Measuring the Wireless Channel

Transcription

1 Measuring the Wireless Channel Author: Prof Roland Küng, Elektrobit AG and HSR University of Applied Sciences, Switzerland Introduction We believe most communications can eventually be available through wireless services The move toward wireless everything is being driven by users, producers of information and wireless operators The same standard of convenience that requires car traffic highways and 24-hour shopping makes us want instant access to our , phone, fa and Internet Current wireless networks are not yet ready to deliver on the promises of wireless everything To move beyond voiceonly, wireless must provide faster and more efficient data transmission In other words, more bandwidth is needed and these can be found at higher frequency bands (eg 57 GHz, GHz) Compared to the cable environment, however, radio waves represent a much more comple world Multipath phenomena, which occurs when two or more propagation paths eist between the transmitter and receiver sites, is one of the most significant transmission impairments Measuring the multipath before designing a new system is mandatory if the channel has to carry the big amount of data for our everything wireless applications The propagation phenomena s The mechanism behind electromagnetic wave propagation are diverse, but can generally be attributed to reflection, diffraction and scattering [siwi] The free space propagation model is used to predict received signal strength when the transmitter and the receiver have an unobstructed line-of-sight (LOS) path between them The received signal power is well known and decreases with the square of the distance and the square of the frequency used In many situation there is no LOS path between transmitter and receiver Instead multiple reflected, diffracted and scattered waves arrive at the receiver site and interact with each other, as depicted in Fig1 When a radio wave impinges another medium with different electrical properties, the wave is partly reflected and partly absorbed The reflection coefficient is a comple function of the material properties and generally depends on the wave s angle of incidence, frequency and polarization Reflection Scattering Diffraction T LOS R Fig1: Propagation Phenomena s Diffraction allows the signal to propagate behind obstructions Although the received signal energy decreases fast when moving deeper into the shadowed region, the diffraction component still produces a useful signal The higher the frequencies used for wireless communications, the rougher the illuminated surfaces look from the wave s point of view Instead of a single reflection, the energy then is spread out in all or at least many directions This process is called scattering and well known from radar techniques In real world situations many objects, still and moving ones, produce reflections, diffraction and scattering This creates a constantly changing environment that modifies the signal energy in amplitude, phase and time The channel can be described by its impulse response as it is well known from filter theory The time-varying impulse response is a wideband characterization of the channel and contains all information necessary to analyze any type of transmission through that channel Before designing any new system it is wise to measure the channel etensively with a so-called sounder The knowledge of the channel response allows one to build models for propagation prediction [leuth] and to take best benefit of the radio channel

2 The multipath impulse response The impulse response h(t,t) is a bandpass function, but can also be described by a comple base band impulse response (CIR) The variable t represents the time variations due to the motion of objects, while T stands for the channel multipath delay for a fied time value t and describes the variable path lengths A typical multipath channel can look as illustrated in Fig2 and is often called power delay profile It is easy to identify the different paths and their strengths and relative delay for each time moment It can also easily be seen that the situation changes while time t goes by In the spectral domain the deep notches which reduce S/N for narrowband systems can be identified at different frequencies and also reflect the time-varying channel behaviour 0 db h 20log [h(t1,t)] 20log [h(t2,t)] 0 db S 20log [S(f, t1)] 20log [S(f, t2)] -40dB 0 ns T 800 ns -30 db f 0 MHz 100 MHz Fig2: Left: Channel impulse response at different times t, right: Frequency amplitude response For a short classification of the channel there are several parameters derived from the power profiles Probably most important ones are RMS delay spread and maimum ecess delay But one can also see how many relevant paths eist and if there is a strong LOS path (shortest delay), which is not the case in Fig2 (red) Other important figures can be calculated from a set of impulse responses, such as coherence bandwidth and doppler spread for an individual path All these parameters are important for the design of optimum communications systems Knowing the statistical and real-time data on the impulse response of a channel is of great importance and a must for the design of all future wireless everything This is especially true for broadband systems which are needed for multimedia applications Multipath Measurements There are three basic sounder architectures to measure the multipath structure Swept frequency channel sounding (Fig 3) makes use of the Fourier relationship between frequency and time domain and is usually done with a vector analyzer plus S-parameter test set The sweeper scans a part of the frequency band and measures S21 Parameter, which describes the frequency response of the channel The result is then transformed to the band-limited impulse response using inverse Fourier transform [pahl] The main disadvantage of this procedure is that hardware cabling between transmitter and receiver is needed to operate the S-parameter test set Other limitations are for real time measurement due to the relatively long sweep time Vector Network Analyzer S21(f) Inverse FFT Processor h(t) ~ IFFT(S21) (t) Amp + IF Strip Envelope Detector Storage/ Display T S-Parameter Set R f LO Fig3: Swept frequency sounder Fig 4: Direct pulse sounder

