Here is the list of my publications and presentations.
Increasing data traffic demands over wireless spectrum has necessitated spectrum sharing and coexistence between heterogeneous systems such as radar and cellular communications systems. In this context, we specifically investigate the coexistence between an air traffic control (ATC) radar and a wide area cellular communication (comms) system. We present a comprehensive characterization and analysis of interference caused by the comms system on the ATC radar with respect to multiple parameters such as radar range, protection radius around the radar, and radar antenna elevation angle. The analysis suggests that maintaining a protection radius of 50 km around the radar will ensure the required INR protection criterion of -10 dB at the radar receiver with 0.9 probability, even when radar beam is in the same horizon as the comms BS. Detailed evaluations of the radar target detection performance provide a framework to choose appropriate protection radii around radar to meet specific performance requirements.
In this paper we investigate the impact that incomplete knowledge regarding user activity can have on the equilibrium transmis- sion strategy for an OFDM-based communication system. The problem is formulated as a two user non-zero sum game for independent fad- ing channel gains, where the equilibrium strategies are derived in closed form. This allows one to show that a decrease in uncertainty about the user activity could reduce the number of subcarriers jointly used by the users. For the boundary case (with complete information, which reflects a classical water-filling game) the equilibrium strategies are given ex- plicitly. The necessary and sufficient conditions, when channels sharing strategies are optimal, is established as well as the set of shared subcar- riers is identified. The stability of the upper bound of the size of this set with respect to power budgets is derived.
Citizen Broadband Radio Service band (3550 - 3700 GHz) is seen as one of the key frequency bands to enable improvements in performance of wireless broadband and cellular systems. A careful study of interference caused by a secondary cellular communication system coexisting with an incumbent naval radar is required to establish a pragmatic protection distance, which not only protects the incumbent from harmful interference but also increases the spectrum access opportunity for the secondary system. In this context, this paper investigates the co-channel and adjacent channel coexistence of a ship-borne naval radar and a wide-area cellular communication system and presents the analysis of interference caused by downlink transmission in the cellular system on the naval radar for different values of radar protection distance. The results of such analysis suggest that maintaining a protection distance of 30 km from the radar will ensure the required INR protection criterion of -6 dB at the radar receiver with > 0.9 probability, even when the secondary network operates in the same channel as the radar. Novel power control algorithms to assign operating powers to the coexisting cellular devices are also proposed to further reduce the protection distance from radar while still meeting the radar INR protection requirement.
Multiple point-to-point (p2p) communication links that share noncontiguously available spectrum via noncontiguous orthogonal frequency division multiple access (NC-OFDMA) are considered in this paper. NC-OFDMA divides the spectrum into orthogonal subcarriers and assigns nonoverlapping subsets of these subcarriers to each of the links. Such a setup is motivated by the need for dynamic and opportunistic access of a large pool of spectrum that is fragmented due to the presence of incumbent transmissions. The transmissions of the p2p links are assumed to be asynchronous, i.e., the start and end of the NC-OFDM symbol is not time synchronized across the links. In such a setting, this paper studies the inter-carrier-interference (ICI) arising from the loss of orthogonality between the subcarriers due to (a) the asynchronous nature of the communications and (b) the frequency offset between the links. An analytical characterization of this ICI reveals that in the absence of transmit power control, the impact of this ICI can be severe enough to disable the concurrent transmissions of the other p2p links. Experimental results using a Universal Software Radio Peripheral (USRP) platform validate our analysis and show that the negative effects of asynchronous transmission which are particularly significant at high SNR. To remedy this, a decentralized synchronization scheme is presented wherein beacons appended to the transmitted signal are used to enable timing coordination between two NC-OFDM links, thereby eliminating the ICI resulting from asynchronous transmission. Effectiveness of such a scheme is validated using experiments. In cases where such timing coordination cannot be enabled, our analysis is then used to provide guidelines on the choice of basic system parameters for the p2p links such as the number of contiguous subcarriers to be allocated to a link and length of guard band and range of operating SNRs to mitigate the effect of asynchronous NC-OFDM transmission.
