Nikkei Electronics Asia -- March 2009
Insights
Trends in RF Testing

Wireless communications is evolving rapidly from mostly voice to mostly data, placing new demands on test systems. The market demand for wireless communications continues to accelerate, along with the shift to data intensive applications, such as texting, Web surfing, and GPS. These applications require higher transmission rates to make them more user friendly, which creates new approaches to the use of a limited frequency spectrum. More spectrally efficient modulation types and digital coding schemes are being used, along with increased signal bandwidths - from 300kHz in the early 1990s to 40MHz today.

Communications Technology Trends

Perhaps the most significant trend is the move away from simple single-input single-output (SISO) configurations to complex multiple-input multiple-output (MIMO) structures. Today's radio devices use a SISO configuration with one transmitter and one receiver; information is sent over a single data channel as one digital symbol at a time. This is typical in Wi-Fi, a communications setup where there is only one radio and one antenna in use at any point in time. With this arrangement, antennas are constantly switched for the best signal path, but there is still only one data stream, and a single data channel at a time per antenna.

The shift from SISO to MIMO technologies allows multiple carriers that can potentially transmit hundreds of symbols simultaneously. MIMO employs more radio carriers to transmit more information, improving spectral efficiency by transmitting all the signals on the same channel, and occupying the same bandwidth.

This change has been driven in large part by both consumer demand for more mobile services and the decreasing cost of the digital signal processor (DSP) technology required to deploy high-bandwidth broadband wireless systems.

MIMO technology can now be used in a wide range of commercial communications devices including mobile phones, PDAs, and laptops. The net result is higher data rates for these consumer devices.

  Challenges in Testing

MIMO takes spectral efficiency to a new level by allowing multi-signal transmission and reception. However, with higher spectral efficiency comes a higher level of complexity. In the case of WiMAX, for example, orthogonal frequency division multiplexing (OFDM) is used to facilitate parallel symbol transmissions. There are a number of significant challenges involved in moving from SISO- to MIMO-based systems that test engineers must consider.

One challenge created by the complexity of MIMO and OFDM is the number of spatial streams that can be supported by the test system. For example, wireless LAN (WLAN) and Long-Term Evolution (LTE) both support four stream configurations, and current WiMAX technology with Matrix A and B configurations support two streams.

A challenge at the test receiver end is to decompose a mixed signal into multiple independent signals or streams.

However, the greatest challenge involves synchronization. Transmission of multiple signals requires accurate synchronization of multiple channels in phase and sampling alignment. This means that signal analyzers and generators must have precise alignment in order to make accurate and repeatable measurements.

In MIMO transmissions the phase of RF carriers must also be controlled. This allows the antenna beam to be steered to different locations. By steering the antenna beam to each of the users in a two-dimensional plane, communications efficiency is further increased. Today, most instrument platforms were designed for SISO applications and do not have the capability to easily control RF carrier phase. Even those with MIMO test capabilities have specifications that only require RF carriers to be stable (i.e. to have low jitter between the RF carriers) and to have adjustable amplitudes.

However, an accurate phase control is becoming a requirement for next-generation test equipment.

Another test equipment challenge is bandwidth (BW). MIMO signals in particular require test instruments with wide BW. For example, WiMAX and LTE have a current 20MHz BW requirement and WLAN, 802.11n, has a 40MHz BW.

There is also growing use of multiple cellular standards in many wireless devices, or a manufacturer may produce multiple devices that use different standards. Therefore, test equipment needs to accommodate all the major formats (eg, GSM, GPRS, EDGE, WCDMA, cdmaOne, and cdma2000). Instrumentation must be able to make the required measurements for any of these and make them accurately with, for example, small error vector magnitudes (EVM), which is particularly important in EDGE signal measurements. When new standards are adopted by a manufacturer, this creates test equipment migration issues. Ideally, one would like to upgrade test equipment for new cellular and modulation formats, easily and cost-effectively - perhaps with only software changes.

Industry Response to Cost Sensitivity

As wireless devices become more complex, competitive pressures are growing, putting downward pressure on profit margins. At the same time testing is becoming more difficult, putting upward pressure on unit costs. Faced with shrinking margins, manufacturers are looking to reduce their costs wherever possible, including test equipment and the cost of testing. This applies not only to the production floor, but also to R&D labs. In both environments, there is a growing need for cost effective test equipment with more functionality, higher throughput, and greater ease of use.

With respect to multiple spatial streams in WLAN, LTE, and WiMAX, a major objective is to keep the cost-per-stream down without sacrificing performance. However, test equipment costs, particularly for MIMO systems, can multiply quickly.

Newer equipment designs are addressing these issues. For example, Keithley's next-generation test platform makes it simple and inexpensive to add support for new signal standards and MIMO configurations. It consists of the firm's 2920 RF vector signal generator, the 2820 vector signal analyzer, the 2895 MIMO synchronization unit, and the signalMeister waveform creation software. This system supports up to 8x8 MIMO measurements currently being used for commercial test applications on signals such as 802.11n Wi-Fi, 802.16e Mobile WiMAX Wave 2, and future standards such as LTE and UMB (Ultra Mobile Broadband).

DSP-Based Architecture

These capabilities are a result of some recent industry innovations. For instance, a DSP-based software-defined radio (SDR) architecture adapts quickly to changing test requirements. SDR-based instruments can generate or demodulate virtually any signal (currently, up to 40MHz of modulation bandwidth) with only a simple software upgrade. This extends equipment longevity by making it easy to upgrade the test system. DSP technology also provides outstanding performance, including EDGE signal generation with extremely low EVM (typically, less than 0.5%). This ensures precise, repeatable signals that help minimize measurement errors. Similarly, DSP-based vector signal analyzers can measure low levels of EVM on a per channel and per symbol basis.

DSP technology also contributes to higher throughput. It allows faster tuning, with frequency switching in less than one millisecond for most frequency steps. Similarly, the settling time after changing signal amplitudes is also within a few milliseconds. A DSP platform is typically complemented by a relatively large waveform memory. This allows the user to have many waveforms in memory simultaneously for instant recall.

By combining all these technologies in the most cost effective manner, the next-generation RF test equipment helps device manufacturers drive down their total cost of test. They can perform more tests, do them faster, and shorten their time to market - while insuring that critical performance parameters are met.

by Mark Elo, Marketing Director, Keithley Instruments Inc

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