Srikanth Pagadarai

Sidelobe Suppression for OFDM-Based Cognitive Radios Using Constellation Expansion

In this paper, we present a novel algorithm for reducing sidelobe interference power levels in OFDM-based cognitive radios. Existing techniques for sidelobe suppression can be computationally intensive when determining the complex-valued amplitude levels for the cancellation subcarriers. Exploiting the fact that different sequences have different sidelobe power levels, the proposed algorithm employs a constellation expansion-based iterative approach in order to suppress the sidelobe power levels. An important advantage of the proposed technique is that, no side information needs to be transmitted. Simulation results show that the proposed algorithm can be employed in a wide range of operating conditions at the cost of a slight increase in the bit error rate and the peak-to-average power ratio characteristics.

A framework for statistical wireless spectrum occupancy modeling

In this paper, we propose a novel spectrum occupancy model designed to generate accurate temporal and frequency behavior of various wireless transmissions. Our proposed work builds upon existing concepts in open literature in order to develop a more accurate time-varying spectrum occupancy model. This model can be employed by wireless researchers for evaluating new wireless communication and networking algorithms and techniques designed to perform dynamic spectrum access (DSA). Using statistical characteristics extracted from actual radio frequency measurements, first- and second-order parameters are employed in a statistical spectrum occupancy model based on a combination of several different probability density functions (PDFs) defining various features of a specific spectrum band with several concurrent transmissions. To assess the accuracy of the model, the output characteristics of the proposed spectrum occupancy model are compared with realtime radio frequency measurements in the television and paging bands.

Characterization of vacant UHF TV channels for vehicular dynamic spectrum access

In this paper, we present quantitative and qualitative results obtained as a result of a TV spectrum measurement campaign. We used these measurements to characterize vacant TV channels a along major interstate highway (I-90) in the state of Massachusetts, USA. By characterizing the availability of vacant TV channels in the 470–806 MHz frequency range, we show the trends in the availability of vacant channels from a vehicular dynamic spectrum access perspective. We also describe the design constraints imposed on a point-to-multipoint communications based architecture in such a setting. Specifically, we described a general geo-location database approach to create a spectral map of available channels in a given geographical area. We presented the results obtained by applying such a technique in the state of MA over several locations on I-90. Furthermore, we presented a discussion on the implications of the non-contiguous channel availability in the TV spectrum on the design of a cognitive radio transceiver from the perspective of vehicular communications. In our future work, we will be presenting a comparative study of the channel availability in the above mentioned geographical area before and after the switch-over carried out on June 12, 2009.

A quantitative assessment of wireless spectrum measurements for dynamic spectrum access

In this paper, we provide a qualitative assessment of the spectrum usage across five geographically separated sites in four mid-size metropolitan centers located in the United States. Specifically, the variations in spectrum occupancy across space, time and frequency are investigated and compared between different sites within the city as well as with other cities. We show that the variation at different locations within the same city is considerably high. Furthermore, we present a simple four-state Markov model that characterizes time-varying behavior of the spectral occupancy of a frequency channel.

Demonstration of Vehicle to Vehicle Communications over TV White Space

Future vehicular communications systems are expected to utilize the vacant channels (white spaces) of the spectrum, otherwise allocated for specific designated use. One such candidate of white space comes from the TV broadcast band. In this demonstration, we will first present animated results of a TV spectrum measurement campaign along the entire portion of Interstate I-90 located in the US state of Massachusetts. Next, we will demonstrate a cyber-physical proof-of-concept lab implementation of our previously developed control and data channel assignment schemes for vehicle-to-vehicle communications over (TV) white space. Finally we will show a video of actual vehicle to vehicle communications field tests conducted in Japan using TV white space.

A sub-optimal sidelobe suppression technique for OFDM-based cognitive radios

Orthogonal frequency division multiplexing (OFDM) is a transceiver technology capable of achieving spectrally efficient, high data rate wireless transmissions. Being also capable of transmitting in a non-contiguous fashion by utilizing several seperate spectral whitespaces, it is an ideal candidate for dynamic spectrum access (DSA)-based transceiver systems. However, the large sidelobes emerging from OFDM signal transmissions need to be reduced in order to conform to the FCC spectrum masks, thus enabling the coexistence between licensed and unlicensed transceivers. In this paper, we propose a cancellation carrier-based sidelobe suppression technique for OFDM-based cognitive radios. A selected number of subcarrier locations on either side of the OFDM spectrum are reserved for inserting cancellation carriers whose amplitude and phase are determined by using a straight-forward algebraic technique. The algorithm performing this algebraic technique is based on the fact that the subcarriers near the edges of the OFDM signal spectrum possess a greater impact on the sidelobe power levels than those that are interior in the transmission spectrum. Simulation results show that with only one cancellation carrier on each side of a 64 subcarrier QPSK-OFDM spectrum, there is a reduction of as much as 10 dB on the peak-interference causing sidelobe by considering the impact of 25% of the total number of number of subcarriers on the interference power levels. With two cancellation carriers on each side, the reduction is around 13 dB.

