Test & Measurement

April 1, 2011 Previous day Next day

No one needs a 4G mobile phone, but it seems as if everybody wants one, if for no other reason than to receive such bandwidth-intensive services as streaming video on demand. So, with consumers clamoring for multimedia-on-the-go and other mobile data traffic on the rise, network operators are looking towards 4G as a more efficient means by which to meet customer expectations. That, in turn, translates into a sharp rise in network complexity and a greater emphasis on innovative test solutions to verify designs and identify and resolve errors at each stage of product development and infrastructure roll-out.


 

In the past 12 months we have seen the introduction of two 4G (so-called because it is fourth generation of cellular wireless standards) formats, WiMAX and LTE (Long Term Evolution) with Sprint and its partner Clearwire using the WiMAX standard for its 4G network and Verizon launching LTE-based 4G service, with AT&T joining in as a 4G provider this year and next.


 

There is little doubt 4G networks will be fast right out of the gate with real-world  LTE download speeds of 5 to 12 Mbps and 2 to 5 Mbps via the uplink on, for example,  the Verizon  network.  Demonstrations of Sprint's WiMAX network in Boston last year achieved speeds of around 4 Mbps streaming live video feeds. (Three to five Mbps is generally considered the starting point for streaming live HD movies with smooth, uninterrupted viewing.)


 

In future, LTE-Advanced promises to deliver potential download speeds up to 100 Mbps and uploads to 50 Mbps. LTE-Advanced is a fully the backward-compatible enhancement of LTE whose specs are only now being finalized.


 

Ensuring the operational readiness of LTE networks is imperative not only during the initial technology launch, but also as the network and LTE services continue to grow and mature. From a testing standpoint this means more complex conformance testing.  Testing the components of a 4G network requires comprehensive test coverage of RF, protocol and system-level elements including base stations, cell sites, handsets and network infrastructure.


 

As you might imagine, 4G test challenges are numerous. Here’s just a sampling:


 

Conformance Tests determine the extent to which LTE User Equipment, Base Stations and Network Gateways conform to standards specified by 3GPP (the LTE standards body). For any standards-based technology, the goal of conformance testing is to ensure a device meets a minimum level of performance; as such it is an important and essential step towards the successful deployment of a new system.


 

Protocol testing can involve as much effort in generating test cases as in creating the protocol stack itself. Protocol test diagnostic features also are essential when tracing faults. Several test-equipment manufacturers offer test instruments that cover all protocol layers and physical interfaces (including air and fixed line interfaces) in one integrated offering. You also want to make certain protocol stacks are designed for optimal power consumption and processor utilization.


 

In LTE systems conformance tests are required where the test equipment must make dynamic adjustments in response to hybrid automatic repeat requests (HARQ) and timing adjustment feedback from the eNB--which is the basic access network element that interfaces with the user equipment (UE) and hosts the PHYsical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers. The UE—the mobile phone--and the eNB are required to make physical layer measurements of the radio characteristics. These measurements are used for a variety  of purposes including intra- and inter-frequency handover, inter-radio access  technology (inter-RAT) handover, timing measurements, and other functions.


 

Multiple-Input Multiple-Output (MIMO) Transmission Testing. The specified RF environment for LTE includes not only basic signal transmission and reception, but also MIMO scenarios with up to four separate transmitters and receivers. 4G cellular systems will have to provide a large number of users with very high data transmission rates and range, and MIMO techniques are useful because the base station and mobile handset can communicate using two or more spatial streams. In one MIMO technique, multiple antennas can transmit the same data stream to improve data-transmission reliability. In another, the different antennas can transmit different data streams simultaneously to increase throughput. Base stations and terminals are equipped with multiple antenna elements intended to be used in transmission and reception to make MIMO capabilities available at both the downlink and the uplink.


 

Because the number of transmitters and receivers can vary in MIMO topologies, test systems need to be highly flexible to accommodate this variability. A MIMO system-on-a-chip (SoC) includes receivers, transmitters, converters, filters, switches and a processor. In addition, this SoC features software IP such as coding, modulation, encryption, and communication protocols and this has to be verified as well.


 

UE Device Testing. When the RF, baseband, protocol stack and application layer have been integrated, overall device performance needs to be fully determined. Power consumption, electromagnetic compatibility (EMC) emissions and susceptibility and thermal characteristics all need to be measured under full load. Engineers must also address performance aspects via verification of throughput in order to make sure that the unit is capable of handling the high-data-rate requirements of LTE.


 

What is more, test instruments intended to evaluate a 4G phone must be able to emulate the transmit and receive properties of a 4G base station and reproduce environmental conditions such as signal fading and multipath. Software programs are available that can emulate thousands of unique UE configurations running multiple applications per each unique UE and connecting to real or emulated hosted services. Indeed, it is estimated that software development costs are often 2X to 10X more than capital costs in test systems today.


 

BER Measurement. Communications networks worldwide are converting to optical fiber to meet increased bandwidth needs and development of optical devices for these networks is a very active enterprise. Test solutions are now available providing all-in-one support for the simultaneous Bit Error Rate (BER) measurement and eye pattern analysis required for evaluating the performance of active optical devices (BER baseband coding and decoding are used to minimize errors as well as enable error correction). To assure the integrity of signals passing via these high-speed devices, the BER and eye pattern are measured using a bit error rate tester (BERT) and sampling oscilloscope.


 

There are also simulation- based systems developed for analysis and BER performance measurement of orthogonal frequency division multiplexing (OFDM) systems such as LTE.


 

Simulation. Simulation is a great way to model both baseband and RF design elements as well as RF path impairments. In an LTE network the handover, fading and mobility can introduce significant delay and affect data rates, causing problems in data transmission and reception. Network simulators and traffic obstacle simulators create a controlled, repeatable test environment that allows engineers to isolate these effects, measure their characteristics and evaluate their impact on the 4G phone user’s experience. As a design is turned into functioning physical blocks, simulation can be combined with test instrumentation to permit analysis of the blocks in a close-to-real-world environment.


 

Interoperability. Working with 2G and 3G systems is a requirement for LTE and needs to be tested carefully. The ability to seamlessly hand over between cells while minimizing the interruption to data throughput needs to be tested and assured as does the ability to hand over between different radio access technologies while maintaining the data connection.

 

To fully validate the functionality of the highly integrated hardware and software subcomponents, engineers need system-level test capabilities. But while 4G networks promise dramatic improvements in data throughput and spectral efficiency, the complexity of the evolving LTE standard has forced many system architects to reconsider their use of general-purpose toolsets.


 

A good way to address these and many other complex measurement issues is to use an LTE test set where design software is combined with test equipment to verify LTE system performance. With a test set, engineers can perform measurements in close to real operating conditions, with all the protocol layers working and the handsets communicating with a network as opposed to measurements done with individual instruments, which generally is limited to test signals produced by user equipment.


 

Test equipment vendors are providing needed capability with both new and upgraded instruments, test sets, and software already available. Below is a (non-exhaustive) list of test and measurement suppliers that can provide more information. Internal links are included for those vendors with pages in the manufacturers section of element 14.


 

Aeroflex

Agilent Technologies  (element 14 page)

Anritsu

Fluke  (element 14 page)

Keithley

LeCroy

National Instruments

Rohde & Schwartz

Tektronix  (element14 page)

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