## 1 Introduction
Cognitive radio (CR) provides an alternative (a new paradigm) to systems such as the third generation (3G) and the fourth generation (4G). As a result of the Department of Defense (DoD) focusing on the Joint Tactical Radio System (JTRS), US has a clear technical leadership in cognitive radio. Cognitive radar [14], on the other hand, has similar demand for dynamic spectrum sharing. It is our conviction that it is, indeed, feasible to build a cognitive radar system using today's technology. The advent of (multi-GHz) arbitrary waveform generators has made it possible to change waveforms from pulse to pulse [15]. Until recently, sensor hardware was not capable of changing the transmitted waveform in real time. We believe that the sensor hardware can be leveraged by jointly considering wideband spectrum sensing and waveform design. Anti-jamming--an example of electronic warfare--is critical. The multi-GHz wideband platform proposed for both cognitive radio and cognitive radar may be further leveraged by including anti-jamming strategy (e.g., frequency hopping): it is much harder to jam the multi-GHz wideband communication and radar, compared with their multi-MHz counterparts. Our proposed experimental platform is one of the first of such integrated platforms. There are two frequency bands where the cognitive radios might operate in the near future [11, 13]: 54-862 MHz (VHF and UHF TV bands) and 3-10 GHz (Ultra-wideband (UWB) radios) [16]. The FCC has noted that in the lower UHF bands almost every geographic area has several unused 6 MHz-wide TV channels. In 2002, the FCC approval of UWB underlay networks in 3-10 GHz indicates that this frequency range might be opened for opportunistic use. Figure 1: DARPA XG cognitive radio. Since CR uses opportunistic transmission, it is desirable to operate over the widest possible bandwidth to give the highest probability of detecting unused spectra [17]. The unique sensing function forces the front-end to have several GHz sampling rate with high resolution (of 12 or more bits), if GHz bandwidths are to be searched [18]. One of the most demanding challenge is posed by wide bandwidth: make UWB RF front-end able to access spectrum dynamically. There exist two basic problems for system concepts: (1) How do we deal with baseband signals of several GHz bandwidth, say 3 GHz (0-3 GHz) or 7.5 GHz (3.1-10.6 GHz)? (2) How do we handle the dynamic range of spectrum sensing over the bandwidth of several GHz? The objective of this paper is to address these two problems. The system is designed using a revolutionary new theory, also known as compressed sensing [19, 20, 21]. By exploiting the _structure_ of the natural signal, a sampling rate that is much lower than the Shannon/Nyquist rate can be used to recover the \"information\" of the analog signal with overwhelming probability. We have demonstrated a UWB system baseband bandwidth (5 GHz) that would take decades for the industry to reach with the conventional sampling technology. We could use standard converters at the level of 125 megasamples per second (MS/s), for which excellent high dynamic range commercial solutions are available--a big advantage of the proposed approach. UWB radios are revolutionary due to its unprecedented bandwidth--three orders of magnitude higher than the typical wireless systems. Their signals exhibit many unique properties such as transient and impulsiveness--they are sparse in some domain (e.g., time). The sparseness--the very fundamental notion underlying compressed sensing--can be exploited to reduce sampling rate. Compressed sensing framework provides a universal measurement approach for signal detection and estimation, without reconstructing the signal--a quasi-digital receiver. Unlike the analog-intensive correlation receivers (popular for UWB), extremely wideband analog delay element is not required. The fact that space-time signals are essentially always significantly compressible in some representation promises huge benefits.