Heralds of the New Backhaul Technologies
By John McNicol, Director of Marketing, MMIC Solutions UK

The “backhaul” market is booming. Mobile operators’ investment in equipment to connect their base stations to the core networks increased 60% in 2008 and as much again in 20091 and growth like this seems set to continue for the next few years. Interestingly, U.S. operators are leading the way, with AT&T making another $2Bn increase in capex on wireless equipment to upgrade its networks in 2010.2

Driving these investments is the exploding demand for mobile data capacity for media-rich applications such as YouTube, and new mobile devices like Apple®’s iPhone. In 2009, Cisco estimates global mobile data traffic increased 160%; data on the UK’s O2 mobile network doubled every 3 months. AT&T reports a 5000% increase over the last 3 years3.

Behind the good news of these statistics hides the challenges in upgrading 2G & 2½G networks designed for only a few hundred kilobits per second (kb/sec) from each mobile handset. New 3G phones using High Speed Packet Access (HSPA) techniques now offer data download speeds of more than 10 Megabits per second (Mb/sec) to a single handset, and forthcoming 4G technologies are expected to offer downlink rates of >60 Mb/sec.

Since many base stations are served by only a couple of 2 Mb/sec T1 connections to support voice traffic from several simultaneous users, how will operators offer good quality 4G services from a downtown cell site which will need peak backhaul capacity of ~400 Mb/sec?

The Backhaul Challenge
Connecting cell sites to the core network is the “backhaul” challenge because the nearest optical fibre may be several miles away. Connections can be wireline, such as copper or fibre, or wireless at various microwave, and now millimetre wave, frequencies.

Copper
In the U.S., the backhaul connection has often been copper due to the relatively low cost of leased 2 Mb/sec E1/T1 lines. Copper can support high capacity transmission such as Gigabit Ethernet but, as anyone who lives too far from their local exchange to receive good DSL broadband knows, high data rates are difficult to maintain reliably over long range. Also note that if there is no suitable cabling near the cell site, the cost of digging up roads to install new cables, and traffic disruption, is very high.

Fibre
Fibre offers enormous range, and new laser and modulation techniques are also enabling very high capacity on a single fibre. It’s an obvious solution, but it is not available everywhere. In the U.S., still less than 10% of cellular towers have a fibre connection within easy reach. Operators are extending their fibre networks, but installing new fibre is very expensive. Estimates for metropolitan urban areas range from $200,000 to as much as $500,000 per mile4.

Microwave
One can easily appreciate from the large number of microwave dish antennas mounted on cellular towers that more than 50% of all mobile backhaul worldwide is by microwave5. Microwave links can be quickly and easily installed without the huge cost and disruption of laying fibre or cable, which makes them an obvious choice when building new networks. A third of operators building out new 4G WiMAX™ networks will use only microwave for backhaul. Over the years, microwave links have also proved to be as reliable as fibre equipment and to have a Mean Time Before Failure (MTBF) better than copper connections.

Microwave links for backhaul operate in regulated frequency bands from 5 or 6GHz up to around 40GHz, depending on the country’s governing authority. Allocated bands are often quite narrow (28MHz) to drive efficient usage of precious spectrum resources, and are subject to a local government planning and licensing process to enable frequency re-use in dense urban areas. How can microwave radios support the data demands of new 4G services?

In many countries, more channels and wider channels are being allocated for microwave backhaul (56MHz and more in some cases). Microwave equipment vendors are also innovating. Cross-Polar Interference Cancellation (XPIC) techniques eliminate the usual interference between signals on two polarizations at the same frequency, thus doubling the data capacity. Higher order modulation schemes such as Quadrature Amplitude Modulation (QAM) also increase data rate in a given frequency channel. Microwave vendors now offer equipment with adaptive modulation, switching between high order modulation to drive high capacity in good weather, and scaling back to low order modulation to preserve the link in poor weather. Although some microwave links are duplicated to provide redundancy for critical connections, space on cell towers to mount radios is expensive, so simple multiplication of links for extra capacity is not commercially viable.

The Capacity Crunch
Increasing backhaul capacity is not only a technical challenge, it is also a commercial one, too.
Although the demand for data capacity is sky rocketing, the network operators’ revenues do not grow in proportion. Many of the new applications are free to the users, and operators must compete with fixed monthly pricing for wireline broadband. In this market, operators can attract and retain subscribers with “unlimited” mobile data and smart phones, but cannot charge users by the megabyte.

