Ultrawideband (UWB) communications systems offer great promise of fast data rates and video transmissions without RF carriers. Based on short, time-sequenced pulses, these systems exhibit wide bandwidths, but considerable engineering challenges for reliable spectrum shaping. Engineering challenges also exist in testing UWB devices, since traditional test instruments are based on evaluating narrowband signals. Fortunately, with advances in RF signal generators, analyzers, and digital oscilloscopes, engineers currently have the tools they need to test UWB transmitters.

In February 2004, the United States Federal Communications Commission (FCC) opened 7500 MHz of bandwidth for UWB applications, from 3.1 to 10.6 GHz. The FCC regulation calls an UWB transmitter an intentional radiator with a fractional occupied bandwidth of greater than 20 percent or an instantaneous bandwidth of greater than 500 MHz.¹ The FCC limits its equivalent isotropically radiated power (EIRP) to −41.3 dBm from 3.1 to 10.6 GHz for both indoor and handheld devices.² Additional restrictions are placed on the radiated power outside of this band to minimize interference with Global Positioning Systems (GPS), aviation systems, and other communication systems (see table). The EIRP limits are measured using a spectrum analyzer with a 1-MHz resolution bandwidth (RBW) and average detection or root-mean-square (RMS) detection. In addition, the regulations limit the average radiated power over the 1164-to-1240-MHz and 1559-to-1610-MHz bands to −85.3 dBm when measured using a 1-kHz RBW.

The FCC does not specify the RF physical layer interface in the regulations but rather dictates the spectrum content to balance the needs of the many licensed and unlicensed users. The RF physical layer interface is then defined through the standards community as in the IEEE 802.15.3a standard for UWB communications, and industry groups like the Multi-Band OFDM Alliance (MBOA).³

The candidate proposals for UWB wireless personal area network systems can be divided into two groups: single-carrier and multiband devices. Single-carrier devices use a pulse-modulated RF carrier and a direct sequence (DS) spread-spectrum technique to realize the wideband requirements.⁴ Multiband devices, based on the MBOA proposal, use 528-MHz-wide orthogonal-frequency-division-multiplexing (OFDM) modulation to meet the 500-MHz minimum bandwidth requirements. The multiband RF carrier is then frequency hopped within a specified band group to spread the signal over a much larger operating bandwidth. The MBOA proposal specifies that the 3.1-to-10.6-GHz frequency range is divided into 14 channels or bands. These bands are grouped into five Band Groups (1-5) consisting of four groups of three bands each and one group of two bands.

At introduction, the first MB-OFDM devices will operate in Band Group 1, which specifies three usable RF carriers of 3432, 3960, and 4488 MHz. Assigning frequency-hop patterns or time-frequency codes (TFC) to each radio system will provide multiple user access and allow piconets to be formed. By implementing switching of the OFDM carrier, the system can use the full frequency band and the electronics need only to operate at speeds relative to the 528-MHz modulation bandwidth. By comparison, the single-carrier DS-UWB system would require the electronics to operate at a much higher chip rate in order to instantaneously cover the wide frequency band.

The MB-OFDM switches frequency for every OFDM symbol, at the very high rate of 312.5 ns. The frequency must settle within 9.5 ns, which makes using a tunable phase-locked oscillator impractical. An alternative is to generate the carrier frequency from a single phase-locked oscillator and a single-sideband (SSB) beat product from another frequency derived from that same oscillator (Fig. 1).⁵ In this configuration, if the oscillator frequency is set to 4224 MHz, then any one of the three carrier frequencies, 3432, 3960, and 4488 MHz can be rapidly selected.

Due to the wideband properties and rapid frequency switching of MB-OFDM signals, it is important to understand the types of measurements available to accurately characterize MB-OFDM device performance. The rest of this article will focus on several measurements useful when characterizing MB-OFDM transmitters such as average and peak power, error vector magnitude, and power density measurements.

UWB signals are meant to have noise-like properties in order to underlay narrowband radio systems with minimal interference. To accurately measure the time-averaged power using a spectrum analyzer, the instrument should be configured with an RMS detector. The time-averaged power spectral density (PSD) is the main regulatory test for an UWB transmitter. The FCC requires that the PSD be measured using a swept-tuned spectrum analyzer with a specified RBW and rms detector. The PSD is typically reported in dBm/Hz or normalized to a 1-Hz bandwidth. It is also acceptable to report PSD with an alternate RBW.

