Multichannel RF supports smart antennas

In the never-ending quest for greater capacity and reliability, cellular and other wireless data communications systems are beginning to incorporate multiple transmit and receive channels to support so-called smart antennas. Whether they're being used to help develop sophisticated channel models or characterize components, instruments with multiple coherent RF measurement channels are important to the development and testing of systems using smart antennas.

Smart antennas include diversity, beam forming, multiple-input multiple-output (MIMO) and spatial division multiple access (SDMA) devices. These technologies present new challenges at all levels-from system design to component selection.

For example, take SDMA. It can require very tight control over the magnitude and phase relationships among transmit channels as well as receive channels, with poor control resulting in lower system capacity. This places new demands on component manufacturers who not only have to deal with linearity, but must also ensure stable gain and phase performance under dynamic loading conditions.

Even before a digital communications system can be designed, accurate propagation channel models must be developed. And in MIMO systems, it's important to understand the degree of correlation among propagation channels. Channel correlation will be a function of the environment as well as the antenna design-including antenna spacing and polarization.

While digitizing scopes and vector network analyzers can be used for some of these measurements, multichannel vector signals analyzers are particularly well suited to the task. They offer frequency selectivity, high dynamic range and deep waveform capture memory-and they can work with almost any signal, including those that are naturally part of the smart-antenna system.

However, until recently, spectrum analyzers and vector signal analyzers (VSAs) have been limited to a single RF measurement channel, but the latest version of Agilent's 89600 Series Vector Signal Analyzer supports two RF measurement channels. These two channels are phase-coherent and time-aligned with adjustable bandwidths to 36 megahertz and frequency coverage to 6 gigahertz.

The VSA can tune to a specified frequency and accurately digitize two signals over a specified bandwidth with a sampling frequency that is proportional to the signal bandwidth. The digitized signals are then captured directly into a high-speed, deep-capture waveform memory, where they can be exported to another application, or analyzed directly. Built-in measurements that combine data from both measurement channels include the frequency-response, cross-spectrum, cross-correlation and coherence functions.

In one application, the two measurement channels would be used to simultaneously observe complex modulated signals at the input and output of an active two-port device, such as a power amplifier. This allows both linear and nonlinear behavior to be accurately characterized. In another application, the two measurement channels would simultaneously acquire any pairing of transmit and receive signals in a smart-antenna radio system.

For example, the two measurement channels could digitize both received signals in a 2 x 2 MIMO system. In an eight-antenna beam-forming system, the two measurement channels might be used to determine the magnitude and phase relationships between any two of the eight transmit signals as a function of time, steering inputs, signal level or signal power statistics.

Channel measures

An area of significant interest today is developing accurate channel models as more complex channel models are required for designing and testing communications systems employing smart-antenna technology. A common way to measure a propagation channel's response is to use a network analyzer and a movable antenna.

Here, one measurement channel is used to observe the transmitted signal and another to observe the received signal at various locations in space. These measurements, taken over frequency, location and polarization, then can be combined to create a channel model.

When a two-channel VSA is used in place of the network analyzer, signals other than sinusoids can be used to stimulate the channel. Whereas the network analyzer was limited to using sinusoidal stimulus signals, almost any signal can be used with the VSA, including broadband random noise. For most systems, the VSA's 36-MHz information bandwidth is sufficiently wide to allow one or more radio channels to be accurately digitized. The advantage of this approach is that all frequencies are simultaneously observed.

This is especially critical in dynamic environments where the channel response changes with time. The deep capture memory can be especially useful for rapidly changing environments as a full 36-MHz channel can be digitized continuously for up to eight seconds. Narrower bandwidths can be digitized proportionally longer.

For MIMO systems specifically, both measurement channels can be connected to two separate antennas at the receive location. The spacing and polarization of the antennas can be varied and cross-channel results computed. Alternatively, one of the VSA measurement channels can be connected to any one of the radio's transmitter channels and the second measurement channel to any one of the receive antennas.

If the signals from each transmit channel are uncorrelated, then the propagation channel response between the selected transmitter and the selected receive antenna can be easily computed, as can the percentage of power that is coming from the selected transmitter. The latter can be readily determined from the coherence function. Spectral averaging techniques are used to significantly reduce the influence of the other uncorrelated transmitters.

Signals captured on the time-aligned, phase-coherent measurement channels can also be easily transferred to simulation tools such as Agilent's Advanced Design System, Matlab or custom software. The captured signals are represented as complex IQ pairs. This unique capability allows simulated receiver designs to be tested with real-world signals, improving the accuracy of the simulation when accurate channel models have not been developed. Alternatively, receiver hardware could be tested using the captured signals. The captured signals would be transferred directly into time-synchronized Agilent signal generators as arbitrary waveforms, where they can be reproduced as baseband or RF signals.

Component stability

Accurate beam forming requires precise control of the magnitude and phase relationships among antenna elements. In many of these systems, active elements exist between the antenna and the steering control. While many beam-forming and SDMA systems may be self-calibrating, problems can still arise when components in the signal path aren't stable in the time interval between calibrations.

To put that into perspective, at 2.7 GHz the carrier phase will shift by one degree per picosecond of delay change in the signal path. This shift might occur, for example, with changes in the mean power or power statistics of a signal passing through a power amp. A two-channel VSA with fully phase-coherent measurement channels is suited to detecting small phase and gain shifts.

Bob Cutler is senior member of the technical staff at the Signal Analysis Division of Agilent Technologies (Palo Alto, Calif.).

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