Caught between a rock and a hard place, operators keen to exploit the exploding demand for services and data in an environment of falling revenue are looking for network upgrade equipment providing the lowest possible cost per bit per second. The so called “capacity crunch” in backhaul is already with us, and is only going to get worse.

Millimeter Wave Links
A new low cost per bit solution becomes possible by taking advantage of substantially wider channels in the millimetre wave (MMW) bands. The U.S. FCC allocated 10GHz in the “E-bands” from 71 to 76GHz and 81 to 86GHz for use by point-to-point communications links, and has implemented a “light licensing” regime in which users register the location of their link on a central web-based database on a “first-come-first-served” basis. This avoids the often protracted and expensive frequency planning process for microwave links, but still provides operators with regulated protection against a later installation in close proximity. CEPT & ETSI are also well down the road of allocating these bands, also under a “light licensing” regime, in Europe.

MMW links continue to offer the advantages of microwave backhaul – easier installation than fibre and copper and so, faster rollout for new networks – alongside a much larger capacity and smaller dish antennas due to the higher frequency of operation. In these 5GHz wide channels, simple modulation such as Binary Phase Shift Keying (BPSK) or even On-Off Keying (OOK) delivers more than 1 Gb/sec in both forward and reverse directions simultaneously by Frequency Division Duplex (FDD) using a low cost diplexer. MMW technology is already proven in the U.S., Japan, and Europe in applications extending Gigabit Ethernet networks across campuses, business districts, and to backhaul CCTV video from city centres and traffic intersections to central control rooms.

But there is no such thing as a free lunch. Although the MMW bandwidth available is very much higher than it is in the microwave bands, absorption by rain is also much higher. The increased Bit Error Rate during “rain fade” reduces capacity and can result in a complete loss of signal. Links for mobile network operators must be operational 99.999% of the time at their location, with less than 5 minutes downtime per year. In most temperate climates, the range of such a “five nines” link at 13GHz can be 25 miles, a 38GHz link up to 3 miles, and an E-band link is around 1 mile. Even so, this isn’t as much of a disadvantage as it might seem, because much of the new high capacity infrastructure is needed in urban and suburban areas where most of the high data rate users live and work, and where the length of the backhaul “hops” to the core fibre network are much smaller.

Cost Per Bit
Low cost per bit radios are the key to tackling the backhaul capacity crunch. Microwave links can now cost less than copper or fibre. In Europe, a 2 Mb/sec leased line (E1) costs $800 to $1,000 per month, compared to the typical $500 per month cost of a 2Mb/sec microwave link.
However, upgrading microwave links within narrow 28MHz or 56MHz channels to achieve the high data rates needed for future LTE systems requires high order modulation schemes, XPIC techniques, very linear microwave components and complex digital processing. This increasing complexity drives their cost up.

However, new technologies and increasing volumes are driving down the historically higher cost of MMW radios. MMW semiconductors have been more expensive than microwave integrated circuits for the 20 to 40GHz bands, but new lower cost processes manufactured on larger wafers, such as Silicon Germanium CMOS, are now being used to radically reduce the cost of MMW radios6 to equal, if not beat, that of microwave radios with much less capacity.

Traditionally, MMW radio components have also demanded expensive precision machined metal with tight tolerances, hand-tuned to avoid a number of highly destructive effects that arise when wavelengths approach one millimetre (the same size as cavities, chips, and bonding wires). However, technology is now available to eliminate these issues. For example, MMIC Solutions in the UK uses lower cost materials, simple circuit-board construction, and automated manufacturing to supply low cost components for 1 Gb/sec MMW radios7.

High gain dish antennas for MMW links are also smaller and cheaper than for microwave, simply due to the higher wavelengths used. With a little training they can be mounted unobtrusively on buildings, which reduces tower mounting costs while also avoiding concern from planning officials and local residents.

The Move to Ethernet Protocol
Upgrading backhaul capacity to provide new high data rate services is not only about wide bandwidth and low cost per bit. Most of the data to be carried is no longer voice, but transported in packets. As everyone who has made a call using Voice over Internet Protocol knows, voice is very sensitive to delays in a packet-based network. So, data transport for previous mobile standards is Time Division Multiplexed (TDM) and time synchronised to provide guaranteed Quality of Service (QoS) for voice communication.

