Alpaslan Demir

Matched filtering assisted energy detection for sensing weak primary user signals

Energy detection is widely used by cognitive radios for spectrum sensing. During a silent period, secondary users (SUs) are kept silent so that the energy detector does not confuse SU signals for primary user (PU) signals. Due to imperfect coordination, an SU may transmit during a silent period and cause possible false alarms. We propose to leverage matched filters that already exist in many SUs to alleviate the impact of such SU interference by combining the matched filtering result and the energy detection result. The analysis shows that for practical purposes, our algorithm virtually eliminates all of the negative impact of SU interference with only negligible penalty in delay and energy consumption.

Cross-Layer Optimization for Opportunistic Multi-MAC Aggregation

In this paper, we describe a RAT (radio access technology) agnostic approach referred to as OMMA (Opportunistic Multi-MAC Aggregation) that can aggregate multiple RATs operating over independent spectral bands. Aggregation and/or traffic shaping is performed above the air interface protocol stacks but below the IP layer. We investigate the problem of minimizing the average packet latency (the sum of queuing delay and serving delay) under certain constraints and propose an algorithm to compute optimal packet distribution ratio across RATs to be implemented by OMMA. Moreover, we statistically characterize the reordering delay for the OMMA system and propose an algorithm to compute a modified optimal packet distribution ratio which minimizes the packet end-to-end delay, i.e., the sum of average packet latency and average packet reordering delay. Finally, some numerical results are presented to study the average packet latency, average packet reordering delay and average packet end-to-end delay for different system parameters.

On the Packet Allocation of Multi-Band Aggregation Wireless Networks

Abstract The use of heterogeneous networks with multiple radio access technologies (RATs) is a system concept that both academia and industry are studying. In such system, integrated use of available multiple RATs is essential to achieve beyond additive throughput and connectivity gains using multi-dimensional diversity. This paper considers an aggregation module called opportunistic multi-MAC aggregation (OMMA). It resides between the IP layer and the air interface protocol stacks, common to all RATs in the device. We present a theoretical framework for such system while considering a special case of multi-RAT systems, i.e., a multi-band wireless LAN (WLAN) system. An optimal packet distribution approach is derived which minimizes the average packet latency (the sum of queueing delay and serving delay) over multiple bands. It supports multiple user terminals with different QoS classes simultaneously. We further propose a packet scheduling algorithm, OMMA Leaky Bucket, which minimizes the packet end-to-end delay, i.e., the sum of average packet latency and average packet reordering delay. We also describe the system architecture of the proposed OMMA system, which is applicable for the general case of the multi-RAT devices. It includes functional description, discovery and association processes, and dynamic RAT update management. We finally present simulation results for a multi-band WLAN system. It shows the performance gains of the proposed OMMA Leaky Bucket scheme in comparison to other existing packet scheduling mechanisms.

Exploring the possibility of Full-Duplex operations in mmWave 5G systems

Recent developments in the communications industry to accommodate user demand for ubiquitous, instantaneous over the air access to multimedia rich content has made spectrum utilization efficiency one of the key research topics. Full-Duplex radios have great potential to improve spectrum utilization efficiency and are well-studied for below 6 GHz applications. In this paper, we explore the possibility of mmWave Full-Duplex operation in 5G. We group the Full-Duplex radio system components into four modules: antenna systems, analog frontend and digital baseband self-interference cancellers, and protocol stack enhancements. We create a system model to analyze link-budget and determine the usable range for Full-Duplex operation by focusing on antenna systems, analog RF frontend design assumptions, and the channel models. We also highlight tradeoffs for employing cancellation techniques and criteria for protocol stack enhancements.

