Time-frequency hopping sequences with three no hit zones

Based on the idea of no hit zone (NHZ) in frequency hopping (FH) systems, new sequences with three no hit zones (T-NHZ) in time-frequency hopping (TFH) systems are presented. With the T-NHZ in the time- frequency (TF) correlation functions, the proposed T-NHZ sequences can be directly employed in time-frequency hopping code division multiple access (TFH-CDMA) communication systems to reduce or eliminate multipath interference. Simulation results show that T-NHZ sequences can achieve much better bit error performance than the NHZ sequences and the traditional FH sequences.

Modified Guard Band Power Detection for Coarse Carrier Frequency Offset Recovery in OFDM System

Conventional guard band power detection (GPD) method for orthogonal frequency division multiplexing (OFDM) coarse carrier frequency offset (CFO) estimation has to be operated after the fine CFO recovery due to its sensitivity to the inter-channel interference (ICI). In the paper, modified GPD methods are presented for OFDM system to obtain more accurate estimation of CFO. The accuracy of GPD estimation can be first improved with a simple operation of power average. Then, extending the idea of Reference symbol Power Adjustment (RPA), two general patterns of sub-carrier power adjustment to improve the estimation performance under nonzero fine CFO are presented. According to complete simulations under the COST 207 multi-path fading channel, estimation performance with the presented modified GPD methods can be significantly improved and therefore can be concurrently operated with the fine carrier offset adjustment.

Adaptive Channel Allocation Spectrum Etiquette for Cognitive Radio Networks

In this work, we propose a game theoretic framework to analyze the behavior of cognitive radios for distributed adaptive channel allocation. We define two different objective functions for the spectrum sharing games, which capture the utility of selfish users and cooperative users, respectively. Based on the utility definition for cooperative users, we show that the channel allocation problem can be formulated as a potential game, and thus converges to a deterministic channel allocation Nash equilibrium point. Alternatively, a no-regret learning implementation is proposed for both scenarios and it is shown to have similar performance with the potential game when cooperation is enforced, but with a higher variability across users. The no-regret learning formulation is particularly useful to accommodate selfish users. Non-cooperative learning games have the advantage of a very low overhead for information exchange in the network. We show that cooperation based spectrum sharing etiquette improves the overall network performance at the expense of an increased overhead required for information exchange.

Bio-Inspired Algorithms for Dynamic Resource Allocation in Cognitive Wireless Networks

Regulation will experience enormous changes in the near future resulting in seamless connectivity by spectrum borders. A promising approach in this context is dynamic spectrum allocation which leads to a more flexible access to spectral resources by employing intelligent radio devices called cognitive radios. This paper is concerned with bio-inspired approaches that exploit distribution in multi-radio environments where many users have to share a finite resource harmoniously. Three applications of bio-inspired techniques are described. The first one deals with the detection of spectrum holes whereas the second one describes resource allocation in orthogonal frequency division multiple access based systems. The third one is concerned with distributed resource auctioning.

Adaptive Max SNR Packet Scheduling for OFDM Wireless Systems

In this paper, we consider scheduling and resource allocation for a downlink in a wireless OFDM system. If each broadcast sub channel is allocated to a user according to max SNR selection, optimal system throughput is obtained for the cost of a significant loss in fairness among users. As a solution to resolve this issue and in an attempt at reaching a compromise between fairness and throughput, we propose to add to the max SNR scheme a weak control based on user QoS requirements. In this work, user latency between two successive channel accesses is considered as a parameter for the control. The feedback of quantized channel state information (CSI) is proposed to reduce the feedback burden. Performance analysis of the proposed scheme has been presented to illustrate the capacity-fairness-feedback trade-off of the considered scheme compared to max SNR and proportional fair algorithms used as benchmark.

Dynamic Channel Selection for Cognitive Radios with Heterogenous Primary Bands

Technological advances coupled with regulatory initiative for more efficient utilization of radio spectrum resulted in the introduction of dynamic spectrum access enabled radio called Cognitive Radio (CR). A CR network is allowed to access a frequency band owned by primary user, which can provide higher throughput and better serviceability in wireless networks. The availability of multiple primary bands requires a CR network to select the best operating band which can maximize the total system performance. The selection should be made according to heterogenous properties of primary bands which offer different maximum data rate for secondary use and generate diverse traffic pattern. In this paper, assuming such heterogenous primary bands for secondary use, simple and distributed dynamic channel selection strategies are proposed and evaluated. In addition, we introduce a measurement metric for interference experienced by primary users due to secondary network’s access in primary band. We investigate the impact of different channel selection strategies and parameters on the primary and secondary performance.

