The use of mobile broadband data services has increased rapidly with the advent of smartphones and tablets on the broad-base market. In 2015 alone, global mobile data traffic grew by 74 per cent. And the bandwidth required in wireless networks is expected to continue to rise. Technology giant Cisco forecasts a CAGR of 53 per cent from 2015 to 2020.
Network operators have been forced to respond to this trend by further expanding and optimising their networks. In doing so they rely on a variety of strategies:
- They occupy new frequencies;
- They implement new wireless communications standards and technologies with higher spectral efficiency such as LTE, LTE-Advanced and multiple input multiple output (MIMO);
- They use newly released frequency blocks for broadband technologies (re-farming); and
- They also use small cells to improve efficiency, supplemented by large numbers of WLAN hotspots to reduce wireless network loads.
These strategies take into account that the majority of data traffic is generated in buildings. To improve network quality operators, infrastructure manufacturers and service providers need to get a comprehensive picture of the quality of available wireless networks – as quickly as possible. In Europe, for example, this includes UMTS and GSM in the 900 MHz band and LTE and GSM in the 1800 MHz band, plus UMTS again in the 2100 MHz band and LTE in the 2600 MHz band.
The best way to properly assess coverage and capacity of wireless networks is with drive testing. The measurement range of drive test scanners covers all the aforementioned frequency bands and is also capable of measuring in parallel all wireless standards used in each individual band. A measurement range of 350 MHz to 4.4 GHz covers all bands specified for wireless communications standards. In each band the test system needs to simultaneously measure any combination of signals from different wireless communications standards. A mobile test solution with these capabilities allows test engineers to move through indoor spaces unencumbered by bulky equipment.
Spectrum re-farming
When planning additional LTE networks for a frequency band in which a UMTS network is already operating, the prevalent propagation conditions must be measured. Such data can be used to fine-tune the propagation model used in LTE planning (model tuning). Since the UMTS network cannot be turned off for these measurements, scanners with a high dynamic range are required to detect the signal of a distant radio cell in the neighbouring cell’s coverage area.
Spectrum reallocation
The concentration of digital TV channels has made new frequency bands available for wireless communications worldwide. Network operators first need to gain experience with these bands to be able to optimally utilise them. The introduction of LTE in the 800 MHz band, for example, shows that this band differs significantly from the 900 MHz band. Its lower frequency significantly expands the range of LTE 800 wireless cells. This increases the risk of interference, which can occur when too many neighbouring cells are received too strongly (pilot pollution). To prevent this the LTE network must be planned differently from LTE deployments in the 900 MHz band. Base stations are positioned further apart to take the larger cell radii into account.
sing LTE and MIMO
Combining LTE and MIMO increases the spectral efficiency of a wireless network. LTE is based on orthogonal frequency division multiplexing (OFDM) and can therefore be flexibly deployed in different bandwidths. With 64QAM modulation data rates up to 75 Mbit/s can be achieved in a 20 MHz channel. Under good channel conditions MIMO can double the data rate without increasing bandwidth. However, this requires an appropriately optimised network. Using a scanner during drive tests makes it possible to check for problems with the radio transmission. Interference measurements are typically performed at the same time.
Even externally induced interference, for example from defective set-top boxes, strong TV transmitters or faulty cable TV lines, can be detected with scanner measurements. The network operator or regulatory authority can then take action against the sources of interference.
Assessing the channel quality of LTE MIMO additionally requires special MIMO measurements to optimise the network. Up to four scanners must be cascaded in order to measure the data streams of the individual channels in MIMO systems. Network configurations ranging from 2 × 2 MIMO to 4 × 4 MIMO should be supported.
The LTE-Advanced carrier aggregation mode makes it possible to increase the maximum data rate by combining several LTE cells. These cells generally lie in different frequency bands. In order to assess the quality of all radio channels being used, scanners must have sufficiently large measurement bandwidth.
The compact R&S TSME drive test scanner weighs only 650 g. This makes it possible to fit four scanners in the R&S TSME-Z3 backpack system for measurements in 4 x 4 MIMO systems for example.
In-building measurements
Buildings have the highest subscriber density and the highest data service usage. This is why network operators use dedicated infrastructures such as microcells, picocells and distributed antenna systems in airports, railway stations, trains, event venues (exhibition halls, sports stadiums), shopping centres, office buildings and the like.
A further trend is to supplement these small cell networks with WLAN hotspots that reduce the load on wireless networks (Wi-Fi offloading). Scanner measurements are essential for planning, commissioning and optimising this type of infrastructure. When planning an in-building network test transmitters help to determine the signal propagation. After the network has been set up scanner measurements are used to check coverage.
A major challenge here is that high cell density in buildings leads to increased interference. For this reason, network optimisation is essential for standards such as UMTS – and especially LTE – because it is rarely possible to reserve a specific frequency band for microcells and picocells.
Whatever strategies are used in addressing the growth of mobile data, accurate test and measurement plays a vital role from installation and commissioning to regulatory measures and policing. With such broad requirements, measurement systems must be capable of measuring all wireless communications standards in current and future frequency bands and, where MIMO measurements are involved, also require the ability to cascade multiple measurement instruments. Compact backpack systems with these technical capabilities promise faster testing both on-site and in the field.
About the author
Jordan Schilbach is part of the Product Management team for Wireless Network Optimisation team at Rohde & Schwarz in Munich, Germany.