Link adaptation on aggregated TVWS channels

The availability of Television White Space (TVWS) [1] has attracted considerable attention in industry and academia. Given the relatively small amount of bandwidth allocated to each TVWS channel (e.g., 6, 7 or 8 MHz), and the non-contiguous nature of the availability, channel aggregation ([2], [3]) is an attractive way to make full use of the available TVWS spectrum and achieve higher throughput. In this paper we propose and analyze several link adaptation schemes in the context of MAC layer aggregation. We first analyze the SINR conditions of multiple aggregated channels using a model of a modified 802.11n system that supports TVWS operation. Then, we investigate three different link adaptation mechanisms and compute their MAC layer throughputs. Our results show that the SINR variance on the aggregated channels can be high, mainly due to the unique characteristics of TVWS spectrum, i.e., different maximum transmit powers imposed on different types of TVWS channels, and significant Digital TV (DTV) out-of-band emissions to adjacent TVWS channels. Both analytical and numerical results show that independent link adaptation based on each channel SINR provides much higher throughput than the single adaptation schemes, where a common rate is applied on the aggregated channels.

A robust algorithm for step 2 in 3G FDD W-CDMA synchronization

3G FDD W-CDMA initial synchronization follows a 3-step procedure to find a primary scrambling code while locating the frame boundary. In step 2, simplified coherent combining of secondary codes has been introduced for group and slot offset identification. The proposed method shows significant performance improvement against frequency offset. The simulation results show that there is approximately 3 dB performance improvement compared to conventional coherent integration for 6 kHz carrier offset while the other results follow almost the same values as those of conventional coherent integration.

Cyclic Prefix Adaptation with Constant Overall Symbol Time for DFT-Spread-OFDM and OFDM

For DFT-spread-OFDM or OFDM, if the delay spread varies in a wide range and the symbol duration is relatively short, adapting the cyclic prefix (CP) duration rather than using a fixed one may significantly improve the spectral efficiency while preventing inter-symbol interference (ISI). In practice, it may be beneficial to have a constant overall DFT-spread-OFDM/OFDM symbol time, which is the sum of the duration of a CP and the duration of a data portion. We propose to adapt the CP duration to the delay spread without changing the overall symbol time for DFT-spread-OFDM or OFDM, and address implementation challenges. In particular, we propose changing the clocking rate of ADC and DAC or using a Farrow filter to reduce the computational complexity of arbitrary-size DFT/IDFT resulting from the adaptation.

Preliminary performance evaluation of sensor network with intermediate fusion helper for cognitive radios

In this paper, we consider the problem of using three sensors to detect whether a certain spectrum is occupied or not. Each sensor sends its binary decision to the data fusion center through a wireless fading channel. The data fusion center combines the outcomes for an overall decision. Our analysis shows that a basic sensor network does not result in a high enough correct probability of the overall decision when the wireless fading channels experience low SNR. Then, we observe that this probability could be significantly increased with the deployment of relays in the network. However, a sensor network with relays still suffers from energy and spectral inefficiency. The sensor network with an intermediate fusion helper was recently proposed to reduce the traffic load at the data fusion center. We examine the correct probability of the overall decision resulting from a sensor network with an intermediate fusion helper. Our evaluation establishes that a sensor network with an intermediate fusion helper performs almost as good as the sensor network with relays, but with energy and spectral advantages.

Performance analysis of different types of sensor networks for cognitive radios

We consider the problem of using multiple sensors to detect whether a certain spectrum is occupied or not. Each sensor sends its spectrum sensing result to a data fusion center, which combines all the results for an overall decision. With the existence of wireless fading on the channels from sensors to data fusion center, we examine three different mechanisms on the transmissions from sensors to data fusion center: (1) direct transmissions; (2) transmissions with the assistance of relays and (3) transmissions with the assistance of an intermediate fusion helper which fuses the sensing results from the sensors and sends the fused result to the data fusion center. For each mechanism, we analyze the correct probability of the overall decision made by the data fusion center. Our evaluation establishes that a sensor network with an intermediate fusion helper performs almost as good as a sensor network with relays, but providing energy and spectral advantages. Both networks significantly outperform the sensor network without relay or intermediate fusion helper. Such analysis facilitates the design of sensor networks. Furthermore, we generalize this evaluation to sensor networks with an arbitrary number of sensors and to sensor networks applying various information combining rules. Our simulations validate the analytical results.