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Title. Opportunistic use of UMTS TDD Spectrum. Paulo Marques | pmarques@av.it.pt. © 2006, motion | mobile systems communication group | all rights reserved. Opportunistic Radio.

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Opportunistic use of UMTS TDD Spectrum

  • Paulo Marques | pmarques@av.it.pt

© 2006, motion | mobile systems communication group | all rights reserved


Opportunistic Radio

  • 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.


  • 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.
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.


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.


UMTS coverage scenarios

FDD mode 


TDD mode






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

The goal is to distinguish between 2 hypotheses:

There is a fundamental tradeoff between the probability of missed detection Pm =1-Pd and the probability of false alarm.

A high Pm causes high probability of interference, on the other hand, high Pfa would result in low spectrum utilization, since false alarms increase number of wasted opportunities.

Sensing problem formulation

False Alarm


The Observation Time required to guarantee a sufficiently low Pmis 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




cyclostationary detector
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.
cyclostationary detector11
Cyclostationary detector

S: Spectrum Cyclic Density

α=1/T0 : Cyclic frequency

cyclostationary detector12
Cyclostationary detector
  • In UMTS signals we can exploit cyclostationary features coming from the redundancy between frequency components separated by the symbol rate T0=SFxTc.

Simulation chain

  • The FFT operations are computed over N samples, N>>T0
  • Time smoothing operation is done using an average operation over K sets of N samples.
  • Observation Time = NxKx + Processing Time

Energy detector vs. Cyclostationary detector

PSD of UMTS signal, SNR=25 dB

SCD 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

detection statistic single cyclic detector
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,
detection statistic multiple cyclic detector
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.
simulation results roc receiver operation characteristic
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.

SNR=-15 dB



extension for collaborative sensing
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.
extension for collaborative sensing21
Extension for collaborative sensing
  • CSDU decides signal presence based on all OR sensing information + own OR BS.


Opportunity Manager



Sensing Report: H0 or H1




extension for collaborative sensing22
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 ?
  • 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.