W-CDMA Signals Now Take Center Stage

Emerging wireless standards bear many implementation challenges. A technology like Wideband Code Division Multiple Access (W-CDMA), for example, is new territory for many designers. Its signals are wider in bandwidth and they are more complex in their modulation and access methods. In addition, W-CDMA signals utilize new handover protocols.

To successfully address such complexities, it is critical to select measurement tools that employ specific techniques for these new standards. Traditional measurement equipment includes radio-frequency (RF) signal generators, base-transceiver-station (BTS) emulators, digital modulation analyzers, and spectrum analyzers. This equipment is chosen with performance, cost, and ease of use in mind. With W-CDMA, engineers are clearly at the beginning of the technology life cycle. The choices that they make now can impact long-term business realities for their products.

To meet these demands, emerging tools are using real-time spectrum analysis. These tools help engineers characterize, troubleshoot, and verify new designs under tight time-to-market deadlines. In addition, real-time approaches to analysis are enabling engineers to speed up common measurements and analysis requirements. By offering increased flexibility, they also help engineers find faults.

This article looks at some of the latest toolset advances employing real-time, multidomain analysis. These improvements can help designers develop network elements and W-CDMA user equipment (particularly for mobile stations). The application examples illustrate such features in action, including the acquisition and analysis of a W-CDMA call-setup procedure and a cell-to-cell handover.

Complex W-CDMA signals call for tools that simultaneously look at larger time segments and more frequency ranges. Changes can occur quickly in the signal. Often, they are changes that designers can't afford to miss, such as PRACH signals and handover protocols.

The traditional solution for RF analysis uses a swept spectrum analyzer. This method accumulates a spectrum display by tuning the path to successive frequency steps. It captures the respective amplitudes at each step. The displayed result represents compiled single-frequency acquisitions that were performed sequentially. A steady, uninterrupted signal is required to produce a meaningful measurement, however. For instance, say a transient event—intentional or otherwise—occurs in the seventh increment after the analyzer has moved on to the eighth increment. The event is then lost.

New spectrum-analysis techniques take an entirely different approach. Rather than acquiring one frequency step at a time, they capture a block of frequencies all at once during a user-specified time frame. These frames repeat continuously with a full acquisition every frame. Instead of waiting for each discrete frequency step to be measured, the instrument constantly samples the full frames. As a result, events that occur anywhere in the spectrum are far more likely to be captured.

Figure 1 shows the real-time spectrum-analysis architectural concept. Instead of having one resolution filter like a swept analyzer, it boasts hundreds of filters in parallel. All of the hundreds of frequency components within the real-time bandwidth are filtered concurrently. They appear on-screen simultaneously. To arrive at the spectrum, a wireless communication analyzer uses fast Fourier transform (FFT) techniques instead of parallel hardware filters. This enabling technology depends on very fast and accurate FFT hardware.

This "all-at-once" capture methodology lies at the heart of real-time spectrum analysis. By using this feature with a vast acquisition memory that is supported by innovative triggering tools, instruments can build up a detailed, deep record. This record is the basis for the new display modes that reveal multidimensional signal details. Such details are shown in a graphically intuitive manner. With codograms, spectrograms, and displays that compile the results of numerous acquisitions, designers can quickly characterize signals, events, and frequencies of interest.

How can these analysis techniques help an engineer who is under tremendous time pressure to complete a project, such as a new prototype handset? One answer lies in the use of dual analyzers. Both analyzers are equipped with real-time modulation and multidomain analysis capabilities. They can be combined to monitor both uplink and downlink simultaneously and in perfect synchronization.

VISUALIZATIONS AID VALIDATION
Consider the example of a prototype project in more detail. To prepare for the measurements, the handset is placed in a test stand that connects to a transmission/reception source. This source simulates a BTS. Intervening between the BTS and the handset is a pair of wireless communication analyzers. When the test commences, the apparatus should:

Capture the full W-CDMA call-setup procedure.
Assist in the interpretation of results in the frequency domain, time domain, code domain, and modulation domain.

The pseudo-BTS acts as a "known-good" transmitter. It receives the call data via the RF link. The wireless communication analyzer then captures the result.

To outline the steps above, realize that a W-CDMA time slot is 666.7 ms. A full sequence lasts 720 ms or one super frame. Using conventional spectrum analyzers, it is impossible to acquire a complete W-CDMA call-setup procedure in one sweep.

In contrast, newer wireless communication analyzers can capture up to 10 seconds of data at a 5-MHz span. In one acquisition, they can capture a total frequency range of up to 15 MHz. The 10 second time interval is sufficient to capture a full W-CDMA super frame with ample pre- and post-trigger time. It is faster and easier to evaluate several super frames that are captured at once than it is to look at a sequence's isolated segments. Taking in the full call-setup procedure allows the engineer to see both spurious events and the intended events that are part of the protocol. Both event types can be viewed in the context of the whole call transaction.