As the mobile comms industry develops standards for the next two steps in the evolution of networks, it is timely to examine the fundamental radio and network technologies being introduced and the related test and measurement challenges. These are the next steps in the continuous move to wider bandwidth and higher data rates.
Release 7 of 3GPP introduces MIMO and 64QAM as transmission technologies to increase data rate to the handset in 3G, and IMS phase 2 introduces all IP network capability. This will reach the limit of what is possible in data rates for the existing 3G networks based on 5MHz WCDMA technology.
Beyond this, 3GPP is now developing the standards for a new mobile network, and this is called the long term evolution (LTE) and system architecture evolution (SAE) for next generation mobile networks supporting 100Mbit/s data rates.
LTE technology
LTE refers to a new radio access technology to deliver higher data rates (50-100Mbyte/s), it uses OFDM and MIMO technologies together with high rate (64QAM) modulation. LTE uses the same principles as HSPA for scheduling for shared channel data, HARQ, and fast link adaptation (AMC adaptive modulation and coding). This technology enables the network to dynamically optimise for highest cell performance according to operator demands (for example speed, capacity).
MIMO is an abbreviation of “multiple input multiple output”. This is an antenna technology together with signal processing that can increase capacity in a radio link. LTE will initially use a 2x2 MIMO, where the user data is separated into two data sets, and these are then fed to two separate TX antennas, and received by two separate RX antenna, so the data is sent over multiple separate RF paths. The algorithm used to split and then recombine the paths allows the system to make use of the independence of these RF paths (not the same RF losses and interference on each) to achieve higher data throughput than just sending the same data and two paths. This is done by separating the data sets in both space and time. The received signals are then processed to be able to remove the effects of signal interference on each, to give an increase of data rate of up to two times.
SAE technology
SAE is the network architecture to simplify the network and provide integration of the mobile network to other IP based networks. SAE uses an evolved Node B (eNB) and access gateway (aGW), and this removes the RNC and SGSN from the equivalent 3G network architecture to make a simpler mobile network. SAE also includes entities to allow full inter-working with other related wireless technologies such as WCDMA and WiMAX.
OFDM Radio testing.
OFDM, and the use of high order 64QAM modulation, both require high linearity, phase and amplitude in both TX and RX chains to prevent inter-symbol interference and ensure accurate IQ demodulation. This requires a fast and adaptive EVM measurement capability to track and measure the signals during adaptive frequency channel use. Testing is made on the EVM of both the ‘per tone’ performance of each individual sub-carrier, and then on the ‘composite’ signal where the sub-carriers are combined and the overall performance is seen.
MIMO testing.
In a MIMO system, the electro-magnetic coupling characteristics from the antenna to free space must be fully understood. Data rate and performance of MIMO links depend on how the multiple RF antennas couple to each other. Accurate calibration of antenna paths, factory calibration and then field installation calibration is required to implement a successful MIMO system. During R&D phase, evaluation of phase delay, variation and matching of designs is required to confirm the sensitivity calculations required to find critical performance limiting issues.
The antenna array will use specialist phased array techniques for accurate control of the phase/timing in each antenna path. This requires accurate characterisation of the RF path in terms of electrical path length, coupling and reflections from both ends. This data is then used in the MIMO adaptation algorithms to enable features such as beam steering. A vector network analyser would normally be used for complete characterisation of the antenna paths.
In a MIMO system, it is necessary to calculate the characteristics of the RF path from each TX antenna to each RX antenna. This is required so that the two paths can be separated by the processor and effectively become two separate data paths.
Test requirements in SAE
SAE is based on an “all IP” network concept, with use of IPv6 to provide the IP protocol. This protocol was developed from traditional ‘wired’ networks where the common traffic flow problems are due to overload or broken connections. The capacity (bandwidth) of each data link is usually static, and traffic flow problems arise from the link being overloaded or effectively zero (broken cables, damaged routers etc).
Capacity issues arise usually from volume of traffic from the users, and the network operator can easily control in a static way how many users are connected to a particular hub and what bandwidth they are offered. In a wireless link, and particularly a fast adaptive link like LTE, the capacity of the data link is variable according to the environment (RF path loss, distance to base-station) or according to loading of the cell (the number of users in the cell can vary in real time, without the control of the operator).
Jonathan Borrill is director of strategic marketing at Anritsu
See also: Electronics Weekly's Guide to Wireless Networks, a roundup of content related to wireless networks.