As RF signals become more and more complex, it is necessary to utilize tools that enable RF engineers to synthesize these signals for development and conformance test use. However, as bandwidths widen and the physical layer (PHY) and media access control (MAC) structure become more complex, synthesis becomes more difficult. The most technically challenging signals today are the ultra-wide band (UWB) signals. This article looks at both the UWB signal and the hardware/software tool set that can synthesize signals like this.
Regulators worldwide are approving unlicensed use of short range, low power, UWB RF devices in the 3.1GHz to 10.6GHz spectrum. The regulatory bodies have chosen to define UWB by its bandwidth-to-carrier frequency ratio rather than any specific modulation type.
Because of this ruling, there are several types of UWB applications in use today. Time-Hop UWB (TH-UWB) uses a series of very short impulses and selects a pseudo-random time position for each pulse. Direct Sequence UWB (DS-UWB) assigns positive or negative values to each bit, and pulse shapes the resultant signal for transmission.
Multi-Band Orthogonal Frequency Division Multiplex (MB-OFDM) uses the massively parallel carrier structure of OFDM and further enhances that with optional frequency hopping. This is the modulation method chosen by the WiMedia group for short-range computer links such as wireless USB and the next-generation Bluetooth signals.
On the UWB transmit side, low power and limited range can make an effective UWB signal nearly invisible to existing spectrum users. In addition, some countries have a requirement for detect and avoid (DAA) technology, which allows UWB signals to avoid bands already in use. On the UWB receive side, the wide bandwidth tends to give immunity from single carrier interference.
Current low-cost OFDM modulators can only achieve a little over 500MHz of modulated signal bandwidth. Using a simple frequency hop pattern over three bands, in conjunction with a conventional OFDM modulator, designers can create over 1.5GHz of bandwidth. This allows further spreading of the conventional OFDM signal.
UWB signals, as the name implies, are very wide band. This makes signal generation a challenge, particularly when the signal generator needs to be flexible. Most common laboratory signal generators are capable of generating only a few tens or hundreds of megahertz of bandwidth, which is far short of the bandwidth necessary for most UWB signals.
Different UWB modulation types require different signal synthesis techniques. Designers often create signals like TH-UWB and DS-UWB at baseband. Other signals like MB-OFDM are more typically upconverted to the appropriate RF band. Upconversion methods require less baseband bandwidth from the signal generator, but add the complexity of an external up-converter or modulator.
A simple solution is to use the UWB's own system software to generate test signals, but this approach can have issues. The primary problem is that, early in the development cycle, the design may not be working properly, leading to potentially serious test issues. In addition, the radio system under development usually lacks the ability to add impairments and can be cumbersome to manipulate for test purposes.
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