Should we dig more trenches or build more towers to bring broadband internet to a wider audience? Ten years ago, this would have been a moot point: broadband internet was only practical over wired connections via telephone lines, coax cable networks or, for a lucky few, fibre-optic links. Now, with the rapid evolution of the 3GPP LTE standard – commercial networkslaunched in January 2009 –the idea of cellular broadband backhaul is becoming a reality.
Market pull, technology push
Why is this? The answer is twofold.
Firstly, let’s look at the market demand. Consumers want to access streaming video, social media, and Internet-based services as easily when they’re out and about as when they are at home. Commercial and industrial users see business opportunities in providing cloud access and enterprises services to places where cables can’t go, and to userson the move .
Secondly, the cellular industry and regulators have been very successful in developing LTE to deliver faster broadband in response to this demand. Figure 2shows how rapidly LTE bandwidth is increasing, thanks to the evolving 3GPP LTE specification. LTE’s emergence as the dominant cellular technology, coupled with the redefinition of the smartphone as the internet access device of choice for many,has driven the standard’s evolution much faster and further than was imagined just six or seven years ago.
Figure 2: The rapid evolution of the LTE standard is increasing the bandwidth of cellular connections
Improving wireless bandwidth
Here are some of the techniques that have been added to the LTE specification, since the first version was ratified as 3GPP release 8 in December 2008, to improve its capabilities:
The first LTE spec supported the use of two antennae at the basestation and two at the terminal to double data rates. This multiple input, multiple output (MIMO) approach has been extended to 4×4 and even 8×8 configurations. Although an 8×8 antenna array may be impractical for a handheld device, it would be perfectly possible for a fixed wireless access device, such as a home gateway.
MIMO uses multiple antennae to transmit and receive signals. Beam-forming goes a step further by using an array of antennae (4, 8 or even more) and advanced signal processing to steer a beam of RF energy towards the mobile terminal. This means each terminal can receive more RF energy and therefore a clearer signal with greater bandwidth. The approach also means that multiple beams can be steered to reach multiple devices in parallel – a form of spatial multiplexing that can enable the same amount of spectrum to carry more data.
Another way to increase bandwidth is to use more channelsby aggregating RF bands. If a mobile network operator owns 3 x 10 MHz and 1 x 20 MHz bands, these can be aggregated into a 50MHz band with five times the bandwidth of one 10MHz band. The first LTE specification to support carrier aggregation limited its use to two bands. The most recent, Release 13, allows the aggregation of four bands.
A coordinated multipoint strategy uses simultaneous connections from multiple basestations to a mobile terminal to provide multiple data channels.Users get a more consistent service as they move away from their primary basestation,because stronger signals from other basestations ‘fill in’ for its fading signal.
The wired alternative
Fixed broadband technologies haven’t stood still while LTE has been advancing.
Cable communication has also evolved significantly since the first telephone call in 1876. Initially, the Digital Subscriber Line (DSL) standard was used to provide low-bandwidth data connections over twisted-pair cables, an approach later enhanced through the introduction of VDSL and ADSL variants. Cable TV companies also laid private networks of co-axial cables over which they could offer TV services. These later became the bearer for cable broadband services, implemented using the Data Over Cable Service Interface Specification (DOCSIS) released in 1997.
Like LTE, the DOCSIS standard has been steadily improved, with the introduction of higher data rates and IP signalling. The latest version, DOCSIS 3.1 released in 2013, supports Gbit/s data rates using techniques such as deep modulation and multiple subcarriers – similar approaches to those used by LTE.
However, the performance of cable systemscan degrade quite rapidly with distance from the central office, and so consumers who are a long way away from it may suffer from relatively poor service.
Fibre-optic distribution systems don’tface this issue. Fibre-optic cable can run for tens of kilometres before light dispersion within the fibre affects throughout. Fibre-optic cables can also deliver very high bandwidths, in part because they support a variety of modulation and multiplexing schemes. The major drawback of fibre systems is that they are expensive to deploy and maintain, because it is difficult and time-consuming to make low-loss, optically perfect connections in the field.
Since it usually costs too much to build fibre to the premises (FTTP) schemes for mass adoption, various hybrid approaches have been tried to balance the advantages of fibre networks with their costs. For example, the ‘fibre to the distribution point’approach runs a fibre-optic cable to a small hub or distribution cabinet in the street, from where a connection is made over existing telephone lines to each customer’s premises using a signalling technique called G.fast. This evolution of DSL trades distance for bandwidth, and so can act as a relatively high bandwidth connection of up to 1Gbit/s over up to 400m.
If we compare the achievable bandwidth of LTE with that of contemporary cable technologies (see Figure 3), LTE is clearly catching up with wired alternatives.
LTE is becoming a viable alternative to wired connections, especially for new deployments and to replace ageingcable networks. The choice of approach isn’t just about the technology or capacity: the cost of maintaining a few cellular basestations in well-controlled environments is likely to compare favourably with those of looking after a widely distributed, ageing twisted-pair, coax or fibre network running in trenches and culverts and entering all sorts of customer premises in all sorts of ways.
There are regulatory issues to be considered as well, especially for green-field deployments, where the time and costs involved in securing the rights to run wired networks across multiple owners’ land can be unpredictable. On the other hand, cellular coverage in many environments is already well established. Basestations have often been in place for decades, and there’s a long history of updating them as the cellular standards evolve.
Building LTE-based gateways
If you choose to use the LTE network as backhaul, how do you build the right set-top box, home hub, gateway, or internet access point for your needs? The fact that there are so many names for what used to be known as ‘customer premises equipment’ reflects the way that broadcast entertainment, video on demand, internet access and, more recently, smart home services, are converging and competing for a space in the home. One technical response to this is to build a single, integrated box providing many or all of these options.
Figure 4: The emergence of smart homes is driving a convergence between IT connectivity (gateways), home entertainment (TV, STB) and the IoT (connected home), in which wireless connectivity is key
On the service side, some operators, such as AT&T with its Digital Life initiative, are now offering a package of fixed and mobile telephony, TV, internet and smart-home services. AT&T has become a prime mover in the trend towards LTE internet connectivity with their Wireless Home Phone & Internet offering.
Migrating to LTE
A typical gateway (see Figure 5) consists of a backhaul connection to the TV and/or internet head-endand several types of local connectivity.
Figure 5: The connectivity of a typical home gateway
Moving to LTE means changing the backhaul connection (see Figure 6). Although most of the gateway’s design and operation is well understood, introducing another complex multi-antenna RF system and having its cellular subsystem independently tested and certified by operators adds complexity.
Figure 6: Block diagram for a generic home gateway
To address these issues, u-blox has developed a range of modules for customer premises equipment, set-top boxes, gateways and routers.They include a high-performance, certified LTE CAT6 modem and various pre-integrated local connectivity options (such as WiFi, Ethernet and Telephony) that can significantly reduce development risk and time to market.Furthermore, working with our partner Intel to exploit the advanced features of their LTE chipset u-blox has been able to create an optimised architecture for gateways, routers and CPE (see Figure 7).
Figure 7: Using a u-blox TOBY-L410 module as theheart of a gateway (Source: u-blox)
Cellular technology began as a way to cut the cord for telephony services. As LTE evolves, the same thing is about to happen to the broadband services we have all come to rely on at home and at work.
Charles Sturman, Senior Principal Product Strategy, u-blox
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