*Albert Szent-Gyorgyi*[source]

(last updated: 2014-07-23)

"Research is to see what everybody else has seen, and to think what nobody else has thought."
—*Albert Szent-Gyorgyi*
[source]

"Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution. It is, strictly speaking, a real factor in scientific research."
—*Albert Einstein*
[source]

my favorite quotes on 'Research'.

Research, in my opinion, is about finding simple, effective, and not-necessarily-optimal solutions to a problem. It involves imagination, the attention to details, and disregarding the inessential; technical skills/concepts are mere tools, one may have to learn, relearn, or even unlearn as means to an end. This page highlights my research activities prior to joining Qualcomm.

Reliable data transmission is arguably the most important consideration
in a communication system. Wireless communication systems employ advanced signal processing techniques
that combat the transmission impairments caused by the communication channel
and ensure reliability while delivering the ever increasing data rates the end users demand.

OFDM technology,
which is used by the latest cellular network standards (e.g., 3GPP LTE) and wireless data network standards (e.g., WiFi, WiMAX),
is effective against frequency-selective fading.
However, it is quite fragile
under time-selective fading. Applying frequency-domain equalization techniques,
which are typically computationally intensive, on top of OFDM can mitigate only mild time-selective fading.
However, this approach is not effective where time-selective fading is severe.

Doubly-selective fading,
i.e., the simultaneous occurrence of frequency-selective fading and time-selective fading,
becomes severe with increasing data rates, carrier frequencies, and mobility.
Therefore, combating doubly-selective fading would become crucial in next generation cellular networks,
mobile wireless data networks, and vehicular ad-hoc networks, where all these factors are significant.

Hence, it is necessary to develop new technologies to mitigate 'doubly-selective fading' in multi-carrier transmissions.
The focus of my ongoing research is developing such signal processing techniques anew,
without trying to build them upon OFDM.
'Frequency time block modulation' (FTBM), a patent-pending transmission technology, is an outcome of that effort.
Related R&D on channel modeling, pulse shaping, channel estimation, peak-power reduction, device prototyping, and field testing are underway.

Similarities in the wireless channel in space, time, and frequency domains
indicate that the space-dimension can be exploited for benefits other than the usual diversity and multiplexing—e.g,
to achieve multicasting or for multipath resolution.
With the advancements in antenna and semiconductor technologies,
wireless radio terminals are being equipped with increasingly many antennas,
and spatial DoFs are becoming affordable.
The inclination towards using higher frequency bands (e.g., millimeter waves) also has a catalytic effect on that trend.
Thus, those 'other' benefits of space-dimension are gradually becoming feasible.

The goal of my PhD research project—titled
'*New Signal Processing Techniques for MIMO Physical Layer*'
and conducted under the supervision of Dr. Chintha Tellambura—was
developing signal processing techniques to achieve aforementioned other benefits,
quantifying their performance.

My MSc is a course-based one with a research component,
and the research project —titled
'*Intuitive Reasoning for Epistemic Uncertainty*'
and conducted under the supervision of Dr. Chulantha Kulasekere—
was on intuitive representation and manipulation of *epistemic uncertainty*,
which is associated with subjectiveness and partial availability of information.
Although not directly related to my current research area, the MSc research marks my first exposure to 'research'.

- Damith N. Senaratne, "New Signal Processing Techniques for MIMO Physical Layer," Ph.D. dissertation, University of Alberta, Edmonton AB, Canada, Sep. 2012
[thesis: pdf | from era @ ualberta]
[presentation slides: view | download].
MIMO systems, characterized by multiple antenna transceivers, add a 'space' dimension to signal processing for wireless communication. Conventionally, the DoFs, i.e., the number of independent data streams that can be transmitted or received, available in the space dimension are utilized to improve the quality-of-service and the data rates. In other words, the spatial DoFs are exploited to gain diversity and multiplexing benefits. However, these DoFs may be used for other purposes (including multicasting, duplexing, and multipath resolution), which are conceivable given the emerging trend of accommodating more and more antennas in wireless terminals. Developing new physical layer signal processing techniques to realize such non-conventional benefits and ascertaining their viability through performance analysis are the main goals of this thesis. GSVD beamforming, which generalizes eigenmode transmission and zero forcing beamforming techniques for two-user MIMO downlink channels, and spatial multipath resolution, a unique application of spatial signal processing to mitigate multipath fading, are proposed here for the first time. Moreover, beamforming techniques for physical-layer multicasting and space division duplexing are developed in detail; the exact performance of channel inversion power allocation over eigenmode transmission is characterized. This thesis develops each of those contributions in detail.