Joint Carrier-Frequency Offset and Channel Estimation for MIMO-OFDM Transmission

In this paper, we consider a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system operating over frequency-selective fading channels. We propose a novel scheme for joint carrier-frequency offset (CFO) and channel estimation based on the expectation–maximization (EM) algorithm. Furthermore, the Cramer-Rao bounds (CRBs) for both CFO and channel estimators are exploited to evaluate the performance of the proposed scheme. Computer simulations show that the proposed algorithm achieves almost ideal performance compared with the CRBs for both channel and frequency offset estimations.

A Rigorous Proof of MIMO Channel Capacity’s Increase with Antenna Number

It is well known that adding more antennas at the transmitter or at the receiver offers larger channel capacity in the multiple-input multiple-output communication systems. In this letter, a simple proof is presented for the fact that the channel capacity increases with an increase in the number of receive antennas. The proof is based on the famous capacity formula of Foschini and Gans and uses matrix theory.

Uplink Channel Estimation for Multi-user OFDM-based Systems

In this paper we propose a simple, yet flexible and efficient, channel estimator for the uplink in broadband orthogonal frequency division multiplexing (OFDM) systems. The processing is performed in the time-domain, by extracting the Channel’s Impulse Response (CIR) for each user from a joint training signal. In this OFDM system, the pilot sequence we advocate, where all users share the same pilot sub-carriers, consists of one OFDM-symbol endowed with time-shifted properties per user, which isolates each user’s CIR and is robust against multi-user interference. The feasibility of our approach is substantiated by system simulation results obtained using BRAN-A broadband mobile wireless channel model.

An OFDMA-Based Subcarrier and Power Allocation Scheme in Wireless Cooperative Cellular Systems

An OFDMA-based subcarrier and power allocation scheme is proposed in this paper in the wireless cooperative cellular system. It is assumed that users in this system have the ability to relay information to each other to improve the overall system performance. The formulation of this scheme is to maximize the uplink system throughput subject to the half-duplex, distributed power and queue state constraints of each user. A two-slot resource allocation structure is constructed to deal with the half-duplex constraint. A low-complexity suboptimal algorithm is developed to solve the problem efficiently. This algorithm first greedily allocates subcarriers and power to users and then decides the relay selection. Numerical examples demonstrate the good throughput performance of our propose scheme and the effectiveness of the low-complexity algorithm.

Real-time DSP and FPGA Implementation of Wiener LMS Based Multipath Channel Estimation in 3G CDMA Systems

This paper investigates real-time DSP and FPGA implementations of a low complexity technique for asynchronous multiuser delay acquisition and time varying channel tracking for multipath channels in WCDMA and cdma2000 systems. A multiuser-LMS-like structure along with smoothing/prediction filters to improve tracking quality is reviewed. We investigate an efficient implementation based on FFT/IFFT technique, under fixed-point data representation and computation constraint. The measured BER reveals that fixed-point implementation is feasible at possibly no performance degradation. Based on real time execution made on a fixed-point high performance DSP, the maximum number of users is 15 and 17 for the proposed method and correlator, respectively. Due to the inherent parallelism and regular data flow FPGA implementation is suggested wherein a maximum number of users more than 80 can be afforded in Xilinx Virtex™ II Pro device.

Multi-access Management in Heterogeneous Networks

Communication networks and mobile devices integrate an increasing number of access technologies. At the same time, new business roles emerge, which lead to new cooperation schemes between access providers providing different types of access connectivity. As a result, a variety of access technologies will be available for users at the same time. In this article we present an architecture and a framework capable of integrating different access systems into a multi-access system and selecting the best suited access for users. A utility-based approach is proposed for the evaluation of different access allocation choices, which is based on user and network policies, the performance of access bearers, and the availability of access resources. We present a general multi-access management framework, which integrates the different multi-access related functions: access detection, access evaluation and access selection, which can then lead to an access handover.

Bin-packing with fragile objects and frequency allocation in cellular networks

We consider two related optimization problems: bin-packing with fragile objects and frequency allocation in cellular networks. The former is a generalization of the classical bin-packing problem and is motivated by the latter. The problem is as follows: each object has two attributes, weight and fragility. The goal is to pack objects into bins such that, for every bin, the sum of weights of objects in that bin is no more than the fragility of any object in that bin. We consider approximation algorithms for this problem. We provide a 2-approximation to the problem of minimizing the number of bins. We also show a lower bound of 3/2 on the approximation ratio. Unlike for the classical bin-packing problem, this lower bound holds in the asymptotic case. We then consider the approximation with respect to fragility and provide a 2-approximation algorithm (i.e., our algorithm uses the same number of bins as the optimum, but the weight of objects in a bin can exceed the fragility by a factor of 2). We then consider the frequency allocation problem (which is a special case of bin-packing with fragile objects) and give improved approximation algorithms for it. Finally, we consider a probabilistic setting and show that our algorithm for frequency allocation approaches optimality as the number of users increases.

