Shared Frequency Band Research Articles

Frequency Partitioning Methods to Mitigate Cross-Tier Interference in Two-Tier Femtocell Networks

Femtocells are attractive candidates for future cellular systems since they can greatly improve coverage and data rates. However, severe interference may occur and hinder the improved system performance due to a large number of femtocells sharing frequency bands with the macrocell. In this paper, we consider the interference problem in two-tier femtocell networks and propose two frequency partitioning methods to mitigate the interference between the macrocell and femtocells. The key idea of the frequency partitioning methods is to partition the whole frequency band into several nonoverlapping parts and allocate different parts to the macrocell and femtocells in different regions. In the proposed methods, the downlink and uplink frequency bands are both divided into four nonoverlapping parts. The macrocell and femtocell are divided into three or four regions, with one part of the frequency band allocated to one region. With frequency and region partitioning, the users or base stations in one tier and the interfering users or base stations in the other tier are separated by a certain distance, which can greatly mitigate the interference. The main contribution of the proposed methods is that the macrocell and femtocells can assign allocated resources to their own users independently, and the cross-tier interference can be mitigated without complicated resource allocation or cooperation between the macrocell and femtocells. Moreover, we theoretically analyze the existing interference and average per-channel rate performance of different users in different regions. Simulation results show that the theoretical analysis is reasonable and that the proposed methods can successfully mitigate cross-tier interference.

Study of coexistence between indoor LTE femtocell and outdoor-to-indoor DVB-T2-Lite reception in a shared frequency band

Nowadays, the demand for high-quality multimedia services (video, audio, image, and data) is rapidly increasing. The Digital Video Broadcasting - terrestrial (DVB-T) standard, its second-generation version (DVB-T2), and the Long-Term Evolution (LTE) standard are the most promising systems to fulfill the demand for advanced multimedia services (e.g., high-definition image and video quality), especially in Europe. However, LTE mobile services can operate in a part of the UHF band allocated to DVB-T/T2 TV services previously. The main purpose of this work is to explore the possible coexistences of DVB-T2-Lite and LTE systems in the same shared frequency band (co-channel coexistence) under outdoor-to-indoor and indoor reception conditions. Furthermore, an applicable method for evaluating coexistence scenarios between both systems is shown with a particular example. These coexistence scenarios can be noncritical and critical. In the first case, both systems can coexist without significant performance degradation. In the second one, a partial or full loss of DVB-T2-Lite and/or LTE signals can occur. We consider an indoor LTE femtocell and outdoor-to-indoor DVB-T2-Lite signal reception in a frequency band from 791 up to 821 MHz. Simulations of combined indoor and outdoor signal propagation are performed in MATLAB using 3rd Generation Partnership Project (3GPP) channel models, separately for both DVB-T2-Lite and LTE systems. Correctness of path loss simulation results is verified by measurements. Afterwards, an appropriate linear model is proposed which enables to evaluate the impact of coexistence on performance of both systems in outdoor-to-indoor and indoor-to-indoor reception scenarios. The results are related to an actual location in the building and are presented in floor plans. The floor plans include different coexistence conditions (different power imbalance and different amount of overlay of the radio channels). Service availability of both systems is verified again by measurements. The resulting maps help better understand the effect of coexistence on achievable system performance under different indoor/outdoor reception situations considering real transmission conditions.

A low computational complexity V-BLAST/STBC detection mechanism in MIMO system

Abstract The idea of multiple antenna arrays has evolved into multiple-input multiple-output (MIMO) system, which provides transmit and receive diversities. It increases robustness of the effect of multipath fading in wireless channels, besides yielding higher capacity, spectral efficiency and better bit error rate (BER) performance. The spatial diversity gain is obtained by transmitting or receiving multiple copies of a signal through different antennas to combat fading and improves the system BER performance. However, the computational complexity of MIMO system is inevitably increased. Space-time coding (STC) technique such as Alamouti’s space-time block code (STBC) that combines coding, modulation and signal processing has been used to achieve spatial diversity. Vertical Bell Laboratories Layered Space-Time (V-BLAST) uses antenna arrays at both the transmitter and receiver to achieve spatial multiplexing gain. Independent data streams that share both frequency bands and time slots are transmitted from multiple antennas and jointly detected at the receiver. The theoretical capacity of V-BLAST increases linearly with the number of antennas in rich scattering environments. It's well-known that maximization of spatial diversity gain leads to degradation of spatial multiplexing gain or vice versa. In order to achieve spatial multiplexing and diversity gains simultaneously, the V-BLAST/STBC scheme has been introduced. This hybrid scheme increases MIMO system capacity and maintains reliable BER performance at the same time. However, both V-BLAST and STBC layers, in this hybid scheme, assume each other as an interferer. Thus, the symbols must be decoded with a suitable detection mechanism. In this paper, a new low complexity detection mechanism for V-BLAST/STBC scheme based on QR decomposition, denoted as low complexity QR (LC-QR) decomposition, is presented. The performance of the proposed LC-QR decomposition detection mechanism in V-BLAST/STBC transceiver scheme is compared with other detection mechanisms such as ZF, MMSE and QR decomposition. It is shown that the BER performance in V-BLAST/STBC scheme is better than V-BLAST scheme while its system capacity is higher than orthogonal STBC scheme when the LC-QR decomposition detection mechanism is exploited. Moreover, the computational complexity of proposed LC-QR decomposition mechanism is significantly lower than other abovementioned detection mechanisms.