- Damith Senaratne and Chintha Tellambura, "Frequency time block modulation for mitigating doubly-selective fading," US provisional patent application filed (US 20140105315 A1) [google patents].
The present invention provides systems, methods, and devices for addressing the effects of doubly-selective fading channels. Data symbols to be transmitted are organized into blocks and a Fourier transform-based pre-processing method is then applied to the data blocks. Cyclic padding is then applied to the pre-processed blocks and each symbol in the padded block is then mapped to frequency subcarriers in specific time slots. The resulting signal is then transmitted through a wireless connection. At the receiver, the received signal is processed to thereby reverse the processing performed at the transmission end. As part of the reversal, a post-processing method is applied that reverses the effects of the pre-processing method.

- D. Senaratne and C. Tellambura, "GSVD beamforming for two-user MIMO downlink channel," IEEE Transactions on Vehicular Technology (in print; 11 pages; accepted: Jan. 2013)
[pdf |
doi].
This paper introduces the fundamentals of GSVD beamforming, a non-iterative beamforming technique based on the GSVD for the two-user multiple-input multiple-output downlink. The numbers of private/common channels produced by GSVD beamforming and transmit power normalization are investigated; the channel gains under Rayleigh fading are characterized exactly for configurations involving only common channels. Moreover, symbol error rates are presented for elementary multicasting and two-way relaying applications; the performance distinctions of the private/common channels as well as the effect of channelestimation errors and asymmetries in channel fading are highlighted thereby.
- D. Senaratne and C. Tellambura, "Spatial multipath resolution with space time block codes," IEEE Wireless
Communication Letters, vol. 1, no. 3, pp. 249–252, June 2012
[pdf |
doi].
The use of STBCs over frequency selective MIMO channels is investigated in combination with SMR [1], a novel spatial signal processing technique proposed by the authors for mitigating inter-symbol interference. Neither STBC nor SMR requires transmit channel state information. This fact, along with SMR not requiring any modification to the transmitter, makes the MIMO STBC-SMR hybrid effective. Numerical results are presented, illustrating how the scheme fares under the Alamouti STBC scheme, and highlighting the advantage of adaptive SMR - i.e., adapting the number of multipaths resolved based on the channel state. A practical multipath MIMO channel based on the IEEE 802.15.3c NLOS (CM4) model is used for simulation.
- D. Senaratne, C. Tellambura, and H. Suraweera, "Performance analysis of MIMO channel inversion in
Rayleigh fading," IEEE Transactions on Vehicular Technology, vol. 61, no. 3, pp. 1188–1196, Mar. 2012
[pdf |
doi].
The performance of eigenmode transmission over a MIMO Rayleigh channel under the CI power allocation scheme is investigated. The moment-generating function of the reciprocal of the received SNR is derived. For the special case when the minimum number of antennas at the transmitter and the receiver is two, the exact closed-form expressions for the pdf and cdf are also derived. The average SER is derived as an application of the results. The extension to Rician and semicorrelated Rayleigh fading scenarios is also outlined.
- D. Senaratne and C. Tellambura, "Spatial multipath resolution for MIMO systems," IEEE Wireless Communication
Letters, vol. 1, no. 1, pp. 10–13, Feb. 2012
[pdf |
doi].
Wireless MIMO terminals with a large number of antennas are becoming a reality, increasing the spatial spatial DoFs available at the terminals. This paper proposes exploiting the excess spatial DoFs available at the receiver for spatial multipath resolution. A rake-receiver structure, whose fingers are implemented through spatial signal processing, is introduced for that. The joint computation of beamforming matrices for the transmitter and each receiver finger to achieve single-carrier eigenmode transmission over a multipath MIMO channel is developed. Numerical results on the error performance are provided for a practical multipath MIMO channel based on the IEEE 802.15.3c NLOS (CM4) model.
- C. Tellambura, M. Soysa, and D. Senaratne, "Performance analysis of wireless systems given the MGF of the reciprocal of the signal-to-noise ratio," IEEE Communication Letters, vol. 15, no. 1, pp. 55–57, Jan. 2011
[pdf |
doi].
A class of wireless problems is characterized by the availability of the MGF of the reciprocal of the signal-to-noise ratio. We show how to compute the average error rates and outage probability in this case. The result allows a simple, accurate numerical calculation of the average error rate by using the Gauss-Legendre numerical quadratures. We also derive the exact bit error rate of multihop relays for the special case where the fading index of each hop is an odd multiple of one-half.
- C. Tellambura and D. Senaratne, "Accurate computation of the mgf of the lognormal distribution and its application to sum of lognormals," IEEE Transactions on Communication, vol. 58, no. 5, pp. 1568–1577, May 2010
[pdf |
doi].
Sums of lognormal RVs are of wide interest in wireless communications and other areas of science and engineering. Since the distribution of lognormal sums is not log-normal and does not have a closed-form analytical expression, many approximations and bounds have been developed. This paper develops two computational methods for the MGF or the CHF of a single lognormal RV. The first method uses classical complex integration techniques based on steepest-descent integration. The saddle point of the integrand is explicitly expressed by the Lambert function. The steepest-descent (optimal) contour and two closely-related closed-form contours are derived. A simple integration rule (e.g., the midpoint rule) along any of these contours computes the MGF/CHF with high accuracy. The second approach uses a variation on the trapezoidal rule due to Ooura and Mori. Importantly, the cumulative distribution function of lognormal sums is derived as an alternating series and convergence acceleration via the Epsilon algorithm is used to reduce, in some cases, the computational load by a factor of 10^6! Overall, accuracy levels of 13 to 15 significant digits are readily achievable.
- D. Senaratne and C. Tellambura, "Unified exact performance analysis of two-hop amplify-and-forward relaying in Nakagami fading," IEEE Transactions on Vehicular Technology, vol. 59, no. 3, pp. 1529–1534, Mar. 2010
[pdf |
doi].
We present a general two-parameter received SNR model for two-hop AF relaying. It encompasses AF schemes that select the relay gain as the reciprocal of a linear combination of the channel gain and the noise power of the incoming link, including all channel-noise-assisted, channel-assisted, and blind relay schemes. Moreover, the model is flexible enough to represent independent source and relay power allocations. A unified performance analysis is then developed for AF relaying over independent but nonidentically distributed Nakagami-m faded links, where m is an integer. Exact analytical expressions are derived for the cdf, pdf, and mgf of the received SNR. Monte Carlo simulation results are provided to verify the results.