3 A direct pulse sounder (Fig4) transmits a very short wideband pulse, while a receiver consisting of a ultra fast amplitude detector and a digital storage oscilloscope measures directly the impulse response [rapp] The main disadvantages are its vulnerability to interference, the need for high peak power transmitters, trigger problems of the oscilloscope and the lack of phase information of the individual multipath components Many of these disadvantages can be eliminated by the third approach of a spread spectrum channel sounder A carrier signal is spread over a large bandwidth by miing it with a pseudo-noise sequence (PN) This sequence has typically a high chip rate Rc and a broadband power spectrum envelope proportional to the square of sin()/ with a null-to-null bandwidth of 2Rc It follows just the same principle as it is used in direct sequence spread spectrum systems [dio] The transmitter architecture is quite simple as shown in Fig 5 and transmits a signal continuously In the receiver the signal arriving from multiple paths is correlated with a local copy of the same PN sequence Due to the impulse-like autocorrelation function of PN sequences, the correlator output looks quite similar to the impulse response PN code generator mier amp Ant (t) Amp + IF Strip SAW PN-Code Matched Envelope Detector Storage/ Display f LO signal generator Fig5: Simple spread spectrum transmitter Fig6: SAW based channel sounder Simpler systems use surface acoustic wave (SAW) based correlators like convolvers or PN-code matched filters which directly operate at the IF frequency of the receiver (Fig6) They deliver no phase information Another simple technique without matched filter is the sliding correlator which despreads the received signal with a PN sequence that run at a slightly faster chip rate than the transmitter as described in [talv] Every time the chip sequence is aligned with an eisting path the signal is despread to a narrow band signal which can be processed further by standard methods to determine amplitude and phase relations Due to the sliding the actual delay times are scaled, but can be calculated Real-time measurements for many upcoming channels are not possible - Oscillator PN Code Spread Spectrum Signal T 1 T 8 T 1 T 8 Demodulator Demodulator T - control i Data signal proc and scanner PN Code Fig7: RAKE receiver based spread spectrum sounder

