Can Wi-Fi Really be used to Backhaul Cellular Voice?

the strain of a mobile data onslaught, the move to small cells is opening the
door to new and unexpected uses of smarter Wi-Fi



As operators are confronted with insane
demand for mobile data, there are at least four distinct areas where congestion
may appear:

        * The
access radio network,

           * The
signaling and control portions of the network,

           * The
network packet core and

           * The
backhaul network.


Each of these choke points pose a unique
challenge to the operator and generally can be addressed in one of three ways:


capacity of the affected network resource,

the network resource to relieve congestion, or doing both.


The move to smaller cells to augment existing
macro networksis widely viewed as a potential panacea to the access radio
network congestion these problems but also creates a new one: backhaul. This
has become one of the telecom industry’s hottest debates.


Mobile operators are planning their LTE
networks as a combination of macro cells and an ‘underlay’ network of smaller
micro and Pico cells. To achieve the capacity density required by rapidly
rising mobile Internet bandwidth demand, these small cells will need to be much
larger in number in a given area than is the case in current cellular networks.
This presents a new and very significant backhaul challenge – because the
mounting locations of these small cell nodes (such as utility poles or other
street-level assets) will very rarely be a natural fit for fiber or microwave


The concept of LTE self-backhaul or meshing
is one possible solution, but as with early mesh Wi-Fi networks that attempted
to provide access and meshing all within the same spectrum band, this approach
rapidly consumes scarce (and expensive) LTE access-capable spectrum with
backhaul traffic. The option of using a 5 GHz 802.11n point-to-point solution
is a very attractive alternative- easily delivering the more than 100 Mbps of
backhaul capacity an LTE cell will need.





Small cells are low-powered, multi-radio
access points (cellular/Wi-Fi/backhaul) that improve indoor and outdoor
coverage to increase capacity and offload traffic – as much as 80% during peak
times. While small cells benefit 3G service deployments today, their importance
will only grow as the industry moves towards higher capacity 4G / LTE,
especially in urban environments. According to In-Stat’s latest report, Femtocells and Small Cells: Making the Most of
, small cell shipments will reach $14 billion in 2015.


The problem is, as network operators continue
to increase coverage and capacity and look to offload data to relieve traffic pressures,
they also increase the stress on their cell site backhaul connectivity. In this
small cell world, conventional point-to-point microwave, bonded copper, and
fiber-based backhaul solutions can quickly become impractical or uneconomical.


While microwave point-to-point equipment
costs have come down in recent years, it generally requires a line-of-sight
(LOS) link with the connecting backhaul hub, a condition many small-cell
locations will be unable to meet. Sub-6 GHz NLOS solutions using a point-to-multipoint
architecture are better suited for dense underlays, but when using licensed
spectrum, narrow bandwidth channels put strict limits on backhaul capacity, and
most sub-6 GHz spectrum bands are expensive and frequently not available for


Another choice, fiber, is clearly the
preferred backhaul option for mobile operators (if you can get it). But pulling
fiber to every small cell location is, well, just not going to happen.It’s
simply too expensive, disruptive and time consuming. Consequently, traditional
cellular backhaul solutions must now be rethought in the context of moving to
smaller cells.



New Backhaul Options


New backhaul options, well suited for dense
urban environments and for close-to-the ground equipment (both line of sight
and non-line of sight), are required to make small cells viable.


Counterintuitive to most, unlicensed smart
Wi-Fi has become a viable and affordable option to solve this problem and looks
to play acrucial role in backhauling licensed small cell traffic.  Yes, cellular traffic. Here’s why:


Assume a mobile network operator (MNO)
deploys an infill underlay radio network of small cells to add access capacity
to areas where there is a high density of mobile data users, perhaps in an
urban city center such as in London, New York, or Hong Kong.


Today this small cell network would likely be
comprised of lower-powered 3G and/or Wi-Fi nodes, or possibly in the future LTE
radio nodes. No matter what the access radio technology is used, how does the
operator get the data from the access radio node back to the network?


