07 Nov 2011

Test Issues Affecting LTE Rollout

Mike McHale of Livingston and Jonathan Borrill of Anritsu consider the challenges that still hang over LTE and the practicalities of the use of this technology.

The value proposition Long Tem Evolution (LTE) offers the mobile industry is a highly advantageous one.

LTE networks will more efficient to run, providing telcos with the opportunity to reduce their operational expenditure levels. Provided with far higher data rates (potentially in excess of 100 Mb/s), subscribers will benefit from access to a broader spectrum of functionality and features – allowing telcos in turn to generate greater revenue from their customer base.

Market analysis firm Infonetics predicts that the number of LTE subscribers will be past the 290 million mark by 2015, with ABI Research expecting there to be some 600,000 base stations in operation around the globe by that time. But even now, as the first deployments are underway.

Quality and quantity

According to a report compiled by Infiniti Research, the worldwide market for infrastructure supporting LTE mobile communication will have a value of over $11 billion by 2014. LTE represents a step change in the way that mobile networks operate, with the move to a fully IP based topology, rather than circuit switched one employed by earlier mobile generations. This means that telcos need to completely overhaul the way they assess Quality-of-Service (QoS). It will permit execution of end-to-end QoS monitoring from the user level right through to the backbone, as the headers for carrying QoS information are built directly into its protocol stack. This will enable prioritisation of data traffic transmission, based on the subscriber’s service level agreement and the nature of the data (whether it will be effected by latency or not). New, more complex mechanisms for measuring QoS in terms of service availability and service continuity will result and as a consequence additional training will be required for those who operate the network.

Anritsu’s S820D tester for LTE networks

A forecast published recently by industry analyst iDate states by the year 2020 annual global mobile traffic will amount to a staggering 127 ExaBytes. The level of congestion resulting will put serious strain on LTE network infrastructure. Telcos will look to make use of 1 Gigabit Ethernet direct to a base station and up to 100 Gigabit Ethernet in the backhaul so that the access networks can cope with this vast increase of data without performance being compromised. This will call for major investment over the next few years.

Keeping things covered

LTE will have to be implemented in different parts of the radio frequency (RF) spectrum for different geographic locations: 800 MHz plus 2600 MHz here in Europe, 700 MHz plus 1900 MHz in North America, 2100 MHz in Japan, 2300 MHz in China, and 1800 plus 2600 MHz for the rest of Asia. This is due to the licensing rules of each country and the local regulators. As a result mobile handset manufacturers will need to develop models with multiple antennas, so users can still rely on having global coverage. Conformance testing will therefore be a far lengthier process than it was for previous handset generations.

At the same time more complex antenna configurations will be used in LTE implementations to support Multiple In Multiple Out (MIMO) transmission. MIMO means that data throughput and cell coverage can be improved without any increase to either the bandwidth or the transmission power, since the signal is transmitted through several different paths simultaneously. This will lead to more sophisticated measurements needing to taken by test engineers to calculate throughput performance. Previously over the air test methods meant the data rate that could be supported was directly related to the received signal to noise ratio, furthermore the transmission range would be improved by having a clear ‘line-of-sight’ transmission path. With MIMO, however, these basic principles are no longer valid, and so re-educating engineering staff will be a necessity.

Other concerns

As LTE networks will be initially implemented in ‘hotspots’, to exploit the areas where demand is greatest, inter-technology handovers between LTE and legacy 2.5/3G infrastructure will be occurring frequently. There will therefore be a need for telcos to have a greater breadth of test equipment available to them (so they can handle old and new protocols), as well as equipment with the capacity to deal with both. Where LTE’s 800 MHz band is being used it is likely that GSM/UMTS and LTE will have similar coverage initially, although LTE has the capability to give a theoretically wider coverage as it can adapt well to lower signal to noise levels. Good network characterisation will be vital here, so the telco can select the best network to provide the service requested by the user.

Passive Inter-Modulation (PIM) is another major issue. This basically arises when the receiver on a base station is affected by RF interference caused by mixing together of other wireless signals. This is something that has been a concern with earlier mobile communications generations, but the distortion caused is far more significant in relation to LTE networks, due to the high proportion of data being carried and the wider bandwidths being used. Checking for and solving PIM issues will require deployment of new test methods and test equipment to identify and remove the sources of unwanted signals.

Finally LTE requires a totally new approach to cell planning activities, with increased focus on initial coverage mapping and interference measurements. Through use of Inter-Cell Interference Cooperation (ICIC) mechanisms it will be possible dynamically adjust to an adjacent cell which is utilising the same sub-carrier frequency in an area of overlap. But as there are currently very few LTE cells in operation where overlapping occurs, since they are initially deployed into isolated hotspots, this is effectively uncharted territory. It is only once a larger number of cells are brought online that it will be clear how well this actually works. Once again careful testing will be essential to guarantee correct configuration.

From the various matters discussed in this article it is clear that the widespread implementation of LTE is not, as some would suggest, a fait accompli. Those implementing LTE networks will need to source high-end test hardware to undertake the required modulation, throughput and spectrum measurements. They will also need to be equipped to carry out analysis of cell coverage as well as handover, protocol sequence and protocol conformance tests. This will call for telcos to engage in in-depth consultation with test equipment manufacturers and their sourcing partners, to ensure they are ready for the challenges LTE will present to them.

LTE test solutions

Among the various LTE testing solutions now being stocked by test equipment rental specialist Livingston is the Anritsu S820D tester (as shown in Figure 1). This supports frequencies from 25MHz to 20GHz (with a 10kHz resolution level) and has a dynamic range of 60dB. The vector error correction features included in this tester allow high accuracy measurements to be taken and the lightweight (5 lbs), rugged design means that it is highly suited to use in the field. The S820D can save up to 200 measurement traces on its capacious memory. It has a multilingual user interface, with support for English, French, Chinese, Japanese, Spanish and German. This test solution is complemented by the Anritsu MD8430A signalling tester, which serves as a base station simulator for the conformance testing LTE-based chip-sets and handsets. It has a 100Mbps maximum data rate and a 350Hz to 3000MHz frequency range (with a 100kHz resolution level). It fully supports 2 x 2 MIMO transmission.

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