The ability of HSPA, HSPA+ and LTE to deliver high spectrum efficiency is dependent upon them being able to reach the instantaneous peak data rates from each technology. To achieve high overall average throughputs from a base station, the base station must be able to achieve the maximum data rate possible in excellent radio conditions. For example, the ability for HSPA+ to reach its highest spectrum efficiency of about 1bps/Hz relies on the radio access network being able to reach an instantaneous data rate of about 42Mbps. Insufficient transmission capacity at the base station would inevitably restrict peak instantaneous data rates, thus reducing spectrum efficiency.
The need to support increasing peak speeds of cellular technologies will drive 3G operators to upgrade their backhaul capabilities. Historically, 3G operators have relied on either microwave radio or leased lines (E1 or T1) to meet their backhaul requirements. While these have been sufficient to support voice telephony traffic and the relatively small amount of data traffic, major backhaul enhancements will be necessary, in particular to support the introduction of HSPA+ and LTE.
There is a general trend towards Ethernet as the future transmission solution. It can be overlaid over various transmission methods (for example, fibre, microwave and DSL) and provides efficient management of capacity and resilience.
3G enhancements will necessitate transmission upgrades for many 3G operators within the next two years to support dramatic increases in peak data rates. For early W-CDMA networks, 3G operators have been able to meet transmission requirements with one or two E1/T1 leased lines. In contrast, the adoption of LTE may eventually demand support for peak speeds of between 135 and 978Mbps at some base stations, as shown in the table below. Leased lines would be woefully inadequate to support these speeds, either from a performance or cost perspective.
With the continued rapid take-up of mobile broadband services and the introduction of HSPA+ and LTE services, a rapid evolution to Ethernet transmission will be necessary, which will take different forms:
- optical fibre
For mobile networks operators currently employing leased lines, continuing to use only leased lines would become a much more expensive option if they had to be upgraded to support the necessary peak data rates of HSPA, HSPA+ and LTE.
In Western Europe, DSL services are prevalent, which provide 3G operators with a cost-effective alternative to leased lines for data transmission (although leased lines would still be maintained to support voice telephony services). T-Mobile was one of the first operators to deploy an Ethernet-over-DSL solution (from RAD Data Communications), as a cheaper alternative to deploying a large number of E1 leased lines. In January 2008, T-Mobile announced that it had completed trials in Germany using ADSL2+ for backhaul for 3G base stations. T-Mobile was also testing the solution in two other European markets, including the UK. With RAD’s backhaul gateway in Germany, T-Mobile uses ADSL2+ lines that are supplied by Deutsche Telekom’s fixed-line business T-Com. With the successful completion of the trials, ADSL2+ lines were expected to be deployed across T-Mobile’s network, including the UK, and expected to support growth of HSDPA services for some time.
Fibre potentially solves all backhaul capacity problems, although its relatively limited availability in some countries means that it is not feasible to provide fibre at all base station sites. Fibre deployment may take considerable time and there will be practical constraints if base stations are a significant distance from the fibre access point. In the USA, a common approach among 3G operators is to almost exclusively to use fibre where this is available and to use microwave to fill in the gaps (providing backhaul to the nearest fibre access point).
Microwave Ethernet can scale to over 100Mbps per connection and can be deployed rapidly. It can provide more cost-effective scaling than multiple T1/E1 lines. However, there can be issues with licensing and interference. Legacy microwave equipment can potentially be upgraded with Ethernet. New microwave systems provide native Ethernet, which can be integrated into flexible Ethernet network configurations. In February 2009, Vodafone announced at the Mobile World Congress 2009 that it was trialling Ethernet microwave technology that would quadruple its current backhaul capacity, by supporting peak rates of about 180Mbps compared with the 32Mbps with traditional microwave backhaul.
We expect that the need to support very high peak data rates of HSPA+ and LTE will encourage 3G operators to rapidly migrate their transmission architectures to Ethernet within the next two years.