Cancellation carrier technique using genetic algorithm for OFDM sidelobe suppression

In this paper, we investigate the sidelobe power reduction of non-contiguous orthogonal frequency division multiplexing (OFDM) signals using a genetic algorithm (GA) approach. OFDM is widely deployed in numerous wireless access networks since it can achieve high data rate while simultaneously being robust to the effects of multipath propagation. Despite these advantages, OFDM produces a substantial amount of out-of-band (OOB) interference that can potentially disrupt communications in adjacent wireless channels. A GA is a search technique that is employed to iteratively find the optimal solution given large solution spaces. To reduce the OOB emissions of an OFDM transmission, GAs can be employed to determine the amplitude and phase values of cancellation carriers (CCs), which are signals designed to cancel out the OOB emissions when added to this transmitted signal. Note that the CCs themselves do not carry any data, thus a delicate trade-off between overall data throughput and interference mitigation must be considered. However, with this CCs technique, there is a small loss in bit error rate which is caused by the fact that a certain amount of transmission power is wasted. Performance comparisons of the proposed GA technique with two other techniques presented in the open literature have been conducted.

Queueing Theory Representation and Modeling of Spectrum Occupancy Employing Radio Frequency Measurements

In this paper, we provide a new perspective for an- alyzing spectrum occupancy by introduced an M/M/1 queueing model to generate accurate temporal and frequency behavior of various wireless transmissions. Our proposed research builds upon existing concepts in the open literature in order to develop a more accurate time-varying spectrum occupancy model. This model can be employed by wireless researchers for evaluating new wireless communication and networking algorithms and techniques designed to perform dynamic spectrum access (DSA). Using statistical characteristics extracted from actual radio fre- quency measurements, first- and second-order parameters are employed in a statistical spectrum occupancy model based on a combination of several different probability density functions (PDFs) defining various features of a specific spectrum band with several concurrent transmissions. To assess the accuracy of the model, the output characteristics of the proposed spectrum occupancy model are compared with real-time radio frequency measurements in the paging and ISM bands.

A Novel Sidelobe Suppression Technique for OFDM-Based Cognitive Radio Transmission

In this paper, we propose a novel two-step sidelobe suppression technique for OFDM-based cognitive radios designed to minimize the amount of out-of-band emissions that could potentially interfere with incumbent licensed transmissions in adjacent frequency bands. The proposed technique combines both constellation expansion (CE) and cancellation subcarrier (CC) sidelobe reduction approaches in order to achieve lower out- of-band interference levels relative to their individual application. Being a two-step process, the first step of the proposed technique consists of performing CE, which exploits the combination of different random symbol sequences across the subcarriers in order to yield lower sidelobe levels. The second step is to insert several CCs on either side of contiguous blocks of data-bearing subcarriers such that they combine destructively with the data- bearing subcarrier sidelobe levels, yielding a reduction in out- of-band emissions. Simulation results show that with the CE of four symbols mapped to an 8-PSK signal constellation and with one CC employed on each side of a 32 subcarrier OFDM spectrum, there is a reduction of as much as 16 dB on the peak- interference causing sidelobe. From a probabilistic point of view, this translates to reducing 99.9% of the sidelobe power levels from around -5 dB to around -22 dB.

Feasibility of NC-OFDM transmission in dynamic spectrum access networks

In this paper, we provide a feasibility analysis of non-contiguous orthogonal frequency division multiplexing (NC-OFDM) transceivers based on different operating configurations. The selection of these configurations conform to the constraints defined by the spectral masks of the Federal Communications Commissions (FCC), such as out-of-band radiation. Actual spectrum measurement data collected from several bands including the paging and digital TV bands are considered and several parameters, such as the number of subcarriers, or the power spectrum density (PSD) of the NC-OFDM waveform are modified in order to study the viability of this transmission technique in actual licensed spectrum. Due to the bursty nature of the paging band, an analysis of the interference caused by the changing number of subcarriers is also provided.