Packet-based transport by Internet Protocol (IP) offers huge cost savings for two main reasons. Firstly, there is a wealth of low cost standardized equipment developed for millions of IP networks worldwide rather than the often custom-built switching systems used for TDM. Secondly, statistical multiplexing uses the capacity bandwidth much more efficiently. This makes all-IP transport very attractive, but the lack of time-synchronisation to support high quality voice is a barrier for most established networks.

New standards such as “synchronous Ethernet” and “pseudo-wires” are being developed to transport packets of voice data in a virtual wrapper of time information to provide QoS for voice over IP networks. Network architectures, often a tree and branch structure, may also need to change to realise the cost benefits of IP transport. For example, mesh architectures can allow multiple redundant paths at lower cost than conventional “protected” links; two microwave links side by side. Mobile data traffic is also often very asymmetric, with much higher capacity required for downlink to the mobile device than uplink to the network. This can conflict with a legacy switched-circuit network designed for voice traffic.

WiMAX™ & LTE – New Networks, New Backhaul
Many operators have announced upgrades to their existing 3G networks using the Long Term Evolution (LTE) standard, some launching first limited services during in 2010. LTE8 was developed by the 3G Partnership Project (3GPP) as an evolution from previous standards and deployment is expected to take several years in order to support a gradual transition from legacy TDM services.

The similar WiMAX9 standard was originally conceived and promoted by some non-traditional telecom equipment and semiconductor manufacturers. Now formalised in a family of IEEE 802.16 standards, it is already deployed in more than 140 countries and is forecast to cover 800 million subscribers by the end of this year. In many countries WiMAX has been a brand new network deployment, which avoids many of the difficulties associated with upgrading existing networks and allows the cost advantages of the all-IP transport to be realized more quickly.

The rollout of such new all-IP networks has already provided the opportunity to install low cost all-IP backhaul links using both microwave and MMW radios. The shorter range obviously makes MMW radios unsuitable for some network nodes, but they offer very wide bandwidth capacity, and are already being used in urban and suburban areas where the capacity demand is greatest and small antennas are very desirable. By connecting directly to optical fibre, 1Gb/sec links at E-band have also demonstrated very low latency, which is an important metric for network performance, especially for carrying voice traffic over IP. The complex processing needed for 1Gb/sec through narrow channels makes achieving low latency in a microwave link a nontrivial and often costly problem. Also, MMW costs will fall to those of lower capacity microwave as new semiconductor and module technologies come on stream.

Already hotly debated by various groupings, the relative success of the LTE and WiMAX standards in terms of coverage and number of subscribers will likely be determined in large part by the policies of corporate multi-national operators and the national interests of large equipment makers. However, the commercial success of the brand new networks depends on attracting subscribers and retaining them by providing high quality data and voice services. Millimetre wave links are already a clear winner on cost per bit basis, and will be a major factor in delivering the high capacity backhaul which is a critical component of their commercial success.

Endnotes
1 Infonetics’s 2009 Mobile Backhaul Equipment and Services report: http://www.infonetics.com/pr/2009/1h09-Mobile-Backhaul-Market-Highlights.asp.
2 IDC Research Opinion on AT&T: http://www.att.com/Common/about_us/files/pdf/IDC_report.pdf.
3 Cisco: http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/ns827/white_paper_
c11-520862.html.
4 See for example, ISP Planet: http://www.isp-planet.com/business/fiber_price_bol.html.
5 http://www.techbites.com/201001151755/myblog/articles/z0007-hspa-evdo-wimax-and-then-lt
e-but-what-about-the-mobile-backhaul.html.
6 Siklu Announces Industry’s First SiGe Based E-band Transceiver for Wireless Backhaul: http://www.prnewswire.com/news-releases/siklu-announces-industrys-first-sige-based-e-band-transceiver-for-wireless-backhaul-81240477.html.
7 MMIC Solutions Ltd.: http://www.mmicsolutions.com.
8 3GPP: http://www.3gpp.org/LTE.
9 WiMAX Forum: http://www.wimaxforum.org.

MMIC Solutions UK
www.mmicsolutions.com
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