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SEFDM

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Spectrally efficient frequency division multiplexing (SEFDM)[1] is a multicarrier communication technique developed at UCL in 2003 by Miguel Rodrigues and Izzat Darwazeh. Unlike orthogonal frequency division multiplexing (OFDM), the sub-carriers of SEFDM are packed non-orthogonally for the purpose of improving spectral efficiency. SEFDM originated as an extension of fast-OFDM (F-OFDM) developed by the same UCL group in 2002.[2]

a typical OFDM spectrum
OFDM spectrum
SEFDM spectrum
a typical OFDM spectrum

Advantages[edit]

  • The notable benefit of SEFDM is improved spectral efficiency compared to OFDM.
  • Lower peak to average power ratio (PAPR) compared to OFDM. Although SEFDM is a multicarrier technique, due to its special signal characteristics, SEFDM shows relatively lower PAPR.
  • Although sub-carriers are no longer orthogonal, signal modulation and demodulation can efficiently be implemented using inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT), respectively.
  • Low sensitivity to frequency offset.

Disadvantages[edit]

  • Signal detection is more complex due to the non-orthogonally packed sub-carriers.
  • Typical frequency-domain channel estimation/equalization cannot be used in SEFDM indicating a more complex time-domain equalization has to be used.

Practical applications in wireless and optical systems[edit]

After several years of theoretical investigations, SEFDM has been realized practically in both wireless, optical and their joint systems.

  • 2017: Software defined radio platform USRP based 5G testbed.
  • 2016: A realistic wireless platform setup for the verification of coexistence of 4G and promising 5G signals.
  • 2016: Non-orthogonal millimeter wave radio over fiber (RoF) testbed setup. 3.75 Gbit/s non-orthogonal signals are delivered through 250 meters multimode fiber (MMF), over 3 meters wireless link at 60 GHz mm-wave frequency.
  • 2015: Dual polarization coherent detection optical testbed transmitting non-orthogonal signals at 24 Gbit/s.
  • 2015: Bandwidth compressed carrier aggregation wireless signal transmission at 70 Mbit/s using commercial PXI transceiver and Spirent VR5 channel emulator.
  • 2015: LTE/LTE-A experimental testbed that can reach up to 50 Mbit/s data rate.
  • 2014: Direct detection optical testbed transmitting non-orthogonal signals at 10 Gbit/s.
  • 2013: A real time MIMO/SEFDM detector working at 1.06 Gbit/s was designed and implemented on a Xilinx Vertex-6 FPGA chip.

Mathematical descriptions of SEFDM signals[edit]

Signal generation[edit]

IDFT based SEFDM transmitter models
Three different SEFDM transmitters are included

In a conventional OFDM system, signal generation can be realized straightforwardly using standard IFFT. However, due to the violation of the orthogonality property within SEFDM, the typical IFFT approach is not applicable. In order to use the IFFT algorithm, three alternative algorithms [3]are specially designed for SEFDM.

Signal detection[edit]

The benefit of SEFDM is to reduce bandwidth occupation while it is at the expense of complicated signal detection. The optimal detection scheme is maximum likelihood (ML) while the high complexity prevents its practical use. Since 2008, researchers have been working on simplifying signal detection. A brief history is presented below.

  • Sept. 2008: A combined MMSE-ML signal detector was evaluated[4]
  • Sept. 2009: Investigation of semidefinite programming (SDP) detection[5]
  • Sept. 2009: A pruned sphere decoding (SD) was investigated[6]
  • Sept. 2010: A joint channel equalization and detection[7]
  • Nov. 2010: The use of a fast constrained SD detector[8]
  • Mar. 2011: Proposal of a truncated SVD (TSVD) detector[9]
  • May. 2011: Evaluation of fixed sphere decoding (FSD) detector[10]
  • Oct. 2013: An enhanced FSD detector with iterative soft mapping[11]
  • Jul. 2014: A higher order modulation scheme with iterative soft mapping[12]
  • Jul. 2014: A simplified detector for a multi-band SEFDM architecture[13]
  • Nov. 2014: A soft detector based on Turbo principle[14]

Channel estimation[edit]

Various SEFDM system models[edit]

Different system architectures are proposed in order to facilitate the detection of SEFDM signals.

Precoded SEFDM[edit]

As mentioned before, SEFDM requires complicated detection algorithms to recover signals from ICI. In order to simplify signal processing, a precoding technique [15] for SEFDM was proposed to modify transmitted signals with new properties that can simplify the detection work at the receiver. In general, according to the characteristics of the correlation matrix, signal power is re-distributed on each symbols resulting in 'weak' (low power) and 'strong' (high power) channels. Therefore, the precoding method leads to high performance for the relatively 'strong' channels while lowers the performance for the 'weak' channels. Simple detectors like ZF or MMSE can be applied for the strong channels while more complex high performance detectors such as SD or FSD have to be used for the weak channels.

