Taming Multipath for QoS

A customer whose wireless phone use is hampered by interference or fading doesn't rationalize the technical effects of distance or nearby obstacles. When the voice on the phone fades away, the phone simply "isn't working."

This customer frustration has led to an increasing emphasis on quality of service (QoS). The carrier that builds and sustains a reputation for trouble-free service can offer a powerful inducement to potential customers: mobile phone service without the apologies.

Fading is a prime concern in QoS testing. It's common, it's quantifiable (given the right tools), and once characterized, it can be improved in all but the most extreme circumstances. The traditional tool for monitoring time-variant phenomena such as multipath fading is the swept-spectrum analyzer. However, a newer architecture known as the real-time analyzer is beginning to prove itself as a tool for characterizing multipath effects. Real-time analyzers offer a different kind of signal acquisition, one that is especially well-suited for capturing unpredictable fading characteristics.

CAUSES & EFFECTS Base-station coverage and range limitations are sometimes the culprits of fading, but physical obstructions also degrade wireless phone transactions. A building or a hill situated between the base station and the phone set can attenuate the signal, causing fading. These effects are predictable and can be mitigated by strategic base-station placement.

A building blocking the signal path also can cause wireless signals to "reflect." This is where the thorniest problems begin. Reflections cause unpredictable and uncontrollable fading effects. Again, appropriate base-station placement can help. But the opportunities for interference are as numerous as the paths a signal can take as it ricochets from surface to surface -- thus the term "multipath fading."

Wireless phone fading results when one or more reflected signals, delayed because their indirect path is longer, meet the directly-transmitted signal as it arrives at the receiver. Because of the delay, the reflected signals are slightly out of phase. The signals combine with the direct transmission, additively one moment and subtractively the next, depending on the phase relationship. The result is a signal level that ebbs and flows.

Today's digital wireless phone formats are designed to resist -- actually counteract -- multipath fading as much as possible. With analog cellular systems, the common practice simply is to move the mobile set when fading occurs. GSM technology, however, uses frequency hopping to "sidestep" multipath effects. The phone itself automatically moves the transmit and receive frequencies. Because multipath interference is frequency-dependent, the new frequency may not have the same fading problems. If it does, the GSM signal hops again.

CDMA takes an entirely different approach, known as spread spectrum. It uses complex orthogonal codes to spread the transmission content across a broad (1.23MHz) spectrum. CDMA has a wideband, noise-like signal environment and relies on code-correlation methods to extract the content at the receiving end. The theory is that using a wideband signal spectrum, with the data content distributed over many frequencies, helps offset fading tendencies at any one frequency.

TEST PLATFORMS The primary tool for multipath fading tests has long been the swept-spectrum analyzer. The normal procedure is to roam with an antenna, usually in a service van, taking the off-air signal to the spectrum analyzer input via an external RF amplifier. The swept-spectrum analyzer interprets the signal using a scanning process: It sweeps from one end of a band of frequencies to the other, capturing the amplitude (power) of the signals within that band. The instrument's resolution bandwidth determines how many steps make up the full-band sweep. The spectrum analyzer produces a graph of these amplitudes.

The spectrum analyzer takes time to make its full sweep. If transient fading occurs at a frequency the sweep already has looked at, the transient goes unrecorded. The swept-spectrum analyzer views one frequency at a time, and other activity across the band can go unnoticed.

An alternative to the swept-spectrum analyzer is the real-time analyzer. The real-time instrument is a type of spectrum analyzer in that it can provide the classic frequency-domain plot of signal activity. But also it provides significant new display modes and analysis capabilities, and most important, it spans enough bandwidth with every acquisition to get a real picture of fading phenomena. Real-time analyzers are suited for capturing signal behavior in both the frequency-hopping and CDMA formats.

A 'BIRD'S-EYE VIEW' The real-time analyzer captures a 5MHz block of frequencies during a user-specified time frame of, say, 20 microseconds. These 20-microsecond frames repeat continuously, with a full 5MHz acquisition every frame. Because the instrument samples these frames constantly, the signal can come or go, and the analyzer will detect the change instantly.

Thanks to its continuous frame acquisition, the real-time analyzer's display reveals more signal information than the normal 2-dimensional frequency vs. amplitude display. A 2-dimensional graph of a single frame would look like an ordinary spectrum analyzer plot, but if a succession of these frames were layered behind the front-most plot, the map would begin to accumulate depth. This depth could be viewed only by raising the perspective slightly, which is known as a waterfall display. Now add more layers (frames) and raise the perspective even higher, to the point where you are directly "above" the plot, looking down. The frequency (X) axis runs from left to right; time (Y) runs from top to bottom (most recent acquisitions at the top), and color denotes the amplitude (Z) axis. Although amplitude levels are less explicit in the spectrogram view (compared to those of a 2D spectrum display), the information still is available, and fading effects are much easier to see.

REAL-TIME FADING SPECTROGRAMS Featured is a spectrogram of a GSM signal that bears evidence of fading. The chart is composed of 304 frames, and the center frequency in this case is 1. 8925GHz. Again, amplitude is expressed as color; red indicates the highest power levels. The plot clearly shows the frequency hopping. Take a closer look at the trace nearest the left boundary of the plot. Its green color indicates a lower-amplitude, faded signal.

This plot is rich in detail, but the real-time analyzer provides tools to drill even deeper. A marker can be placed over any point on the map to provide the amplitude and frequency information about that point. The text above the plot explains it all. The marker is positioned at the center frequency, and the amplitude at that point is -26.556dBm. For even more visual resolution of the image, the real-time analyzer has a zoom feature that expands the selected area.

In a CDMA spectrogram taken with a real-time analyzer, the spread-spectrum nature of the signal is obvious. Once again, fading is clearly visible -- the two eyebrow-shaped blue "holes" in the middle of the active signal area. If this looks like too much information to digest, remember that the marker can provide details about every point on the plot. Using the marker, you can characterize the frequency boundaries of the holes and study the amplitude variations around those boundaries.

Multipath fading measurements are a key index of coverage and mobile-signal integrity and are playing a larger role in day-to-day QoS monitoring procedures. Working from a moving vehicle, the real-time analyzer can capture real-time changes in signal dynamics that other measurement tools fail to see.