- D. Senaratne and C. Tellambura, "Beamforming for space division duplexing," in Proc. IEEE International Conference on Communications (ICC), Kyoto, Japan, June 2011
[pdf |
doi]
[presentation slides].
This paper examines SDD in MIMO systems. The antennas each full-duplex node has are partitioned to form two antenna banks - one for transmission, the other for reception. Self-interference is avoided at each full-duplex node by utilizing the nullspace (or the left nullspace) of corresponding self-interference channel for transmission (or reception). Simulation results are provided on the error performance. Useful insights are obtained on the effects finite precision arithmetic, and quantization errors have on the feasibility of SDD.
- D. Senaratne and C. Tellambura, "Beamforming for physical layer multicasting," in Proc. IEEE Wireless
Communications and Networking Conference (WCNC), Cancun, Mexico, Mar. 2011
[pdf |
doi]
[presentation slides]
[MATLAB code (1.11kB)].
A systematic scheme is proposed to facilitate arbitrary VC-to-user mappings in space dimension, through multiple-input multiple-output physical layer multicasting. It is a divide-and-conquer strategy, which breaks down the VC-to-user mapping to manageable orthogonal sub-mappings, each represented in terms of a MAG. A generalized form of block diagonalization is proposed to make transmissions pertaining to distinct MAGs orthogonal. Known non-iterative coordinated beamforming techniques are investigated for intra-MAG beamforming. The approach enables physical layer multicasting with non-iterative beamforming techniques alone.
- D. Senaratne, H. A. Suraweera, and C. Tellambura, "Channel inversion in MIMO systems over Rician fading," in Proc. IEEE Global Communication Conference (GLOBECOM), Miami, FL, Dec. 2010
[pdf |
doi]
[presentation slides].
We examine the distribution of the per virtual-channel received signal-to-noise ratio over a multiple-input multiple-output channel modeled by a rank-2 non-central Wishart matrix. The exact probability density function is derived for all possible non-centrality matrices; and verified through simulation for selected systems. Identities for exact analytic results on the outage probability, the average symbol error rate, and the moment generating function are also derived.
- D. Senaratne and C. Tellambura, "Generalized singular value decomposition for coordinated beamforming in MIMO systems," in Proc. IEEE Global Communication Conference (GLOBECOM), Miami, FL, Dec. 2010
[pdf |
doi]
[presentation slides].
In this paper we examine the use of GSVD for coordinated beamforming in MIMO systems. GSVD facilitates joint decomposition of a class of matrices arising inherently in source-to-2 destination MIMO broadcast scenarios. GSVD allows two channels of suitable dimensionality to be jointly diagonalized, i.e. to be reduced to non-interfering virtual broadcast channels, through the use of jointly determined transmit precoding and receiver reconstruction matrices. Potential applications for GSVD-based beamforming can be found in MIMO broadcasting, as well as in MIMO relaying under all amplify-and-forward, decode-and-forward, and code-and-forward relay processing schemes. Several of them are highlighted here. We also present simulation-based performance analysis results to justify the use of GSVD for coordinated beamforming.
- D. Senaratne and C. Tellambura, "Performance analysis of channel inversion over MIMO channels," in Proc. IEEE Global Telecommunications Conference (GLOBECOM), Honolulu, HI, Dec. 2009
[pdf |
doi]
[presentation slides].
The distribution of the received signal power Λ of a multiple-input multiple-output system, with the associated 2 × 2 complex Wishart matrix having n degrees of freedom, is investigated under the channel inversion power allocation scheme. Exact closed-form expressions are derived for the probability density function and the cumulative distribution function of Λ. The analysis covers systems with transmit-receive antennas {Nt,Nr| min(Nt, Nr) = 2, max(Nt, Nr) ≥ 2}, and independent and identically distributed Rayleigh fading links between each antenna pair. We derive (i) the distribution of Shannon capacity of the multiple-input multiple-output channel (ii) average symbol error rate for a class of modulation schemes that includes Binary Phase Shift Keying.
