Калибрација дистрибуираног вишеканалног пријемног система бежичним путем

Abstract

Predmet ove doktorske disertacije je kalibracija distribuiranog vixekanalnog prijemnog sistema beiqnim putem, tako xto se vrxi obrada primljenih radio signala. Distribuirani prijemnici imaju nezavisne analognodigitalne konvertore i lokalne oscilatore, te su vremenski, frekvencijski i fazno nesinhronizovani. Pod kalibracijom se podrazumijeva procjena i kompenzacija vremenskih, frekvencijskih i faznih pomaka izmeu signala u prijemnim kanalima u cilju postizanja vremenske, frekvencijske i fazne sinhronizacije, koja je preduslov za pravilno funkcionisanje obrade signala sa antenskih nizova. Pretpostavlja se da postoji prostorna koherencija signala, xto znaqi da se faza nosioca predvidivo mijenja po prostoru tako xto je linearna funkcija vremena propagacije talasa i frekvencije nosioca. Pored toga, smatra se da se snaga reflektovanih komponenti signala (Non-Line-of-Sight { NLoS) moe zanemariti u odnosu na direktnu komponentu (Line-of-Sight { LoS). Ove pretpostvake vae pri prenosu u mmWave opsegu u malim elijama, xto je karakteristiqno za nadolazeu petu generaciju mobilne telefonije, gdje se oqekuje primjena rezultata istraivanja u tezi. Pretpostavlja se da se predajnici i prijemnici ne kreu. Prijemnici su povezani sa fuzionim centrom putem digitalnih linkova, posredstvom kojih nije mogu prenos analognih referentnih signala za sinhronizaciju. Zbog jednostavnosti, teza se najveim dijelom bavi dvokanalnim prijemnim sistemima, a proxirenje na vixekanalne sisteme moe se postii posmatranjem parova kanala. Analizirana su dva scenarija. U prvom scenariju, izmeu prijemnih kanala postoje konstantni vremenski pomak, poqetni fazni pomak i promjenljivi frekvencijski pomak. Cilj je razvoj i ispitivanje performansi metoda koje, procjenom i kompenzacijom pomaka izmeu primljenih signala, omoguuju distribuirani digitalni beamforming (BF) i lokalizaciju izvora radio signala korixenjem datog sistema. Sa tom namjerom je u tezi predloena procedura za kalibraciju. U sluqaju lokalizacije, procedura obuhvata zdruenu procjenu i kompenzaciju vremenskog i trenutnog faznog pomaka izmeu prijemnih kanala. U tu svrhu se koristi predajnik za kalibraciju, beacon, koji xalje uskopojasni i xirokopojasni pilot. U sluqaju beamforming-a, procedura obuhvata zdruenu procjenu i kompenzaciju vremenskog i trenutnog faznog pomaka izmeu primljenih korisnih signala i, opciono, ekvalizaciju korisnog signala. Pored uskopojasnog i xirokopojasnog pilota koje xalje beacon, koriste se i uskopojasni pilot (ako se vrxi ekvalizacija) i xirokopojasna preambula, poslati od strane korisniqkog predajnika. Poxto je frekvencijski pomak promjenljiv, ne procjenjuje se eksplicitno, ve je formulisan novi adaptivni algoritam za procjenu trenutnog faznog pomaka. Nekoherentni algoritam tipa maksimalne vjerodostojnosti (Maximum Likelihood { ML), predstavljen u drugom dijelu teze, vrxi procjenu vremenskih pomaka. Numeriqki prost algoritam baziran na korelaciji signala koristi se za procjenu konstantnog faznog pomaka. U svrhu evaluacije predloene procedure i algoritama vrxene su Monte-Karlo simulacije i eksperimenti sa softverski definisanim radio-ureajima. Rezultati eksperimenata su pokazali odliqno slaganje sa rezultatima simulacija, xto potvruje ispravnost matematiqkog modela i usvojenih pretpostavki...

The dissertation deals with over-the-air calibration of a distributed multichannel receiving system, by processing the received radio signals. Distributed receivers have independent analog-digital converters and local oscillators, so they are time, frequency, and phase unsynchronized. The calibration includes estimation and compensation of time, frequency, and phase offsets between the signals in the receiving channels with the aim of achieving time, frequency, and phase synchronization, which is a prerequisite for proper operation of array processing. Spatial coherence of signals is assumed, meaning that the carrier phase changes predictably over the space, being the linear function of the propagation delay and the carrier frequency. Additionally, we assume that the Non-Line-of-Sight components (NLoS) are negligible compared to the Line-of-Sight (LoS) components. These assumptions are valid for communication in mmWave range in small cells, which are typical of 5G, where the application of the investigation in the thesis is expected. All of the transmitters and receivers are assumed to be stationary. The receivers are connected to a fusion center via digital links, which are incapable of conveying analog reference signals for the synchronization. For the sake of simplicity, the thesis mostly addresses twochannel receiving systems, but the generalization to multichannel systems can be achieved by dealing with pairs of the channels. Two scenarios are analyzed. In the first scenario there is a constant time offset, an initial phase offset and a variable frequency offset between the receiving channels. The goal is to develop and investigate the performance of the methods that, by estimating and compensating for the offsets between the received signals, enable distributed digital beamforming (BF) and the radio source localization using that system. To this end a procedure for calibration is proposed in the dissertation. In the localization case, the procedure includes joint estimation and compensation of the time offset and instantaneous phase offset between the receiving channels. For that purpose a calibration transmitter, the beacon, sends a narrowband and a wideband pilot. In the BF case, the procedure includes joint estimation and compensation of the time offset and instantaneous phase offset between the received signals, and, optionally, equalization of a user signal. For that purpose a narrowband pilot (if the equalization takes place) and a wideband preamble sent by the user transmitter are used, in addition to the beacon signals. Since the frequency offset is time-variable, it is not estimated explicitly, but a new adaptive algorithm for instantaneous phase offset estimation is formulated. A non-coherent maximum likelihood (ML) algorithm, presented in the second part of the dissertation, is used for the time shifts estimation. A numerically cheap algorithm based on signal correlation is used for the constant phase offset estimation. Monte-Carlo simulations and experiments using software defined radio devices have been carried out to evaluate the procedure and algorithms. The results of the experiments have shown excellent matching with the simulation results, which confirms the correctness of the signal model and adopted assumptions. The adaptive algorithm for instantaneous phase offset estimation has shown capability of following abrupt changes in frequency offsets. The obtained accuracies show that the proposed procedure and algorithms are especially suited to receive BF and non-coherent/semi-coherent localization. If the frequency offset changes slowly enough, application in transmit BF is also possible. The proposed procedure is modular, i.e. every algorithm can be replaced by another algorithm of the same type...