Design of Power and Area Efficient Digital Down-converters for Broadband Communications Systems

This paper shows that when a digital receiver is designed utilizing two clock scopes, the digital down-converter can be designed to be efficient in terms of area and power consumption. The main design parameter that contributes to make the design efficient is the relationship between the transition band of the designed filter and its sampling frequency.

On the MIMO Channel Capacity Predicted by Kronecker and Müller Models

This paper presents a comparison between the outage capacity of MIMO channels predicted by the Kronecker and Müller models as a function of the number of scatterers, transmit and receive antennas. The Müller model is based on the singly-scattered rays between arrays of transmit- and receive antennas, while the Kronecker model considers only double scattering. The channel capacity predictions by the Müller model were observed to be higher than those by the Kronecker model. Moreover, Müller model is simpler since it is characterized by fewer parameters, and accounts for frequency selective fading whilst the Kronecker model is valid only for frequency flat fading.

Multi-Agent Radio Resource Allocation

This paper presents a spatially distributed and dynamic billing, pricing and allocation mechanism for which a user terminal requires Cognitive Radio abilities. That is, the Cognitive Radio abilities will be applied to the economical environment. The radio resource goods are allocated to the users by a multi-unit sealed-bid auction. Intelligent entities like the bidding strategy have to represent operator’s and users’ behavior and to make decisions for them in order to fulfill the preferences and QoS. The main functionalities, used to execute the dynamic auction sequence and located in the MAC are described, assuming the entities possess Cognitive Radio abilities.

Improved Fractional Fourier Transform Based Receiver for Spatial Multiplexed MIMO Antenna Systems

In this work the performance of a Fractional Fourier transform (FrFT) based Minimum Mean Squared Error receiver for MIMO systems with space time processing over Rayleigh faded channels is presented. The proposed receiver called Optimum FrFT based MIMO receiver (OFMR) shows improved performance outperforming the simple MMSE receiver in Rayleigh faded channel.

Wireless Channel Tracking Based on Optimum Predictive LMS

A frequency domain analysis is presented to optimize the Predictive Least Mean Square (PLMS) algorithm used for wireless channel tracking. Simulation results show that the PLMS offers significant improvement in tracking performance compared to that of the conventional LMS based method. The algorithm parameters should be carefully selected in order to gain such improvements. The objective of this paper is to use frequency domain analysis to determine an expression for the Mean Square Tracking Error (MSTE) and use it to obtain the optimum PLMS algorithm parameters such as step size (μ) and smoothing constant (θ) with numerical optimization methods.

A Novel Carrier Frequency Offset Estimation for OFDM Systems Over Time-Varying Multipath Channels

This paper introduces a novel method to estimate fractional carrier frequency offset (CFO) under time-varying multipath channels in OFDM systems based on the approximation of Basis Expansion Model (BEM). Due to the symmetrical character of the BEM basis, we propose a new design of training sequence with trailing zeros. This training sequence can be used at the receiver to estimate the frequency offset without channel estimation only in one OFDM symbol. We also compare our method with Beek’s ML estimator and Lv’s fine synchronization estimator. The numerical results demonstrate that the proposed method provides an improved performance when the training sequence has a short length.

Predictive Dynamic Channel Allocation Scheme for Improving Power Saving and Mobility in BWA Networks

The radio spectrum of IEEE 802.16 medium access control (MAC) protocol ranges from 2–66 GHz, which is one of potential solutions for broadband wireless access (BWA) or beyond third generation (B3G)/4G networks. The maximum transmission range can reach about 48 km. However, with the property of radio propagation, the maximum transmission distance is proportioned inversely to the frequency the mobile subscriber station (MSS) carries. According to this property, the channel allocation can be based on how far the distance between the MSS and the base station (BS) in a macrocell. Therefore, this paper first proposes a new concept of channel allocation model for BWA system and investigates the relations between the signal propagation and the distance as well as propose a signal-aware dynamic channel allocation (SDCA) scheme for dynamic channel allocation (DCA) in BWA networks (BWANs). The SDCA enables the BS to allocate appropriate channels to MSSs according to the received signal-to-noise ratio (SNR) value from the MSSs. Besides, according to the frequency, the SDCA can estimate a minimum power for MSS to communicate. The SDCA not only increases the capacity of the system but saves the overall power consumption of the system well. We also present a new out-of-service prevention scheme for supporting mobility in the system. Simulation results show that the proposed SDCA can achieve the channel utilization (throughput) by up to 94.4% when the spectrum ranges from 2–11 GHz.