Using a swept-tuned spectrum analyzer with an RMS detector, it is possible to measure the average power over a selected frequency range and obtain the same reading as a traditional power meter. As an example, Fig. 2a shows the averaged transmitted power of a noise modulated, fixed-carrier signal as 0 dBm. This measurement was obtained using the band power function of the analyzer and gives the same result as a power meter. Figure 2a also shows that the maximum PSD of the signal is slightly above the −17 dBm/100 kHz reference line using the RMS detector. If the RF carrier of this signal is now rapidly switched equally in time between two frequencies, the total band power remains the same at 0 dBm but the PSD is reduced by 3 dB (Fig. 2b). This rapid switching allows the MB-OFDM device to reduce the PSD in order to meet regulatory requirements without sacrificing the total transmitted power. Recall, that the regulatory guidelines are put in place in order to allow UWB signals to coexist and underlay existing narrowband radio systems. It is often necessary to verify the proper operation of any narrowband communication system that would co-exist with the MB-OFDM system. Modern signal generators can be used to create UWB interference signals in order to determine the effects of UWB pulsing on any narrowband receiver.

Although an UWB signal has many noiselike properties, the transmitted signal can also contain unwanted spurious signals generated from local oscillators, and clock signals and various other components generated within the transceiver. Therefore, the FCC also places limits on the highest radiated emission at 0 dBm EIRP. These peak measurements are made using a swept tuned spectrum analyzer with a RBW setting of up to 50 MHz. The accuracy of the RBW filter generally degrades for wider bandwidths and the FCC guidelines allow for measurements with RBW settings between 1 and 50 MHz. Should the selected RBW be greater than 3 MHz, the FCC requires a detailed description of the test procedure, equipment calibration, and instrumentation employed when the application for certification is filed.

The following scaling factor is used to determine the peak emissions with RBW settings other than 50 MHz: 20log (RBW/50), where RBW is specified in MHz. As an example, the maximum peak emissions using a 1-MHz RBW would result in an EIRP of −33.9 dBm. The scaling factor is based on the way the peak detector responds to pulsed signals under various RBW settings.¹ The 20log scaling will result in a higher, more conservative value than a 10log scaling, which would be used for pure noise signals.

As a measurement example, Fig. 3 shows a spectrum analyzer response to an OFDM signal using a peak detector (upper trace). As a comparison, the same signal is measured using an rms detector (lower trace). When the signal is noiselike, peak measurements are a very useful indication of spectrum occupancy, but generally not ideal for measuring an absolute power level. On the other hand, time-average measurements allow the total signal power to be easily determined using the band power function. This measurement was made on the Agilent PSA spectrum analyzer that has the capability of simultaneously displaying measurements from the two detector types on the same display.

When characterizing the MB-OFDM symbols and transient responses of the UWB signal, the swept-tuned spectrum analyzer does not have an adequate measurement bandwidth to fully capture all the information contained within the wideband signal. Real-time oscilloscopes with bandwidths to 13.5 GHz are the natural tools for this case. Wideband spectral analysis of the MB-OFDM signal can be obtained using an FFT-based measurement, such as found in the Agilent 89601A vector-signal-analysis (VSA) software.

An FFT-based measurement system can provide the most informative view of the UWB signal but the measured spectrum can look different than the response obtained using a swept-tuned spectrum analyzer, partly because it is giving much more information about the changes in spectrum with time. Several factors cause detailed differences in the displayed data such as the shape and bandwidth of the selected RBW filter, the record length of the time captured waveform, the point in time when the signal is sampled and the way the signal is detected. The FFT-based measurement can be made to look similar to the measurement on a swept-tuned analyzer by selecting a Gaussian filter, increasing the time averaging, using a "max hold" function and proper triggering of the scope in order to capture all the transient effects of the signal. In addition to spectral analysis, the wide measurement bandwidth of the real-time oscilloscope allows additional insight into the physical RF layer of the UWB signal such as time-varying characteristics of the OFDM symbols. The time-varying properties of the MB-OFDM physical layer will now be introduced and related to a series of measurements using a real-time (not under-sampled) measurement system.