAX performance is easily approximated with simple
calculations [19], and WiLD can be theoretically modelled
with 802.11 analytical model that incorporate the SlotTime
and the propagation time [20], [5]. However, the theoretical
analysis of the performance in unsaturated conditions with the
throughput, delay, jitter and packet-loss for variable offered
traffic loads at different distances becomes very complex and
not necessarily reliable. On the other hand, although real
equipments are available for this research, experimental results
with this technologies are extremely difficult to obtain at
arbitrary distances, the range of interest being 0 - 60 km.
A network simulator such as NS3 [21] permits to do
extensive tests under different conditions and obtain results
more flexibly. Although the details of theoretical modeling for
WiLD and the NS3 validation cannot be included here due to
space restrictions, it is valuable to know that reliable theoretical
models based on [20], [5] have been used to characterize the
theoretical behaviour of WiLD systems in order to validate
simulations with NS3. The same thing has been done with
theoretical calculations for WiMAX. Once this has been done,
NS3 is considered a valid tool for simulations in this paper.
For NV2 and AirMAX systems, the lack of information
about the implemented protocols and algorithms in these proprietary solutions makes impossible both theoretical analysis
and software simulation. A testbed in laboratory has permitted
to measure the performance at short distances, and the knowledge about this technology, together with punctual measurements on a few long-distance real links, have been the base for
projections that permit to estimate the expected performance
at any distances in the range of interest. Mikrotik RouterBoard
R52Hn with MIMO 2x2 capabilities have been used for the
NV2 testbed, and Ubiquiti Rocket M5 systems habe been used
for the AirMAX textbed [8], [9]. Both technologies were tested
in the 5 GHz band and injecting traffic from D-ITG tool [22].
As NV2 has shown a better behaviour than AirMAX in all
the experiments, only NV2 has been used in the comparison
with the other technologies. A few WiMAX experiments have
also been run in order to validate theoretical calculations and
simulations at short distances, using Albentia ARBA PRO
4900-5875MHz base station and subscriber stations [10].
Link budget parameters
Transmission Power
24 dBm
Directional Antenna Gain
27 dB
Cable and connectors Attenuation
2 dB
Margin
20 dB
TABLE I: Parameters used to estimate link budgets
Dist. (Km)
0
5
10
15
20
25
30
PER=0%
42.77
37.11
32.53
28.95
26.09
23.74
21.78
Throughput (Mbps)
PER=1%
PER=5%
42.77 (-0.02%)
42.73 (-0.10%)
37.07 (-0.11%)
36.89 (-0.59%)
32.47 (-0.18%)
32.21 (-0.99%)
28.89 (-0.24%)
28.58 (-1.29%)
26.01 (-0.29%)
25.69 (-1.54%)
23.66 (-0.32%)
23.33 (-1.74%)
21.70 (-0.36%)
21.36 (-1.90%)
PER=10%
42.68 (-0.23%)
36.62 (-1.32%)
31.82 (-2.19%)
28.13 (-2.85%)
25.21 (-3.38%)
22.83 (-3.81%)
20.87 (-4.16%)
TABLE II: Saturation throughput in 802.11n MCS7, with
different values of PER
throughput in 802.11n MCS7 and with different values of PER.
[2] indicates sensitivity levels for the different MCSs providing
10% of PER, and Table II shows that, even for long distance
and the maximum value of PER, the loss of performance is
not overly significant.
Fig. 2: Relation between Throughput and distance in a Wifi link
using 20Mhz bandwidth channel, 800ns guard interval and 8192
aggregation threshold.
For simulations of WiLD links with NS3, AckTimeout
and RTS/CTSTimeouts parameter values have been correctly
chosen as explained in [5]. Reference specifications such
as sensitivity and transmission power for WiLD and NV2
equipments have been obtained from the datasheets of the
equipments used in laboratory. Based on those specifications,
a free space propagation model has been used to estimate the
maximum range of each modulation and coding schema. Table
I shows the parameters taken to estimate the link budget. For
both simulations and experimental tests, UDP flows with a
packet size of 1500 bytes have been used.
V.
P ERFORMANCE RESULTS
In general terms, the capacity of WiLD links could be associate to the saturation throughput. However, at the saturation
point the total delay is much higher than required (see Section
III). Hence the analysis is done considering that the delay must
be bounded by limiting the offered traffic load as convenient.
How much the offered load must be reduced to get a suitable
delay depends of the MCS and the link length. Simulation
results of a PtP link are shown in Figure 2 and illustrate
the relation between the throughput achieved and the distance
when the offered load is in the saturation point or above and
when the offered load is configured to set the link delay under
5 ms. As it can be observed, the throughput decreases as the
distance increases, but this decrease is notably higher when the
delay is limited due to the saturation points shifts downwards
in the link load function.
