Designing the RF test instruments of tomorrow
July 01, 2008
Test and measurement suppliers need to think ahead and design RF test instruments suitable for emerging and future wireless technologies.
The demand for wireless communication continues to increase with the number of new users and services continually expanding. Moreover, wireless communication traffic is migrating from mostly voice to mostly data, requiring faster data rates.
With limited frequency spectrum, digital wireless technology has progressed rapidly during the past two decades to address these market demands. More spectrally efficient modulation types and digital coding schemes are being used along with increased signal bandwidths from 300 kHz in the early 1990s to 40 MHz today. New transmission methods such as MIMO are being deployed to further increase data rates. This next generation of RF test equipment adds more layers of complexity, thus posing difficulties for test engineers.
MIMO test equipment complications
MIMO is a growing RF technology that uses multiple radios for both transmitting and receiving data. In wireless communication devices, it can increase data throughput rates or improve transmission quality without requiring additional bandwidth.
In a typical MIMO approach, four independent Orthogonal Frequency Division Multiplexing (OFDM) carriers are placed atop one another, as shown in Figure 1. This MIMO technique allows transmitting up to 3.5x as much information in the same bandwidth as a single carrier.
Testing MIMO presents several key challenges, including the amount of spatial streams that can be supported. 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 supports two streams. Another issue is keeping costs per stream down without sacrificing performance. Costs for test equipment, especially MIMO systems, can multiply quickly. For instance, to get N inputs and M outputs, each I/O requires a separate transmitter and receiver or source and analyzer.
Bandwidth poses an additional problem. MIMO signals in particular require test instruments with wide bandwidths. For example, WiMAX and LTE have a current 20 MHz bandwidth requirement, and 802.11n WLAN has a 40 MHz bandwidth. Instrumentation must be able to perform these measurements while maintaining exceptional Error Vector Magnitude (EVM) performance.
High sensitivity is another critical parameter. The noise floor affects modulation accuracy, measured as the EVM. Higher noise will increase the EVM, reducing communication quality. With wide signal bandwidths, low noise in both the signal generator and the signal analyzer is important. Low-cost instruments have poorer noise performance than their more expensive cousins, which directly affects measurement accuracy. This in turn weakens product quality and production yields, which impacts product cost and competitiveness.
MIMO relies on channel distortion. Without it, MIMO as a transmission technique becomes redundant. Understanding how devices perform under different channel conditions and calculating those conditions with measurements such as Channel Response or the Matrix Condition are important capabilities. Take WLAN as an example. A header is transmitted with a known symbol pattern. The receiver uses this known signal to reveal what the channel distortion looks like and then determines the actual received data symbols.
Measuring wide bandwidth OFDM signals‚Äô channel response, that is, the amplitude and phase changes across the channel, poses another challenge. To accurately characterize a transceiver unit under test, the test equipment must not only have wide bandwidth but also flat response with low amplitude and phase variation.
Next-generation RF test instrument innovations
With the next generation of MIMO, beam-forming applications will become more prevalent. Second-generation MIMO test systems will require that the RF carrier phase and amplitude be accurately controlled. This enables the transmitters to produce different antenna patterns, allowing the antenna beam to be steered to different locations. Steering the antenna beam to each user increases communications efficiency. Most of today‚Äôs instrument platforms were designed for Single Input/Single Output (SISO) applications and cannot easily control RF carrier phase.
Some MIMO test system architectures only support balanced MIMO configurations such as 2x2, 3x3, or 4x4. Future instrumentation must also support unbalanced MIMO configurations, especially for collaborative MIMO such as 1x2, 2x3, and 3x4 configurations. As more advanced beam-forming applications come online, 8x8 and 16x16 configurations could be required.
Time alignment between transmitters and receivers is also critical. As MIMO relies on time discrepancies in the channel to function correctly (multipath), timing misalignment within the signal analyzer or sources will result in increased distortion from the test instruments, reducing measurement accuracy.
Next-generation capabilities will rely on several unique industry innovations. For instance, a DSP-based SDR architecture adapts to the dynamic wireless market‚Äôs quickly changing test requirements, giving the instrument added longevity by making it easily upgradeable. SDR-based instruments can generate or demodulate virtually any signal with up to 40 MHz of modulation bandwidth, which is important for many of today‚Äôs devices and for tomorrow‚Äôs new signal standards such as 4G LTE.
For example, new RF test instruments such as Keithley‚Äôs 4x4 MIMO RF Test System (Figure 2) use a DSP-based SDR architecture. These instruments feature a precise and stable local oscillator locking system with peak-to-peak carrier phase jitter of less than 1 degree, making them ideal for beam-forming applications.
However, phase alignment isn‚Äôt the only important feature. For non-beam-forming MIMO applications, time alignment is imperative. Maintaining high synchronization (time alignment) on more than two signal generators or signal analyzers is difficult with most instrument architectures because they are not designed for MIMO applications. Some instruments are limited to two inputs and/or outputs. In contrast, Keithley instruments are scalable up to eight inputs and outputs with precision sample clocks locking the instrumentation to within a nanosecond of each other.
Thinking ahead a generation
Test engineers will spend a great deal of money on their first-generation MIMO test systems. To ease the burden on test engineering budgets, test and measurement suppliers need to think ahead and design RF test instruments suitable for emerging and future wireless technologies. Test system vendors are addressing this need with next-generation instrument platforms that use state-of-the-art RF and high-speed DSP SDR technology to reduce testing costs and shorten time to market.