Block-SEFDM[edit]

Due to the self-created ICI, the detection of SEFDM signals becomes complex. When the signal dimension is small, exhaustive search can be operated to achieve the ML performance while the complexity of signal detection increases rapidly with the enlargement of the system size. To simplify the detection it is therefore sensible to divide the problem into sub-problems where the detection of each is more manageable. Therefore, a multi-band SEFDM architecture [16] that divides the whole spectrum into several non-orthogonal blocks was proposed. Symbols in each block can be detected separately by using the SD algorithm. The out-of-block interference is minimized in this system through using narrow frequency guard bands. This technique makes it practical to detect large size non-orthogonal SEFDM signals.

Turbo-SEFDM[edit]

Iterative detection has been investigated in a paper [17] where it indicates that signal detection is dependent on previous iterations and a decision error will affect the subsequent signal decisions resulting in performance degradation. The effect is more notable in SEFDM since self-created ICI is introduced. In order to eliminate the iterative errors, error control coding may be considered to improve data reliability. In general, the Turbo principle [18] is employed in Turbo-SEFDM [19] system to improve the reliability of symbol decisions iteratively.

Hardware implementation[edit]

  • May. 2011: A real-time FPGA based SEFDM signal generator[20]
  • Sept. 2011: FPGA implementation of the TSVD detector[21]
  • May. 2012: A reconfigurable SEFDM transmitter in 32-nm CMOS[22]
  • Jun. 2012: A hardware verification methodology for SEFDM signal detection[23]
  • Sept. 2012: A hybrid DSP-FPGA implementation of the TSVD-FSD detector[24]
  • May. 2013: A real-time FPGA based implementation of the TSVD-FSD detector[25]
  • Jun. 2013: A pure DSP implementation of the modified TSVD-FSD detector[26]

References[edit]