Bistatic Scattering due to Hydrometeors on Cochannel Intersystem Communication Links over a Subtropical Path

The performance of fixed satellite systems in the shared frequency band depends on the tolerance level of interference between them. Interference disturbs the functionality of the ground station and causes signal degradation. The knowledge of interference level must therefore be known for an optimal satellite design. In this work, we evaluate interference due to hydrometeors for a situation in which a satellite downlink signal is affected by the signal from a terrestrial microwave network operating at the same frequency as the satellite system in a subtropical station: Durban, South Africa. The evaluation of the transmission loss is based on the modified 3D bistatic radar equation and the exponential rain cell model for the scattering. The results of intersystem interference for different station separation over frequencies variation, terrestrial antenna gains, and exceedance probabilities are presented. The effect of the additional rain attenuation(Aw)on the satellite signals is also examined. The results point out to some remarkable attenuation differences between the effective transmission loss and the transmission loss statistics for small time unavailability at the frequencies considered in this work. This could be detrimental in link budget design if overlooked.

Outage Analysis of Cognitive Two-Way Relaying Network with Physical-Layer Network Coding in Nakagami-m Fading Channels

In spectrum sharing systems, a secondary user (SU) is permitted to share frequency bands with primary user (PU) as long as its transmission does not interfere with the PU's communication. In this paper, the outage probability is investigated for the physical-layer network coding (PNC) based cognitive two-way relay network over Nakagami-m fading channel. By applying the interference temperature (IT) constraints at the transceiver nodes and relay nodes in secondary systems, we analyze the outage performance in underlay spectrum sharing with the best relay selection criterion. The cumulative distribution function (CDF) of the signal to noise ratio (SNR) for each link is derived to obtain the tight upper bound on the outage probability of the secondary relay system. Simulations results demonstrate the validity and accuracy of the theoretical analysis. Index Terms—Cognitive two-way relay network, physical layer network coding, outage probability, Nakagami-m fading channel.

Software Tool for Assessing the Viability of Secondary System Deployments in Whitespace Spectrum Access Scenarios

The increasing demand for wireless capacity has led to a depletion of unallocated spectrum resources in the popular sub-3 GHz frequency bands. In order to overcome this spectrum scarcity, various secondary spectrum access policies have been proposed. Especially for UHF bands where spectrum usage of the incumbent broadcasting system is comparably static, a move towards granting access on a non-interfering basis seems to be a plausible step. Regulators, governments, and industry alike are currently working on technically feasible and economically viable solutions for opening up more potential spectrum whitespaces. In order to allow coexistence between wireless systems in shared frequency bands, the effect of interference from secondary transmitters to primary receivers needs to be evaluated. Besides active practical testing which is conducted in large-scale field tests, regulatory policies are currently being developed that define rules for maximum power emission and minimum separation distances of secondary device installations. The most prominent example of such legislation is the TV whitespace ruling in the US, but other countries are catching up with scenario-dependent proposals. While the proposed rulings are still under evaluation, concerns have been raised as to whether they will scale for entire secondary systems with hundreds to thousands of deployed devices. Evaluation here is still in its infancy, as it encompasses a complex techno-economical eco-system with a large number of unknowns.To our understanding, the envisioned coexistence scenarios of primary and secondary systems need to be analyzed in a holistic manner. Only by jointly studying the technical, economic, and social effects of secondary deployments within potential whitespaces, can the real threats and potential of such deployments be revealed. In this demonstration we present a software tool that, for the first time, supports such studies by combining primary and secondary technology models, application of different regulatory rules, and derivation of key performance indicators. Information generated by our tool may subsequently be used for in-depth studies on the fitness of proposed whitespace policies, feasibility of technical implementation, and business aspects of secondary system deployments. Our tool deliberately goes beyond simple whitespace availability studies by providing extensive analysis opportunities and building on a large-scale primary spectrum usage database. This demonstration aims at raising awareness of the complexity of the required studies and presents a novel methodology that may be used for the necessary evaluations. As such, our tool can be of benefit to regulators, researchers, and industry in assessing the utility of secondary spectrum access.