- D. Senaratne and C. Tellambura, "Numerical computation of the lognormal sum distribution," in Proc. IEEE Global Telecommunications Conference (GLOBECOM), Honolulu, HI, Dec. 2009
[pdf |
doi]
[presentation slides].
This paper develops highly accurate numerical techniques for evaluating the mgf/ chf of a single lognormal variable and for computing the lognormal sum cdf. Complex integration techniques based on the steepest-descent integration are thus developed for evaluating the lognormal mgf/ chf. The saddlepoint of the integrand is explicitly expressed using the Lambert-W function. The optimal steepest-descent contour passing through the saddlepoint is then derived. Even a simple mid-point-rule-based integration technique can be used along this contour to evaluate the mgf/ chf at extremely high precision. A highly efficient, extremely accurate numerical method is then developed for evaluating the cdf of sum of independent lognormal variables. The cdf is expanded as an alternating series, on which the Epsilon algorithm for convergence acceleration is applied. This reduces the computational load significantly.
- D. Senaratne and C. Tellambura, "Unified performance analysis of two hop amplify and forward relaying," in Proc. IEEE International Conference on Communications (ICC), Dresden, Germany, June 2009
[pdf]
[presentation slides].
Wireless relay networks have been studied extensively in the recent literature. AF is one of the most widely used type of relaying. Even though special cases such as channel-noise-assisted, channel-assisted and blind relay modes have been analyzed, a unified performance analysis seems to be not available. In this paper, we present unified performance analysis results for two-hop AF relaying over Nakagami-m fading nonidentical source-to-relay (S→R) and relay-to-destination (R→D) links. A general model for the received signal-to-noise ratio, which covers channel-noise-assisted, channel-assisted and blind relay configurations as special cases, is developed. Closed-form expressions are then derived for the cdf, pdf, and mgf. Exact results are derived for symbol error rate of special cases. All results are verified through Monte Carlo simulation.
- D. Senaratne and C. Tellambura, "A general numerical method for computing the probability of outage," in Proc. IEEE Wireless Communications and Networking Conference (WCNC), Budapest, Hungary, Apr. 2009
[pdf |
doi]
[presentation slides].
The outage probability is a fundamental performance metric, which has been widely investigated in the literature. In this paper, we develop a general method to compute the outage given the MGF. When computing the outage using the MGF, integration along the standard Bromwich contour suffers from a loss of accuracy due to oscillatory nature of the integrand. One can address this difficulty by using the Cauchy's theorem to replace the Bromwich contour by an appropriate equivalent contour. For highly-accurate numerical results, the steepest descent contour is the most suitable replacement. Unfortunately, this optimal contour can not be in general expressed in closed-form. The class of Talbot contours characterized by three parameters provides an alternative. However, it is not clear how these parameters need to be tuned for best results. We propose the use of linear regression to set the parameter values so as to minimize the mismatch between corresponding Talbot contour and the steepest-descent contour. The resulting integral has a smooth and rapidly decaying integrand, making it possible for the outage probability to be evaluated with high accuracy by using a simple numerical integration method. This approach is general in a sense that it works for any system where the MGF is known. Thus, it can handle a wide range of fading distributions and a variety of communication systems.