One obvious high performance solution is
fiber, assuming that it’s available. The operator may have to lease this fiber
from a fixed line carrier which drives up operational costs, but perhaps more
significantly there is the very real possibility that the fiber POPs will not
exist in specific locations where the MNO needs to place the small cell.


The reality is that small cells only increase
network capacity if you place them in close proximity to subscribers trying to
access the network. Therefore site acquisition becomes a major determinant in
the relative effectiveness of the small cell deployment.


But this then poses a very real problem –
given the constraints of where operators must place small cells. It is highly
unlikely that a fiber POP will exist in all of those locations. And given the
cost and time delays of provisioning new fiber runs to each small cell
location, an alternative solution is clearly needed.


Microwave radio links are of course a
well-understood alternative technology that can be used to at least partially
address the problem. But while microwave point-to-point (PtP) links are high
performance, reliable workhorses for backhauling data and voice traffic, they
have issues.


First and foremost, PtP microwave solutions
generally rely on licensed radio bands for transmission. This improves
reliability, howeveracquiring new licensed spectrum takes deep pockets filled
with lots of cash. Also radio capacity is directly related to how much spectrum
is used for the radio transmissions.This means deploying more capacity on the
access radio side exacerbates both the cost and the shortage of spectrum for
the backhaul radio network. Add to this the problem that PtP radio links require
highly skilled installation to aim or align the radio nodes. In a crowded urban
area or near street level, this quickly becomes an onerous task.



the Unlicensed Band for Transporting Licensed Band Traffic?


Exactly.Wi-Fi has evolved to becomean ideal
solution for this small cell backhaul problem – if done properly.


New Wi-Fi technology has been developed that
combines integrated adaptive directional antennas with smart meshing technology
and predictive channel management – all used within the channel-rich 5GHz
802.11n spectrum. The combination of these technologies makes the use of Wi-Fi
for both line-of-sightand non-line of sightbackhaul applications advantageous.

antenna arrays deliver more reliable connectivity at longer ranges by focusing
and steering RF energy only where it helps deliver the best throughput across a
specified link. As the environment changes, these smart antennas mitigate Wi-Fi
and non-Wi-Fi interference, constantly selecting better signal paths that yield
the highest data rates and lowest latency at any given time. When used within
the 5GHz band, these antenna arrays become ideal for constructing highly
resilient, long range, adaptive backhaul connections between Wi-Fi nodes.


channel management is then used to optimize RF channel selection by maximize
network capacity specifically in high-density, noisy public Wi-Fi environments.
It does this by measuring actual channel throughput and building a statistical
model that allows access points to learn over time what channel will yield the
highest capacity. By relying on real-time, observed capacity on all 2.4 and
5GHz frequencies, backhaul links can be automatically moved to a better channel
with less interference thereby realizing higher data rates.


smart mesh techniques with adaptive antenna arrays as an alternative to fixed
PtP links eliminates much of the complexity associated with aiming and
alignment during the installation process. This also results in a much more
affordable solution with greater resiliency in crowded urban environments given
its intrinsic capabilities to dynamically adjust to changing conditions by
choosing alternate paths to the network.


live field trials with multiple network operators today, this small cell Wi-Fi
backhaul approach has proven to deliver reliable, carrier grade transport of 3G
mobile data and circuit switched voice traffic along with the prioritized
transport of timing signals (eg. IEEE 1588v2/PTP or NTP) necessary for small
cell network synchronization.


backhaul technology is currently being built into small cell nodes housing
cellular and Wi-Fi access within a fairly small footprint. This allows
operators to deploy a single box to provide Wi-Fi access, cellular access and
backhaul together.


with small cells and better backhaul, mobile subscribers should enjoy higher
speeds with more coverage in more places. 
In turn, mobile operators can reduce subscriber churn and increase
revenue by having visibility into both cellular and Wi-Fi traffic – giving the
customers more options to connect in more places.


 By Telecom Lead Team

[email protected]