The throughput shown in Figure 2 have been obtained
considering an ideal channel. In non-ideal channels, as the
PER increases, the system performance decreases. However,
this is a slight decrease because a packet loss involves an
artificial (not caused by a collision) increase of the CW, so
that the probability of collision decreases in the following
transmissions. As an example, Table II shows the saturation
A point-to-point NV2 link has been experimentally tested.
A 20 Mhz bandwidth channel, SISO techniques, 800 ns guard
interval and 8192 aggregation threshold have been used. In
order to get maximum and delay-bounded throughput, 10 ms
and 1 ms frame duration has been used respectively, appreciating that the capacity offered by a link with delay limited to
5 ms is not significantly lower than that of a link operating
at the saturation point. Since NV2 has the same PHY layer
than 802.11n, highly similar performance is obtained for short
distance in both technologies. However, while in NV2 same
throughput is obtained independently of the distance due to its
TDMA-based design, in the capacity in WiLD links decreases
significantly with the distance.
Regarding WiMAX, its design permits to achieve a performance almost constant with the distance while the same
modulation and coding scheme can be used. The delay is
mainly determined by the frame duration chosen and,for this
reason,lower throughput values are achieved when the delay is
limited. While maximum throughput is achieved using 20 ms
frame duration, this rises the link delay over the 50 ms. Using
2.5 ms frame duration, the delay is bounded to 5 ms.
Once the different technologies have been tested it is
possible compare them in order to get a clarify view of what
are the advantages of each for use them as backhaul segments.
Following the methodology described in Section IV and the
same parameters used in the previous analysis, Figure 3 has
been composed to achieve this. While for short distances Wifi
achieves higher throughputs than NV2 and WiMAX, NV2
generally gives better performance when distance increases.
This is due to NV2 uses 20Mhz bandwidth and have not the
CSMA/CA problematic.
Although it may seem that WiMAX is the worst option to
use as backhaul segment, it must to been recalled that WiMAX
has better spectral efficiency than Wifi and NV2 and the worse
performance is due to WiMAX is using a 10Mhz bandwidth.
This could be advantageous for those scenarios where the
frequency scheduling is a constraint. NV2 seems to be the
better option, but its use limit notably the equipment to be
used in the backhaul network, since only Mikrotik implements
it. Last, Wifi gives high performance for short distances but
it decreases when the distance increase due to its CSMA/CA
nature. Although this issue could be solved using higher frame
aggregation threshold, few manufacturers allow increase it
the contributions of the colleagues from TUCAN3G Consortium (http://www.ict-tucan3g.eu).
R EFERENCES
[1]
[2]
[3]
[4]
Fig. 3:
Throughput vs distance comparison for Wifi, NV2 and
WiMAX when link delay is limited to 5 ms. 20Mhz bandwidth is
used for Wifi and NV2 and 10Mhz bandwidth for WiMAX. MIMO
2x2 techniques are used
above 8192 bytes. Even so, Wifi 802.11n has a huge amount
of standardized equipment available and the price is relatively
low comparing with other technologies which makes Wifi a
interesting alternative in those cases when traffic requisites are
not demanding and the CAPEX is limited.
[5]
[6]
[7]
[8]
[9]
VI.
C ONCLUSIONS AND F UTURE W ORKS
[10]
Extending the coverage of rural 3G access networks to
remote and sparsely populate areas require of low-cost wireless
broadband technologies for backhaul. Both 802.11 (WiFi) and
802.16 (WiMAX) have been studied as alternatives. For WiFi,
the standard CSMA/CA MAC has been considered as well as
propietary TDMA solutions that some researchers and vendors
propose as efficient replacements of the standard MAC for
outdoor setups. For IEEE 802.16, only the WirelessHUMAN
PHY has been considered for operation in non-licensed bands.
After a detailed comparison conducted through theoretical
analysis, simulation and practical experiments in laboratory,
all these technologies seem to be adequate for backhauling
rural 3G femtocells. All of them require traffic control at a
higher layer in order to keep the wireless link working under
unsaturated conditions, which in turn permits to keep the delay
and the packet-loss probability very low. While WiMAX is
the most efficient solution, the use of larger channels permit
that WiFi exhibits higher capacities at short and medium
distances, and WiFi-based TDMA solutions outperform the
other alternative at long distances.
Hence, all these technologies are constructive elements that
can be considered in a future work for a heterogeneous rural
transport networks that may act as the backhaul of rural access
networks. The specific choice among them will depend on the
requirements of each specific scenario. Further works must
advance in the integration of these technologies in multi-hop
networks and on the traffic control they require in order to
offer quality of service guarantees.
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ACKNOWLEDGMENT
This work has been performed in the framework of the FP7
project TUCAN3G IST-601102 STP, which is funded by the
European Community. The authors would like to acknowledge
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