  1. "A Spectrally Efficient Frequency Division Multiplexing Based Communication System – UCL Discovery". discovery.ucl.ac.uk. Retrieved 2015-10-07.
  2. MRD, Rodrigues,; I, Darwazeh, (2002-06-01). "Fast OFDM: A proposal for doubling the data rate of OFDM schemes". discovery.ucl.ac.uk. Retrieved 2015-10-10.
  3. Whatmough, P. N.; Perrett, M. R.; Isam, S.; Darwazeh, I. (May 2012). "VLSI Architecture for a Reconfigurable Spectrally Efficient FDM Baseband Transmitter". IEEE Transactions on Circuits and Systems I: Regular Papers. 59 (5): 1107–1118. doi:10.1109/TCSI.2012.2185304. ISSN 1549-8328.
  4. Kanaras, I.; Chorti, A.; Rodrigues, M.R.D.; Darwazeh, I. (2008-09-01). "A combined MMSE-ML detection for a spectrally efficient non orthogonal FDM signal". 5th International Conference on Broadband Communications, Networks and Systems, 2008. BROADNETS 2008: 421–425. doi:10.1109/BROADNETS.2008.4769119.
  5. Kanaras, I.; Chorti, A.; Rodrigues, M.; Darwazeh, I. (2009-09-01). "Investigation of a Semidefinite Programming detection for a spectrally efficient FDM system". 2009 IEEE 20th International Symposium on Personal, Indoor and Mobile Radio Communications: 2827–2832. doi:10.1109/PIMRC.2009.5449891.
  6. Kanaras, I.; Chorti, A.; Rodrigues, M.; Darwazeh, I. (2009-09-01). "A new quasi-optimal detection algorithm for a non orthogonal Spectrally Efficient FDM". 9th International Symposium on Communications and Information Technology, 2009. ISCIT 2009: 460–465. doi:10.1109/ISCIT.2009.5341206.
  7. Chorti, A.; Kanaras, I.; Rodrigues, M.R.D.; Darwazeh, I. (2010-09-01). "Joint channel equalization and detection of Spectrally Efficient FDM signals". 2010 IEEE 21st International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC): 177–182. doi:10.1109/PIMRC.2010.5671644.
  8. Kanaras, I.; Chorti, A.; Rodrigues, M.R.D.; Darwazeh, I. (2010-11-01). "A Fast Constrained Sphere Decoder for Ill Conditioned Communication Systems". IEEE Communications Letters. 14 (11): 999–1001. doi:10.1109/LCOMM.2010.093010.100918. ISSN 1089-7798.
  9. Isam, S.; Kanaras, I.; Darwazeh, I. (2011-03-01). "A Truncated SVD approach for fixed complexity spectrally efficient FDM receivers". 2011 IEEE Wireless Communications and Networking Conference (WCNC): 1584–1589. doi:10.1109/WCNC.2011.5779400.
  10. Isam, S.; Darwazeh, I. (2011-05-01). "Design and Performance Assessment of Fixed Complexity Spectrally Efficient FDM Receivers". Vehicular Technology Conference (VTC Spring), 2011 IEEE 73rd: 1–5. doi:10.1109/VETECS.2011.5956738.
  11. Xu, Tongyang; Grammenos, R.C.; Marvasti, F.; Darwazeh, I. (2013-10-01). "An Improved Fixed Sphere Decoder Employing Soft Decision for the Detection of Non-orthogonal Signals". IEEE Communications Letters. 17 (10): 1964–1967. doi:10.1109/LCOMM.2013.090213.131573. ISSN 1089-7798.
  12. Xu, Tongyang; Darwazeh, I. (2014-07-01). "M-QAM signal detection for a non-orthogonal system using an improved fixed sphere decoder". 2014 9th International Symposium on Communication Systems, Networks Digital Signal Processing (CSNDSP): 623–627. doi:10.1109/CSNDSP.2014.6923903.
  13. Xu, Tongyang; Darwazeh, I. (2014-07-01). "Multi-band reduced complexity spectrally efficient FDM systems". 2014 9th International Symposium on Communication Systems, Networks Digital Signal Processing (CSNDSP): 982–987. doi:10.1109/CSNDSP.2014.6923972.
  14. Xu, Tongyang; Darwazeh, I. (2014-10-01). "A Soft Detector for Spectrally Efficient Systems With Non-Orthogonal Overlapped Sub-Carriers". IEEE Communications Letters. 18 (10): 1847–1850. doi:10.1109/LCOMM.2014.2352294. ISSN 1089-7798.
  15. Isam, S.; Darwazeh, I. (2010-09-01). "Precoded Spectrally Efficient FDM system". 2010 IEEE 21st International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC): 99–104. doi:10.1109/PIMRC.2010.5671886.
  16. Xu, Tongyang; Darwazeh, I. (2014-07-01). "Multi-band reduced complexity spectrally efficient FDM systems". 2014 9th International Symposium on Communication Systems, Networks Digital Signal Processing (CSNDSP): 982–987. doi:10.1109/CSNDSP.2014.6923972.
  17. Xu, Tongyang; Grammenos, R.C.; Marvasti, F.; Darwazeh, I. (2013-10-01). "An Improved Fixed Sphere Decoder Employing Soft Decision for the Detection of Non-orthogonal Signals". IEEE Communications Letters. 17 (10): 1964–1967. doi:10.1109/LCOMM.2013.090213.131573. ISSN 1089-7798.
  18. Hagenauer, Joachim (1997). "The turbo principle: tutorial introduction and state of the art". in Proc. Int. Symp. Turbo Codes.
  19. Xu, Tongyang; Darwazeh, I. (2014-10-01). "A Soft Detector for Spectrally Efficient Systems With Non-Orthogonal Overlapped Sub-Carriers". IEEE Communications Letters. 18 (10): 1847–1850. doi:10.1109/LCOMM.2014.2352294. ISSN 1089-7798.
  20. Perrett, M.R.; Darwazeh, I. (2011-05-01). "Flexible hardware architecture of SEFDM transmitters with real-time non-orthogonal adjustment". 2011 18th International Conference on Telecommunications (ICT): 369–374. doi:10.1109/CTS.2011.5898952.
  21. Grammenos, R.C.; Isam, S.; Darwazeh, I. (2011-09-01). "FPGA design of a truncated SVD based receiver for the detection of SEFDM signals". 2011 IEEE 22nd International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC): 2085–2090. doi:10.1109/PIMRC.2011.6139882.
  22. Whatmough, P.N.; Perrett, M.R.; Isam, S.; Darwazeh, I. (2012-05-01). "VLSI Architecture for a Reconfigurable Spectrally Efficient FDM Baseband Transmitter". IEEE Transactions on Circuits and Systems I: Regular Papers. 59 (5): 1107–1118. doi:10.1109/TCSI.2012.2185304. ISSN 1549-8328.
  23. Perrett, M.R.; Grammenos, R.C.; Darwazeh, I. (2012-06-01). "A verification methodology for the detection of spectrally efficient FDM signals generated using reconfigurable hardware". 2012 IEEE International Conference on Communications (ICC): 3686–3691. doi:10.1109/ICC.2012.6364570.
  24. Grammenos, R.C.; Darwazeh, I. (2012-09-01). "Hardware implementation of a practical complexity Spectrally Efficient FDM reconfigurable receiver". 2012 IEEE 23rd International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC): 2401–2407. doi:10.1109/PIMRC.2012.6362759.
  25. Xu, Tongyang; Grammenos, R.C.; Darwazeh, I. (2013-05-01). "FPGA implementations of real-time detectors for a spectrally efficient FDM system". 2013 20th International Conference on Telecommunications (ICT): 1–5. doi:10.1109/ICTEL.2013.6632117.
  26. Grammenos, R.C.; Darwazeh, I. (2013-06-01). "Performance trade-offs and DSP evaluation of spectrally efficient FDM detection techniques". 2013 IEEE International Conference on Communications (ICC): 4781–4786. doi:10.1109/ICC.2013.6655330.


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