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Ultra Wideband (UWB) radio communications are fast becoming the media of choice for short range, high speed, Wireless personal area networks (PANS). UWB offers connectivity for mobile handsets, desktop peripherals, and personal devices such as PDAs and Smartphones. While the main application today for UWB is file transfer, high-resolution streaming video is an intriguing emerging application that takes full advantage of UWB's bandwidth capabilities.
UWB signals have, as their name implies, a very wide bandwidth, often several GHz wide. While UWB signals simply cannot have their own spectrum in today's crowded RF environment, it is possible to transmit UWB signals over spectrum dedicated to other uses as long as the power levels are sufficiently low, just above the noise floor.
Interference by UWB signals has been a controversial issue. By keeping the UWB power levels low, and accepting the resultant low range, interference with other signals becomes unlikely while retaining the ability to handle short-range file transfers. In some cases, Detect And Avoid (DAA) technology enables UWB signals avoid bands used by other services.
UWB signals are immune to many types of interference. The very wide bandwidth of UWB signals makes single narrowband interference sources and narrow multi-path cancelation notches less destructive since only a small part of the signal's energy is affected.
On the other hand, testing of UWB signals is challenging, to say the least. Like many of today's more complicated digital transmission standards, UWB receiver test requires signal sources that can produce a valid signal, create repeatable poor signals, and simulate a variety of interference scenarios. Transmitter test requirements include the ability to make traditional RF and signal quality measurements, and in the case of conformance testing, even count bits in various parts of the packets under specific conditions. To make all of this more interesting, the UWB signal's wideband nature creates a very wide bandwidth requirement for both the signal source and the measurement device.
UWB's high data rate, multi-path immunity, and robustness to interference make it an attractive wireless technology for today's bandwidth-hungry computer peripherals. This, in turn, is already driving development of a number of UWB devices. Let's take a closer look at UWB technology, the testing challenges, and some test techniques used to make conformance testing simpler.
MB-OFDM
The United States Federal Communications Commission (FCC) created an open definition for UWB in 2002, which did not specify the techniques used to generate and detect RF energy. Instead, the FCC defined UWB as any radio transmission with a spectrum that occupies more than 20 percent of the center frequency, or a minimum of 500 MHz, while adhering to low output power limits.
Figure 1
This gave rise to a number of competing UWB technologies, many of which are in use today. The WiMedia Alliance has successfully promoted one of these technologies, Multi-Band Orthogonal Frequency Domain Multiplex (MB-OFDM), for short-range computer peripheral use for both USB and Bluetooth applications.
Orthogonal Frequency Division Multiplex (OFDM) is composed of many RF carriers spaced closely together while keeping their phase components orthogonal. This allows each carrier to have a much slower data rate than a traditional single carrier modulation while retaining a high total throughput. Individually slower carriers also allow longer symbol times, which greatly reduce Inter-Symbol Interference (ISI) caused by time spreading from multi-path.
Click for larger image
Figure 2
OFDM modulators are currently limited to approximately a 500 MHz bandwidth. To gain more of the advantages of UWB, multiband OFDM (MB-OFDM) uses a frequency hopping technique. This allows further spreading of the conventional OFDM signal and makes it UWB for regulatory purposes. Designers hop the OFDM signal in a simple three-channel pattern to create over 1.5 GHz of bandwidth. The result is a signal that is attractive for indoor environments, where interference and multipath are prevalent.
The WiMedia Signal
The WiMedia signal is composed of an OFDM modulation with 128 carriers, using either Quadrature Phase Shift Keying (QPSK) or Dual Carrier Modulation (DCM) on each carrier. This modulation format allows at least eight data rates ranging from 53 Mb/s to 480 Mb/s.
Figure 3
The WiMedia OFDM modulation is frequency hopped over a band group composed of 528 MHz wide bands. A Time Frequency Code (TFC) controls the hopping of the OFDM signal across the band group. Relative to most Frequency Hop Spread Spectrum (FHSS) signals, the MB-OFDM WiMedia very quickly, with an uncomplicated hopping pattern, sending one symbol during each hop.
The WiMedia standard specifies a Detect And Avoid (DAA) scheme where transceivers listen to the band for other signals before transmitting, and avoid transmissions in busy bands to help mitigate interference to other users. This addresses a lingering concern about using UWB signals even at low power levels and, in some countries, is a regulatory requirement.
Conformance Test for WiMedia - A Challenging Test Problem
WiMedia signals rely on complex protocols, like many digital communication signals, and can be difficult to test with older traditional instruments. The unusual nature of the signal, combined with radically different hardware architectures, present unique challenges for the engineer.
In particular, conformance testing of the WiMedia signal presents a number of challenges. The signal's bandwidth, power level, hopping characteristics, required RF tests, and packet structure testing requirements all create their own issues. Let's take a look at these challenges.
UWB Conformance Test Challenges
The first challenge is the difficulty of creating test signals and measurements with amplitude and phase flatness accuracy over an ultra-wide bandwidth. Flatness issues create pulse distortions, which affects the spectral properties of UWB signals. The normal way to minimize flatness issues is to choose test equipment with a significantly wider bandwidth than the signal under test. This just does not work with UWB signals.
Another challenge when measuring UWB signals is that they require wide resolution bandwidths (RBWs) during testing. UWB signals cover large swaths of spectrum and some of the licensed channels contained in this spectrum can be up to 50 MHz wide. This requires RBWs of 50 MHz for accurate measurements of power spectral density. Many popular spectrum analyzers cannot accommodate this requirement.
Often, the lack of test ports in the equipment under test is a challenge. Internal test point connections may not exist or may not reflect the attenuation characteristics of an ultra-broadband antenna. This creates a need to measure radiated signals, with all the variables antennas bring into play. Adding to this, the transmit signal is likely to be low, near the noise floor in many cases, requiring a very sensitive spectrum analyzer or external preamplifier.
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