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28.6.2017 : 0:21

Cognitive Radio for White Spaces

Incumbent Protection

Since the start of the Digital Switch Over (DSO) in the TV broadcast bands, an increasing number of European  and  world-wide  regulatory  bodies  and  authorities  are  investigating  the  unlicensed,  opportunistic use of certain parts of the spectrum. As regulatory figures have started to converge in  the TV whitespace (TVWS), the implementation of a flexible transceiver, which meets regulatory figures and  deployment  driven  requirements,  has  become  an  opportunity  for  a  potential commercial application  of  Cognitive  Radio  (CR)  concepts. 

Cognitive radio for white spaces

In order to avoid possible harmful interference with incumbent TV broadcasting systems and wireless microphones, these opportunistic devices need to comply with certain limitations. These include  their effective  radiated  power  and  adjacent-channel  leakage  requirements.  Current  regulations  specify relatively  strict  requirements  which  pose  challenges  on  the  implementation  using  conventional modulation schemes and signal processing techniques. 

Different scenarios for CR applications

QoSMOS Aim

As stipulated in the P1900.7 contribution, the LTE downlink waveform requires additional filtering to meet the -55 dB ACLR requirement which in turn leads to significant complexity increase in the implementation. Furthermore, Dynamic Spectrum Access (DSA) in TVWS is likely to result in heavily fragmented spectrum. Filter Bank Multicarrier Modulation (FBMC) has been considered well suited and a first implementation geared towards these applications is done in the project.

 

What do standards say?!                                                                          

IEEE DYSPAN P1900.7: Radio Interface for White space Dynamic Spectrum Access Radio Systems Supporting Fixed and Mobile Operation – requirements

  • “The P1900.7 shall provide means to protect primary systems according to the national and international radio regulations.”
  • “The P1900.7 shall support operation in interleaved spectrum access, for example, by utilizing vacant sub-channels in a multi-carrier system scenario such as OFDM.”

What can we do?!                                                                          

 • Flexible PHY which can adapt its spectrum profile to allowed and / or available spectral resource

• Multi-Carrier approaches have such flexibility; thanks to the possibility to adjust each subcarrier transmit power in order to shape the overall spectrum profile

• OFDM is the initial choice for CR PHY for the following reasons:

–        OFDM is deployed in broadcast applications, as well as WLAN and in mobile wireless   communication

–        Simple equalization over frequency selective channels leads to simple receiver implementation

–        High spectrum efficiency

Recapitulating OFDM

In the frequency domain, the sum of sinc functions result in adjacent leakage power

• Typically, first side-lobe of an OFDM spectrum is at -13dB compared to the main lobe

• Classical turnarounds are:

–        to add filtering
–        to decrease signal bandwidth 

OFDM power spectral density

What can be done?!

Balian-Low theorem states that we cannot have simultaneously:

• A prototype filter well localized in time and frequency

• A maximal spectrum efficiency

• Complex orthogonality        <--  Change prototype filter

 

 

I-Q diagram of CP-OFDM and FBMC-OQAM symbols

 

 

Filter Bank Multi Carrier (FBMC)

• Keep the flexibility of Multi-Carrier modulation

• Control frequency response of each carrier by introducing a filter bank centered on every active carrier and based on the same prototype response

• This prototype filter can be selected to minimize (null) adjacent channel interference

• The filtering is embedded in the digital modulation scheme

• No additional filter is required

• More flexibility

 

FBMC prototype pulse vs. OFDM sync

The FBMC transceiver shown can be implemented through usual iFFT/FFT for time frequency domain conversion

 

FBMC transmitter - receiver

 

 

Low out of band leakage in FBMC

FBMC shows spectacular spectral efficiency gain relative to filtered OFDM. Regarding complexity, FBMC has lower complexity vs filtered OFDM when we want higher suppression of side lobes. The complexity calculations are given below:

 

Where, N is the number of carriers in FFT and the FBMC prototype filter spans over k OFDM symbols. The plot of the complexity comparison is given below!

 

 

Complexity comparison between FBMC and filtered OFDM

 

Spectrum pooling

• Concept of spectrum pooling where sub carriers are switched ‘on’ or ‘off’ according to available spectrum resource

• This shapes the spectrum to fill the available gaps, while avoiding interference in the band used by other systems (e.g. primary systems)

 

 

Spectrum pooling

 

Spectrum pooling with FBMC

• Since OFDM does not meet ACLR performance, only filtered OFDM is considered for comparison

• In the example hereunder, 15kHz subcarrier spacing is considered in both cases

–   The benefit of filtering on top of OFDM mitigates interference on both sides of the overall band
–   It does not reject the signal inside the notch channels
–   With FBMC, ACLR requirement is met both on adjacents and in the notch

 

 

FBMC ACLR requirements are met both in adjacent channels and inband notch

Hardware Architecture of QoSMOS Flexible Transceiver

The final results of the QoSMOS flexible transceiver hardware implementation and  the  measurements  results  of  the  blocks  are presented  here. The  QoSMOS  flexible  transceiver hardware portrayed in the following Figure consists of several modules: 

  • a baseband board,
  • a RF transmitter board, and
  • up to two RF receiver boards.

The RF boards are designed allowing to stack one board on top of another.

A detailed description of the hardware concept as seen in Figure and the hardware implementation

can be found in [D7.2], as this is the basis for the 2nd  Proof-of-Concept demonstration.

 

Layout of QoSMOS flexible transceiver hardware

 

The  QoSMOS  RF  front-end  adopts  the  architecture  of  a  heterodyne  signal  chain,  possessing  a conversion from an intermediate frequency (IF) to radio frequency (RF) for the transmitting case or from RF to IF for the receiving case. The RF is supposed to be tuneable within 470MHz to 860MHz.

As  a  design  of  a  single  RF  branch  was  targeted,  a  high  IF  of  280MHz  was  chosen  to  relax  the requirements of the pre-selection filter that needs to be tuneable. For the receiving case, for instance, if a  low  IF  were  used,  the  RF  signal  band  would  appear  close  to  its  image  frequency  band.  This eventually  results  in  tighter  requirements  on  the  pre-selection  filter  in  terms  of  out-of-band suppression, since a high out-of-band suppression contradicts a wide tuning range.

 

 

Prototype TVWS flexible tx-rx implementation

 

The following two figures show the demo scan shots of the gain of ACLR in FBMC over OFDM. FBMC can achieve -55.6 dB of out of band leakage.

 

 

OFDM vs FBMC TX ACLR

 

The following figure demonstrates the spectral pooling properties of FBMC and the fact that FBMC is better in this respect over OFDM.

 

OFDM vs FBMC spectrum pooling

 

Conclusion

 •TVWS is a context where spectrum efficiency and incumbent  protection cannot be guaranteed simultaneously by mainstream OFDM technology

• QoSMOS has investigated alternatives to tackle these issues

• Three approaches have been investigated IA-PFT ; GFDM and FBMC-OQAM

• FBMC was thoroughly analyzed down to actual implementation

• High ACLR and flexible access to fragmented spectrum have been demonstrated

• This technology is under discussion in IEEE DYSPAN P1900.7  

The final results of the QoSMOS flexible transceiver hardware implementation and  the  measurements  results  of  the  blocks  are presented  here. The  QoSMOS  flexible  transceiver hardware portrayed in the following Figure consists of several modules: