MIMO/CDMA Detection Strategies
We have developed /are currently developing a number of Bayesian detection methods that are applicable to both synchronous code division multiple access (CDMA) and multiple-input multiple-output (MIMO) communication systems. Markov chain Monte Carlo (MCMC) simulation techniques are used to obtain Bayesian estimates (soft information) of the transmitted bits. We have demonstrated that such detectors perform very close to channel capacity. Comparisons with sphere decoding and previous MCMC based method have shown that our detectors are superior.
Our on going research includes the followings:
Relevant Publications:
[1] B. Farhang-Boroujeny, H. Zhu, and Z. Shi, "Markov chain Monte Carlo algorithms for CDMA and MIMO communication systems," Submitted to IEEE Trans. Signal Processing, Nov. 2004.
[2] H. Zhu, B. Farhang-Boroujeny, and R-R. Chen, "On performance of sphere decoding and Markov chain Monte Carlo detection methods," IEEE Signal Processing Letters, Accepted. [pdf]
[3] R-R. Chen, B. Farhang-Boroujeny and A. Ashikhmin, "Capacity-approaching LDPC codes based on Markov chain Monte Carlo MIMO detection," Submitted to IEEE Communications Letters, March 2005.[pdf]
[4] H. Zhu, Z. Shi, and B. Farhang-Boroujeny, "MIMO detection using Markov chain Monte Carlo techniques for near-capacity performance," Int. Conf. Acoustics, Speech and Signal Processing, ICASSP’05, Philadelphia, March 18 – 23, 2005.
[5] Z. Shi, Haidong Zhu, and B. Farhang-Boroujeny, Markov chain Monte Carlo techniques in iterative detectors: a novel approach based on Monte Carlo integration, IEEE Global Telecommunications Conference, GLOBECOM'04., vol. 2 , 29 Nov.-3 Dec., 2004, pp. 325 – 329.
[6] H. Zhu, B. Farhang-Boroujeny, and R-R. Chen, "On performance of sphere decoding and Markov chain Monte Carlo detection methods," SPAWC 2005, the sixth IEEE International Workshop on Signal Processing Advances for Wireless Communications, June 5-8, 2005, Invited. [pdf]
[7] R-R. Chen, B. Farhang-Boroujeny and A. Ashikhmin, "Capacity-approaching LDPC codes based on Markov chain Monte Carlo MIMO detection," SPAWC 2005, the sixth IEEE International Workshop on Signal Processing Advances for Wireless Communications, June 5-8, 2005. [pdf]
[8] Contribution to IEEE 802.11n, A Novel Soft MIMO Detector for MIMO-OFDM (802.11n) Receivers: [pdf]
Filter Bank – Based Multicarrier Modulation
Even though we do have on going research on traditional OFDM/DMT techniques, part of our research in multicarrier communication concentrates on filter bank-based multicarrier modulation. We refer to this as cosine-modulated filter bank multicarrier modulation (CMFB-MCM) or cosine-modulated multitone (CMT), for a shorter terminology. In the past, under the name discrete wavelet multitone (DWMT) modulation, CMT has been considered for data transmission over digital subscriber lines (DSL). We propose a new receiver structure that is different from those proposed in the previous publications. The new structure simplifies the task of channel equalization, by reducing the number of equalizer parameters by a order of magnitude. We also propose a novel blind equalization algorithm that fits very nicely in the proposed structure. Moreover, we discuss bandwidth efficiency of the proposed CMT and show that it is superior to the convention (single carrier) quadrature amplitude modulation (QAM) and OFDM/CMT. The CMFB is found to be a signal processing block that stacks a number of vestigial sideband (VSB) modulated signals in a number of overlapping subchannels in the most efficient way. We have compared the CMFB-MCM/CMT with OFDM/DMT and shown that the former has a number of advantages over the latter. The details of these comparisons can be found in:
[1] B. Farhang-Boroujeny, "Muticarrier modulation with blind detection capability using cosine modulated filterbanks," IEEE Trans. Commun., Dec. 2003, pp. 2057 - 2070.
[2] L. Lin and B. Farhang-Boroujeny, "Cosine modulated multitone for very high-speed digital subscriber lines," Submitted to EURASIP, Nov. 2004.[pdf]
[3] B. Farhang-Boroujeny and L. Lin, ""Cosine modulated multitone for very high-speed digital subscriber lines," ICASSP 2005, Philadephia, March 21-23, 2005.
CMT equalizer can be adapted blindly. Study of the performance surface of this blind equalizer can be found in:
[4] L. Lin and B. Farhang-Boroujeny, "Analytical study of the performance surface of blind equalizer in a cosine modulated multicarrier communication system," ICASSP 2005, Philadephia, March 21-23, 2005. Also, submitted to IEEE Trans. Signal Processing.
An analysis of DWMT that has led to the development of CMFB-MCM/CMT is presented in:
[5] B. Farhang-Boroujeny, Lekun Lin, "Analysis of post-combiner equalizers in cosine modulated filter bank based transmultiplexer systems," IEEE Trans. Signal Proc., Dec. 2003, pp. 3249 - 3262.
Our on going research includes the followings:
· Carrier and timing recovery.
· CMFB for cognitive radio.
See the next two pages for examples of performance of CMFB-MCM/CMT compared to OFDM/DMT.
Example 1:
Fig. 10 of the first paper above shows the bit-error-rate (BER) results of CMFB-MCMC and OFDM in a wireless channel.
Example 2:
Fig. 7 of the second paper above (presented below) compares the achievable bit rates of DMT, zipper DMT (z-DMT; a proposed method in the VDSL draft standard), filtered multitone (FMT; another proposed method in the VDSL draft standard), and CMT. CMT results are shown for two prototype filter designs; one based on perfect reconstruction (PR) design and one based on a near perfect reconstruction design, proposed in the paper. Upper bound shows the result that would be obtained if there was no intersymbol and intercarrier interference.
Filter Bank Multitone for Cognitive Radio
The demand for ubiquitous wireless services has been on rise in the past and is expected to remain the same in future. As a result, the vast majority of the available spectral resources have already been licensed. It thus appears that there is little or no room to add any new services, unless some of the existing licenses are discontinued. On the other hand, studies have shown that vast portions of the licensed spectra are rarely used [1]. This has initiated the idea of cognitive radio (CR), where secondary (i.e., unlicensed) users are allowed to transmit and receive data over portions of spectra when primary (i.e., licensed) users are inactive. This is done in a way that the secondary users (SUs) are invisible to the primary users (PUs). In such a setting PUs are ordinary mobile terminals and their associated base-stations. They thus do not possess much intelligence. The SUs, on the other hand, should possess the intelligence of sensing the spectrum and use whatever resources are available when they need them. At the same time, the SUs should give up the spectrum when a PU begins transmission.
A recent proposal [2] has suggested multicarrier communication for CR. The rationale is that any CR needs to sense the spectrum, and this involves some sort of spectral analysis. On the other hand, since FFT can be used for the spectral analysis and at the same time it can act as the demodulator of an OFDM (orthogonal frequency division multiplexing) signal, OFDM has been suggested as the candidate for multicarrier-based CR systems. At the same time, a number of short-comings of OFDM in the application of CR has been noted in [3] and solutions to them have been proposed. The short-comings of the OFDM solution originate from the large side-lobs of the frequency response of filters that characterize the channel associated with each subcarrier. The large side-lobs result in significant interference among the subcarriers that originate from different SUs and between PUs and SUs. To resolve/ease this problem, [3] suggests extension of each OFDM block with long cyclic prefix and suffix samples and application of some windowing to reduce the side-lobs of the subcarrier channels. Obviously, this solution is at the cost of bandwidth loss because excessive time should be allocated to cyclic extensions that otherwise could be used for data transmission.
We study methods of using filter banks for multicarrier communication in a CR setup. Two solutions are discussed. The first solution uses subcarrier bands that are non-overlapping. The method is referred to as filter multitone (FMT) and was originally developed for bi-directional transmission of data over digital subscriber lines (DSL) [4], [5]. From the bandwidth efficiency point of view, this solution may be non-attractive because of guard/transition bands between adjacent subcarriers. However, it offers advantages from the simplicity point of view. The second solutions uses cosine modulated filter bank. We call this cosine modulated multitone (CMT). This method also has roots in DSL [6], and has recently been revisited and applied to wireless applications as well [7]. This solution offers the advantages of bandwidth efficiency and blind equalization capability [7]. When multiple adjacent bands are used to carry the data of one user, overlapped adjacent bands can be separated perfectly thanks to perfect reconstruction property of CMFB [8]. We also develop random medium access control (MAC) protocols for the proposed CRs.
References
[1] R.W. Brodersen, A. Wolisz, D. Cabric, S.M. Mishra, and D. Willkomm, "CORVUS: A cognitive radio approach for usage of virtual unlicensed spectrum," White paper, Berkeley, July 29, 2004, available at http://bwrc.eecs.berkeley.edu/Research/MCMA/CR_White_paper_final1.pdf
[2] T.A. Weiss and F.K. Jondral, "Spectrum pooling: an innovative strategy for the enhancement of spectrum efficiency," IEEE Communications Magazine, Vol. 42, No. 3, March 2004, pp. S8 - S14.
