measurements is the understand what is happening in the propagation-physics use of a spectrum analyser as RX [PLVP02]. The phase point of view and further to be able to create realistic information can be recovered from the amplitude spec- channel models for system studies. trum by using Hilbert transform. On the other hand, there are lots of measurements Various kinds of wideband waveforms are used in the related to the systems aspects of the current and future channel sounders, such as a PSK signal modulated by PN mobile radio systems. In a radio system, the radios have to sequence [Prop], [KSPH01], [ZeTS04], a multitone deal with several simultaneous connections between BSs signal with low crest factor [RUSK], and a chirp signal (or access points) and mobiles. Thus one should have in- [SFIH02], [LPEO02]. formation not only on a single link but also on several radio All the signals are designed to have small envelope connections and their mutual relationships. Networks are fluctuation to reduce the backoff of the TX high power probably the ultimate example of the complexity of the amplifier. VNAs use step frequency Continuous Wave propagation scenarios in the future mobile radio systems. (CW) signal. Hybrid stepped frequency and PN se- quence is used to realise very wide bandwidth [KaWe00], 4.7.4.2 Channel sounding techniques [VLSK03]. The phase and timing at both sides must be Several new architectures of the channel sounders have synchronised by some means. When TX and RX are close been proposed for the double-directional channel in distance, the local signal can be shared via coaxial cable Propagation modelling and channel characterisation CHAPTER 4.7 or optical fibre. Otherwise, high stability frequency orthogonal feeds in a single patch element [KSPH01], standards such as rubidium oscillators are usually used for [Rich05], [SLRT03]. the purpose. For high accuracy timing synchronisation, use of Global Positioning System (GPS) and Transmission Array geometries Control Protocol (TCP)/Internet Protocol (IP) co