Performance Evaluation of Multimedia Traffic Transmission Under Binomial and Poissonian Primary Traffics in Cognitive Radio Networks

Cognitive Radio is an enabling agile technology for communication and channels sharing in multimedia and wireless networks. The authors consider multimedia traffic transmission over Cognitive Radio (CR) networks using a progressive source coding associated to fountain codes. For a Cognitive Radio infrastructure where the spectrum is owned by Primary Users (PUs) that have Binomial and/or Poissonian traffics, Secondary Users (SUs) are allowed to share these spectral resources using the Opportunistic Spectrum Sharing feature of the Cognitive Radio concept. The objective of this paper lies in characterizing the reliability and as a result evaluating the performance of the secondary multimedia service provision under both primary traffic types in lossy Cognitive Radio environments. The authors have conducted a general analysis for many still open issues in Cognitive Radio and tackle three critical factors responsible for secondary service disruption: primary traffic interruptions, concurrent access to shared frequency bands leading to collisions and subchannels characteristics. Numerical simulations, in view of the average Spectral Efficiency, examine the performance of the multimedia transmission under the primary interruptions caused by the Binomial and the Poissonian primary applications and consider also the dynamics between the competing secondary peers in addition to subchannels fading and noise.

Cost-Effective Frequency Planning for Capacity Enhancement of Femtocellular Networks

The femto-access-point, a low-cost and small-size cellular base-station, is envisioned to be widely deployed in subscribers homes, as to provide high data-rate communications with improved quality of service. As femtocellular networks will co-exist with macrocellular networks, mitigation of the interference between these two network types is a key challenge for successful integration of these two technologies. In particular, there are several interference mechanisms between the femtocellular and the macrocellular networks, and the effects of the resulting interference depend on the density of femtocells and the overlaid macrocells in a particular coverage area. While improper interference management can cause a significant reduction in the system capacity and can increase the outage probability, effective and efficient frequency allocation among femtocells and macrocells can result in a successful co-existence of these two technologies. Furthermore, highly dense femtocellular deployments—the ultimate goal of the femtocellular technology—will require significant degree of self-organization in lieu of manual configuration. In this paper, we present various femtocellular network deployment scenarios, and we propose a number of frequency-allocation schemes to mitigate the interference and to increase the spectral efficiency of the integrated network. These schemes include: shared frequency band, dedicated frequency band, sub-frequency band, static frequency-reuse, and dynamic frequency-reuse. We derive an analytical model, which allows us to analyze in details the users outage probability, and we compare the performance of the proposed schemes using numerical analysis.

Integrated Common Radio Resource Management with Spectrum Aggregation Over Non-Contiguous Frequency Bands

This paper proposes an Integrated Common Radio Resource Management (iCRRM). The iCRRM performs classic CRRM functionalities jointly with Spectrum Aggregation (SA), being able to switch users between non-contiguous frequency bands. The SA scheduling is obtained with an optimised General Multi-Band Scheduling algorithm with the aim of cell throughput maximisation. In particular, we investigate the dependence of the throughput on the cell coverage distance for the allocation of users over the 2 and 5 GHz bands for a single operator scenario under a constant average Signal to Interference-plus-Noise Ratio. For the performed evaluation, the same type of Radio Access Technology is considered for both frequency bands. The operator has the availability of a non-shared 2 GHz band and has access to part (or all) of a shared frequency band at 5 GHz. The performance gain, analysed in terms of data throughput, depends on the channel quality for each user in the considered bands which, in turn, is a function of the path loss, interference, noise, and the distance from the Base Station. An almost constant gain near 30% was obtained with the proposed optimal solution compared to a system where users are first allocated in one of the two bands and later not able to handover between the bands.

Coexistence of mobile-satellite service system with mobile service system in shared frequency bands

Interference scenarios and a methodology between mobile-satellite service (MSS) system and terrestrial mobile service (MS) system in a co-frequency environment are established. Taking into account a practical deployment situation for both systems, computational simulation of interference in terms of the percentage increase in equivalent satellite link noise temperature (DeltaT/T) and the ratio of carrier power to interference power (C/I) is performed to evaluate the co-frequency interference from the MS system into the MSS system. The methodology and results could be used for planning an MSS deployment without an unacceptable interference impact from MS.