This paper examines the low-probability-of- exploitation (LPE) characteristics of a noncontiguous orthogonal frequency division multiplexing (NC-OFDM) system. NC-OFDM transmission is similar to OFDM transmission but only uses a subset of the frequencies either to avoid incumbent transmissions or due to tactical considerations. This paper considers an NC- OFDM transmission with a given set of active subcarriers and examines how an eavesdropper can infer transmission parameters such as total duration of an NC-OFDM symbol, length of the cyclic prefix, etc., using tools like the cyclostationary analysis. Such an analysis reveals that difficulty in estimating the total nominal bandwidth of NC-OFDM transmissions (bandwidth that includes frequencies occupied by inactive subcarriers) poses a fundamental challenge in determining the correct sampling rate and the subsequent retrieval of the transmitted signal. The analysis also shows that the features of the cyclic autocorrelation function (CAF) of an NC-OFDM transmission depend closely on the set of active subcarriers. Procedures for inferring the transmission parameters from the CAF are discussed while noting that the choice of an interleaved set of subcarriers introduces additional ambiguity in determining the transmission parameters. A PCA-based offline timing recovery scheme is proposed and used as a guidepost in determining the minimum rate at which an active set of subcarriers must be refreshed to avoid easy exploitation. Finally, key advantages of an NC-OFDM system over an OFDM system from an LPE-standpoint are discussed and suggestions for an LPE-centric design of NC-OFDM systems are made.
This paper designs an underlay control channel for noncontiguous-OFDM-based cognitive networks. Noncontiguous OFDM (NC-OFDM) provides a fast and flexible manner of accessing disjoint parts of the spectrum and is ideally suited for dynamic spectrum access. While similar to OFDM, NC-OFDM explicitly restricts transmission to only certain subcarriers that are free of incumbent transmissions. In particular, this paper considers designing a control channel for a cognitive network consisting of multiple point-to-point (p2p) links that operate over a wide bandwidth that might encompass some primary transmissions. In such a scenario, control channel becomes vital not only to share basic transmission parameters but also to aid timing and frequency recovery of NC-OFDM transmission; a nontrivial problem in itself. The proposed design is a low-power underlay transmission that spans the entire bandwidth regardless of any incumbent transmissions and uses direct sequence spread spectrum (DSSS). The control channel operates in one of two modes. The first mode aids timing and frequency recovery through a two-step process, while the second mode is used for control data transmission. To enable multiple access, the p2p links use orthogonal pseudo-noise (PN) sequences. The proposed control channel is implemented on USRPs in the ORBIT testbed using GNU Radio. Experimental results suggest robust timing and frequency offset recovery even in the presence of concurrent primary transmissions and support for about 10 to 20kbps over a 1 MHz bandwidth at an uncoded symbol-error-rate of about 10−2 under typical operating conditions.
We envision a scenario of opportunistic spectrum access among multiple links when the available spectrum is not contiguous due to the presence of external interference sources. Non-contiguous Orthogonal Frequency Division Multiplexing (NC-OFDM) is a promising technique to utilize such disjoint frequency bands in an efficient manner. In this paper we study the problem of fair spectrum allocation across multiple NCOFDM-enabled point-to-point cognitive radio links under certain practical considerations that arise from such non-contiguous access. When using NC-OFDMA, the channels allocated to a cognitive link are spread across several disjoint frequency bands leading to a large spectral span for that link. Increased spectral span requires higher sampling rates, leading to increased power consumption in the ADC/DAC of the transmit/receive nodes. In this context, this paper proposes a spectrum allocation that maximizes the minimum rate achieved by the cognitive radio links, under a constraint on the maximum permissible spectral span. Under constant transmit powers and orthogonal spectrum allocation, such an optimization is a mixed-integer linear program and can be solved efficiently. There exists a clear trade-off between the max-min rate achieved and the maximum permissible spectral span. The spectral allocation obtained from the proposed optimization framework is shown to be close to the tradeoff boundary, thus showing the effectiveness of the proposed technique. We find that it is possible to limit the spectrum span without incurring a significant penalty on the max-min rate under different interference environments. We also discuss an experimental evaluation of the techniques developed here using the Universal Software Radio Peripheral (USRP) enabled ORBIT radio network testbed.
The penetration of wireless broadband services in remote areas has primarily been limited due to the lack of economic incentives that service providers encounter in sparsely populated areas. Besides, wireless backhaul links like satellite and microwave are either expensive or require strict line of sight communication making them unattractive. TV white space channels with their desirable radio propagation characteristics can provide an excellent alternative for engineering backhaul networks in areas that lack abundant infrastructure. Specifically, TV white space channels can provide "free wireless backhaul pipes" to transport aggregated traffic from broadband sources to fiber access points. In this paper, we investigate the feasibility of multi-hop wireless backhaul in the available white space channels by using noncontiguous Orthogonal Frequency Division Multiple Access (NC-OFDMA) transmissions between fixed backhaul towers. Specifically, we consider joint power control, scheduling and routing strategies to maximize the minimum rate across broadband towers in the network. Depending on the population density and traffic demands of the location under consideration, we discuss the suitable choice of cell size for the backhaul network. Using the example of available TV white space channels in Wichita, Kansas (a small city located in central USA), we provide illustrative numerical examples for designing such wireless backhaul network.