4 In [kauf] a spread spectrum communication system, called Scanning DS RAKE Receiver (Fig7) is described, which uses the transmitted data signal itself for real time channel measurement It determines the strongest paths which then are fed to a combiner / demodulator The chip rate of the system is 16 MHz which gives a multipath resolution of 66 ns Each path can be characterized by relative delay, amplitude and phase and is updated roughly every millisecond through a patented scanning process A communications receiver which scans the impulse response continuously as described in [kuen] is efficient in that it makes profitable use of the natural path diversity in the channel But the demands for the design of future wireless everything are growing and growing More bandwidth, higher frequency bands and more users per Hz bandwidth are needed This makes it necessary to gain additional information on the channel as for eample the spatial information on the different multipath components The question about the direction of arrival of each propagation path becomes important, because it allows focusing the transmitter signal more efficiently in the direction of the mobile user with the aid of intelligent antennas Elektrobit s new high performance channel sounder PropSOUND The demand for delay spread resolutions down to 5 ns, frequencies up to 60 GHz and a multiple antenna receiver input has led to the development of a new channel sounder All the radio channel epertise of Elektrobit is coupled with latest ASIC and DSP technology to define a new class of channel measurement equipment Principally it operates according to the spread spectrum approach A multiple antenna input in the receiver for gaining spatial information on the channel, a modular -frontend for different frequency bands, a rubidium time reference and GPS time tagging are added The design is modular and can be adapted to many of today s and future channel measurement needs With switchable PN code lengths, different system bandwidths and programmable real-time capabilities it can cope even with channels showing rapid changes, which means having high doppler spread A concept using rubidium time standard and GPS allows for true cableless measurements and correct stamping of all samples in the receiver on a PC A rough blockdiagram is depicted in Fig 7 PN code generator amp Ant (t) Antenna Selector Amp Broadband IF-strip power divider I(t) 0 o f IF 90 o ADC ADC Q(t) DSP correlator I Q Sync Rubidium frequency standard signal generator Rubidium frequency standard Sync Fig 8: Blockdiagram of new Elektrobit sounder system Left: transmitter, right: receiver The following table lists the most important parameters of Elektrobit s sounder: Item Standard Options Remarks Bandwidth 5200 MHz ( null-to-null bandwidth) up to 400 MHz; I/Q - modulation with PN s Measurement Rate 30'000 CIR/ s sustained (at 256 chip codelength) Variable Codes any code with length any codelength chips Multiple Antennas 1256 receive antenna channels bidirectional measurements antennas/ polarization multipleed Multiple Frequency up link / down link more frequencies frequencies multipleed Measurements Frequency Bands 1825 GHz 5159 GHz 09 GHz and 17 GHz bands or any other band downconverters are echangeable Post Processing Comple channel impulse response(cir) per antenna spatial information could be done online using the multi-dsp baseband unit Delay Resolution down to 10 ns 5 ns Instant Dynamic Range 30 db 3540 db within impulse response Output Power adjustable, up to 05 W eternal PA

5 The architecture delivers full comple impulse response (CIR) data in the form of sampled I/Q baseband data, which are stored for further post-processing on a PC/workstation Modularity allows for adding heavy number-crunching signal processors The storage feature gives the possibility for multiple analysis of the same measured data, enabling new research on channel behavior The data can be processed for real-time emulation of radio channels with Elektrobit s channel simulator PropSIM, which is very useful in wireless product development and testing phases The new sounder under development will allow for the precise characterization of the radio channel needed for the future design of wireless everything The higher the frequency bands for new services are, the more important is measuring to get insight into the channel behavior This is due to the increased compleity of propagation mechanisms, for eample in the 60 GHz band more and more reflection points turn to rough scatterers Other influences gain weight like attenuation in the atmosphere in function of temperature and humidity [Kim9] References: [siwi] Radio wave propagation and antennas for communications, K Siwiak, Artech House, 1995 [leuth] Radiowave propagation in mobile communications, BH Fleury, PE Leuthold, IEEE Communications Magazine, Vol34, No2, Feb96, pp70-81 [kuen] Digital spread spectrum multipath-diversity receiver for indoor communications, R Küng, H Kaufmann, U Fawer, IEEE 42nd VTS Conference, Denver 1992, pp [talv] A wideband channel measurement system for aircraft air-to-ground links, Jaakko Talvitie, Pentti Leppänen, Torsti Poutanen, IEEE Proceedings ISSSTA92, 1992, pp [dio] Dion, RC,Spread spectrum systems with commercial applications, John Wiley & sons, 1994 [pahl] Pahlavan K, Levesque AH, Wireless Information networks, Chapter 5, John Wiley & Sons, 1995 [rapp] Rappaport, TS, Characterisation of UHF Multipath Radio Channels in Factory Buildings, IEEE transaction on [kauf] antennas and propagation, Vol37, No8, pp , 1989 A wireless data modem for local communications, H Kaufmann, R Küng, Wireless Personal Communications, Chapter 8 pp , 1993 Kluwer Academic Publishers, ISBN [kimm] Application of the 60 GHz band to wireless networks, Kimmo Kaitala, Microwave Engineering Europe, April 1999, pp 25-32