Page 1Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Title Paulo Marques | Opportunistic use of UMTS TDD Spectrum © 2006, motion | mobile systems communication group | all rights reserved

Page 2Paulo Marques, Atílio Gameiro3rd COST 289 Workshop  Independent studies carried out in different places have shown that most of the assigned spectrum is under-utilized.  Opportunistic radios improve the efficiency of spectrum utilization.  They typically operate at frequencies that were originally licensed to other primary radio services.  OR will change the way the radio spectrum is regulated, but also requires new enabling techniques such as improved spectrum sensing and dynamic spectrum allocation. Opportunistic Radio

Page 3Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Motivation  UMTS radio spectrum is a very expensive resource: in 2000 in the UK, US$ 154 per citizen was paid for 2x15 MHz radio frequency.  UMTS frequency allocation comprises of a paired band (FDD) and an unpaired band (TDD).  Nearly all UMTS operators have allocated, depending on the country, a number of FDD frequencies and a block of TDD frequencies.

Page 4Paulo Marques, Atílio Gameiro3rd COST 289 Workshop  To date, most UMTS deployments have focused on FDD utilizing the paired frequency bands.  Over 120 of Europe’s and Asia’s largest mobile operators have spectrum that is specifically allocated to 3G TDD technologies. 1920

Page 5Paulo Marques, Atílio Gameiro3rd COST 289 Workshop  TDD fits for pico-cell hot spots, used mainly within buildings and are therefore often shielded by walls.  This results in a natural limitation of a coverage area and consequently we can predict some 5 MHz spectrum opportunities in spatial dimension for OR use.  Opportunistic use of UMTS TDD bands could be a new source of revenue for actual 3G license owners.

Page 6Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Satellite In-Building Macro-Cell Pico-Cell Micro-Cell  TDD mode FDD mode  UTRA UMTS coverage scenarios

Page 7Paulo Marques, Atílio Gameiro3rd COST 289 Workshop  Basic requirements: –OR must not cause harmful interference in a UMTS. –UMTS system does not need to be changed.  UMTS sensing challenges: –UMTS is a DS-Spread Spectrum system. –Classical Energy Detector doesn't work due to low PSD of UMTS signals. –Focus on the detection of UMTS TDD signals through the use of a cyclostationary detector. OR operation

Page 8Paulo Marques, Atílio Gameiro3rd COST 289 Workshop  The goal is to distinguish between 2 hypotheses:  There is a fundamental tradeoff between the probability of missed detection P m =1-P d and the probability of false alarm.  A high P m causes high probability of interference, on the other hand, high P fa would result in low spectrum utilization, since false alarms increase number of wasted opportunities. Sensing problem formulation False Alarm Detection

Page 9Paulo Marques, Atílio Gameiro3rd COST 289 Workshop  The Observation Time required to guarantee a sufficiently low P m is a crucial metric to evaluate the ability of OR not to disturb the UMTS.  The Observation Time until detection the primary user is very critical because, during that time the OR is creating harmful interference to the UMTS system. Observation Time Licensed system OR

Page 10Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Cyclostationary detector  A large class of signals like AM, FM, VSB, PSK, QAM, OFDM, CDMA in fact exhibit underlying periodicities in their signal structure.  That periodicity is not always obvious. It is introduced in the signal format so that an OR can exploit it for detecting a random signal in a background of noise.  Noise doesn't exhibit cyclostationarity features because it is a random stationary process, that is, its statistic isn't a function of time.

Page 11Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Cyclostationary detector S: Spectrum Cyclic Density α=1/T 0 : Cyclic frequency

Page 12Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Cyclostationary detector  In UMTS signals we can exploit cyclostationary features coming from the redundancy between frequency components separated by the symbol rate T 0 =SFxTc.

Page 13Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Simulation chain  The FFT operations are computed over N samples, N>>T 0  Time smoothing operation is done using an average operation over K sets of N samples.  Observation Time = NxKx  + Processing Time

Page 14Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Energy detector vs. Cyclostationary detector PSD of UMTS signal, SNR=25 dBSCD of UMTS signal, SNR=25 dB PSD of UMTS signal, SNR= -15 dB SCD of UMTS signal, SNR= -15 dB SF=16, Tobs=300 ms

Page 15Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Detection statistic: Single cyclic detector  We propose as detection statistic the SNR measured in a specific cyclic spectral line n.  Sw is a noise floor estimated using an average over Q spectrum lines with α≠nα,

Page 16Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Detection statistic: Multiple cyclic detector  It is possible to increase the sensitivity of the cyclostationary detector exploiting other contributions from different M cyclic frequencies.  The detector complexity increases M times.

Page 17Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Simulation parameters Asymmetry’s service level14 time slot Downlink:1 time slot Uplink SF16 Codes per time slot8 00 240 KHz qq Q=2; 120 KHz and 360 KHz N2560 samples  Tc Noise variance error1 dB ChannelAWGN

Page 18Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Simulation results

Page 19Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Simulation results: ROC (Receiver Operation Characteristic)  Sensing is harder in presence of Multipath and Shadowing.  In order to achieve short detection times we may be more tolerant with false alarms, trade a short Tobs against more false alarms. Or increase the complexity, M. Pfa Pd SNR=-15 dB

Page 20Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Extension for collaborative sensing  “ A Pd of 99.9 % need to be assured to license owners. Otherwise, licensed users are not likely to be willing to share their spectrum with others. ” [T. Weiss, F. Jondral, Spectrum Polling an innovative strategy for the enhancement of spectrum efficiency, IEEE Radio Communications, March 2004.]  In heavily shadowed / fading environments, different OR may need to cooperate in order to detect the presence of a UMTS terminal.

Page 21Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Sensing Report: H0 or H1 OR BS CSDU OR2 OR1 Look up table Local sensing OR1 SNR-15 dB Tobs300 ms DetectorSingle Cyc. OR1 Pd98% Pfa20% BS,OR1,OR2 Pd 99.9% Pfa 1% UMTS TDD BS MAC Opportunity Manager Extension for collaborative sensing  CSDU decides signal presence based on all OR sensing information + own OR BS.

Page 22Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Extension for collaborative sensing  Future research topics not addressed yet.  Collaborative sensing depends: –Local sensing sensitivity –Number of OR sharing sensing information –Propagation environment –Which is the optimal combination rule ?

Page 23Paulo Marques, Atílio Gameiro3rd COST 289 Workshop Conclusions  Simulation results showed that using a local cyclostationary detector it is possible to sense a weak UMTS signal, with a SNR= -15 dB, and keep a Pd equal to 98%, even taking into account the shadowing and multipath propagation effects.  With extra detection gain coming from possible collaborative sensing architectures and new cognitive functions we can envisage that UMTS and OR can coexist without a degradation of the UMTS in real scenarios.