Interference Mitigation Technique for Coexistence of Pulse-Based UWB and OFDM

Ultra-wideband (UWB) is a useful radio technique for sharing frequency bands between radio systems. It uses very short pulses to spread spectrum. However, there is a potential for interference between systems using the same frequency bands at close range. In some regulatory systems, interference detection and avoidance (DAA) techniques are required to prevent interference with existing radio systems. In this paper, the effect of interference on orthogonal frequency division multiplexing (OFDM) signals from pulse-based UWB is discussed, and an interference mitigation technique is proposed. This technique focuses on the pulse repetition cycle of UWB. The pulse repetition interval is set the same or half the period of the OFDM symbol excluding the guard interval to mitigate interference. These proposals are also made for direct sequence (DS)-UWB. Bit error rate (BER) performance is illustrated through both simulation and theoretical approximations.

Spectrum Co-existence of IEEE 802.11b and 802.16a Networks Using Reactive and Proactive Etiquette Policies

This paper presents an investigation of spectrum co-existence between IEEE 802.11b and 802.16a networks in the same shared frequency band using cognitive radio techniques with different levels of complexity. Simple reactive interference avoidance algorithms as well as proactive spectrum coordination policies based on etiquette protocols are proposed and compared in terms of achievable spectrum efficiency in a shared Wi-Fi/Wi-Max scenario. In reactive interference avoidance methods, radio nodes coordinate spectrum usage without exchange of explicit control information-this is done by adaptively adjusting transmit PHY parameters such as frequency, power and time occupancy based on local observations of the radio band. Because local observations provide information only about transmitters, they may not be sufficient for resolving spectrum contention in scenarios with "hidden receivers". Proactive coordination techniques solve the hidden-receiver problem by utilizing a common spectrum coordination channel (CSCC) for exchange of transmitter and receiver parameters. Radio nodes can cooperatively select key PHY-layer variables such as frequency and power by broadcasting messages in the CSCC channel and then following specified spectrum etiquette policies. An ns2 simulation model is developed to evaluate both reactive and proactive etiquette policies in scenarios with co-existing IEEE 802.11b and 802.16a networks. The density of radio nodes in the coverage region, and their degree of spatial clustering are key parameters in the system evaluation. Detailed simulation studies were carried out for a variety of scenarios including both single and multiple 802.11b hotspots per 802.16a cell with and without spatial clustering. Our results show that simple reactive algorithms can improve system throughput when sufficient "free space" (in frequency, power or time) is available for PHY adaptation. In more congested scenarios with spatially clustered nodes and hidden receivers, the proposed CSCC etiquette can significantly improve overall system performance over reactive schemes.

Throughput of Strongly Interfering Slow Frequency-Hopping Networks

With the increasing use of short-range wireless devices for high-data-rate communication in the shared frequency bands, the level of interference can be expected to increase. The ability to estimate the performance degradation of these devices due to increasing interference is, therefore, important. In this paper, the throughput of devices that perform frequency hops after each transmitted packet in order to achieve diversity is investigated. The system model allows for an analysis of systems where packets of variable durations are used, and the throughput derivation is based on the assumption that collisions result in a total loss of the data in the colliding packets. The resulting expression for the throughput is given as a function of the number of frequency channels used for frequency hopping, the number of interfering networks, the durations of the packet types available, and the probability of networks selecting a certain packet type for transmission. An approximation of the exact expression for the throughput is also derived, and the results are applied to an example system consisting of Bluetooth piconets.

Capacity of DS/CDMA communication systems with optimum spectral overlap

We consider frequency overlapped DS/CDMA (FO/CDMA) communication systems where a number of DS/CDMA systems share frequency bands with adjacent systems. We analyze the multiple-access interference from adjacent systems with arbitrary amount of frequency overlap and compare the capacity of FO/CDMA with that of single wideband CDMA (WCDMA). The optimum amount of overlap increases with the number of overlapped systems. However the maximum capacity is obtained when the optimum number of systems are overlapped by half null-to-null bandwidth. It is shown that the FO/CDMA yields higher capacity in the case of relaxed bit error rate requirement and low decay rate of multipath intensity profile. Otherwise, WCDMA is superior.

Bistatic radar measurements of ice-cloud reflectivities in the upper troposphere

Results of 15.7 GHz bistatic radar measurements of scattering from ice clouds are presented and the vertical profiles of the reflectivity factor exceeded for small percentages of time inferred. The results have implications for terrestrial and earth-space communications systems sharing frequency bands above 10GHz and for earth-space systems utilising orthogonal polarisations to increase capacity.

Radio-Relay Antenna Pointing for Controlled Interference with Geostationary Satellites

We present analytical methods (i) for calculating microwave radio refraction for negative and positive initial ray angles accounting for station height and (ii) for determining refraction-corrected ranges of antenna pointing azimuth within which mutual interference with geostationary satellites in shared frequency bands is likely.