[3] T.A. Weiss, J. Hillenbrand, A. Krohn, and F.K. Jondral, "Mutual Interference in OFDM-based Spectrum Pooling Systems," IEEE 59th Vehicular Technology Conference, 2004, VTC 2004-Spring, vol. 4, May 17-19, pp. 1873 – 1877.
[4] G. Cherubini, E. Eleftheriou, S. Olcer, "Filtered multitone modulation for VDSL," in Proc. IEEE Globecom’99, vol. 2, pp. 1139-1144, 1999.
[5] G. Cherubini, E. Eleftheriou, S. Olcer, J.M. Cioffi, "Filter bank modulation techniques for very high speed digital subscriber lines," IEEE Commun. Mag., vol. 38, no. 5, pp. 98-104, May 2000.
[6] S.D. Sandberg and M.A. Tzannes, "Overlapped Discrete Multitone Modulation for High Speed Copper Wire Communications," IEEE Journal on Selected Areas in Commun., vol. 13, no. 9, pp. 1571-1585, Dec. 1995.
[7] B. Farhang-Boroujeny, "Multicarrier modulation with blind detection capability using cosine modulated filter banks," IEEE Trans. Commun., vol. 51 ,no. 12, pp. 2057-2070, Dec. 2003.
[8] P.P. Vaidyanathan, Multirate Systems and Filter Banks, Englewood Cliffs, New Jersey, Prentice Hall, 1993.
Relevant Publications:
P. Amini, R. Kempter, R-R. Chen, L. Lin and B. Farhang-Boroujeny, "Filter Bank Multitone: A Candidate for Physical Layer of Cognitive Radio," Presented in the SDR Forum Technical Conference, Hyatt Regency, Orange County, California, 14-17 November 2005. [pdf] [Presentation]
B. Farhang-Boroujeny, P. Amini, and R. Kempter, "Filter bank techniques for cognitive radio," Under preparation for submission to JSAC special issue on Adaptive, spectrum agile and cognitive wireless networks.
CDMA detection requires the knowledge of the user-specific spreading
sequences. Thus, if additional paging is impossible or unwanted such as in
(mobile) CDMA Ad-hoc networks, typically Spread ALOHA is believed to be the only
means for data transmission. However, Spread ALOHA is collision and interference
limited and multiuser detection cannot be used to improve system performance
[1], [2], [3]. As a solution, we present a novel fully asynchronous packet
transmission system termed Random Packet CDMA [4]. In Random Packet CDMA (or RP-CDMA),
every packet consists of a header frame which is spread with a common sequence
as well as a data portion with random spreading. Information which allows to
recover the random sequence is contained in the header portion of the packet
which is also used for packet specific timing recovery. As a result, RP-CDMA is
no longer collision limited and multiuser detection can be used to improve
system performance. In the next step, we introduce a novel CDMA detection scheme
called partitioned spreading based on iterative decoding. In contrast to other
CDMA detection schemes, partitioned spreading detection is near-far resistant.
We compare partitioned spreading to the matched filter receiver, the
decorrelator, the MMSE filter and a successive cancellation detector in randomly
generated networks as well as in an Ad-hoc scenario based on a snapshot of the
FLUX Mobile Robot testbed [5]. We show that as the load increases, only
partitioned spreading detection is able to maintain original network coverage
and prevent network fragmentation. As a result, we motivate that in Ad-hoc
networks, advanced joint detection might be a necessity -- not only to achieve
higher transmission rates but merely to maintain connectivity.
References
[1] Yener, A. and Yates, R. D., "Multiuser Access Detection for CDMA
Systems", Proceedings of Conference on Information Sciences and Systems CISS 98,
1998
[2] Yener, A. and Yates, R. D., "Multiuser Access Capacity of Packet Switched
CDMA Systems", IEEE 49th Vehicular Technology Conference, 1999, vol. 3, pp.
1846-1850, May 16-20, 1999
[3] Yener, A. and Yates, R. D., "Acquisition dependent random access for
connectionless CDMA systems", In Proceedings of WCNC'00, September 2000
[4] C. Schlegel and R. Kempter and P. Kota, "A Novel Random Wireless Packet
Multiple Access Method Using CDMA", IEEE Transactions on Wireless
Communications, to appear September 2006
[5] Johnson, D. and Stack, T and Fish, R and Flickinger, D. and Ricci, R. and
Lepreau, J., "TrueMobile: A Mobile Robotic Wireless and Sensor Network Testbed",
University of Utah Flux Group Technical Note 2005-02, Revised version to
appear in INFOCOMM 2006