Showing papers on "Co-channel interference published in 2012"

Retrospective Interference Alignment Over Interference Networks

Abstract: Maddah-Ali and Tse recently introduced the idea of retrospective interference alignment, i.e., achieving interference alignment with only outdated (stale) channel state information at the transmitter (CSIT), in the context of the vector broadcast channel. Since the scheme relies on the centralized transmitter's ability to reconstruct all the interference seen in previous symbols, it is not clear if retrospective interference alignment can be applied in interference networks consisting of distributed transmitters and receivers, where interference is contributed by multiple transmitters, each of whom can reconstruct only the part of the interference caused by themselves. In this work, we prove that even in such settings, retrospective interference alignment is feasible. Specifically, we show that it is possible to achieve more than 1 degrees of freedom (DoF) based on only delayed CSIT in the 3-user interference channel and the 2-user X channel consisting of only single antenna nodes. Retrospective interference alignment is also shown to be possible in other settings, such as the 2-user multiple-input and multiple-output (MIMO) interference channel and with delayed channel output feedback.

Ergodic Interference Alignment

Abstract: This paper develops a new communication strategy, ergodic interference alignment, for the K-user interference channel with time-varying fading. At any particular time, each receiver will see a superposition of the transmitted signals plus noise. The standard approach to such a scenario results in each transmitter-receiver pair achieving a rate proportional to 1/K its interference-free ergodic capacity. However, given two well-chosen time indices, the channel coefficients from interfering users can be made to exactly cancel. By adding up these two observations, each receiver can obtain its desired signal without any interference. If the channel gains have independent, uniform phases, this technique allows each user to achieve at least 1/2 its interference-free ergodic capacity at any signal-to-noise ratio. Prior interference alignment techniques were only able to attain this performance as the signal-to-noise ratio tended to infinity. Extensions are given for the case where each receiver wants a message from more than one transmitter as well as the “X channel” case (with two receivers) where each transmitter has an independent message for each receiver. Finally, it is shown how to generalize this strategy beyond Gaussian channel models. For a class of finite field interference channels, this approach yields the ergodic capacity region.

GFDM Interference Cancellation for Flexible Cognitive Radio PHY Design

Abstract: Generalized frequency division multiplexing (GFDM) is a new digital multicarrier concept. The GFDM modulation technique is extremely attractive for applications in a fragmented spectrum, as it provides the flexibility to choose a pulse shape and thus allows reduction of the out-of-band leakage of opportunistic cognitive radio signals into incumbent frequency space. However, this degree of freedom is obtained at the cost of loss of subcarrier orthogonality, which leads to self-inter-carrier-interference. This paper will explain how self-interference can be reduced by a basic and a double-sided serial interference cancellation technique and show that these interference cancellation techniques improve the GFDM bit error rate to match the theoretical performance of the well studied orthogonal frequency division multiplexing (OFDM).

Outage Probability of Decode-and-Forward Cooperative Relaying Systems with Co-Channel Interference

Abstract: This paper analyzes the outage probability of multi-branch dual-hop decode-and-forward (DF) cooperative relaying systems over non-identical Nakagami/Nakagami fading channels. The paper derives an exact closed-form expression for the outage probability with maximum ratio combining considering both the effect of co-channel interference (CCI) and addictive white Gaussian noise. The results of this paper show that a DF cooperative relaying system is more vulnerable to noise than interference for low and moderate SINR, whereas it is more susceptible to interference than noise for high SINR and high diversity order. Moreover, this paper shows that the robustness of the destination against CCI is more important than that of the relay. The results of this paper are verified by Monte Carlo simulations.

Adaptive Threshold Control for Energy Detection Based Spectrum Sensing in Cognitive Radios

Abstract: We study energy detection based spectrum sensing of SU (Secondary User) in cognitive radios. Due to the time varying nature of wireless channels and PU (Primary User) activities, the SINR (Signal to Interference plus Noise Ratio) at the SU receiver changes from slot to slot in a time-slotted system. Unlike the conventional energy detector which uses a fixed energy threshold for spectrum sensing, we dynamically adjust the energy threshold according to the SINR. By limiting the average interference to the PU within a target level, the objective is to maximize the SU's average rate using an optimized policy function, which formulates the threshold as a linear increasing function of the SINR. Simulation results show that the SU's average rate can be significantly improved using the optimized function for dynamic threshold control.

Full- or half-duplex? A capacity analysis with bounded radio resources

Abstract: Full duplex communication requires nodes to cancel their own signal which appears as an interference at their receive antennas. Recent work has experimentally demonstrated the feasibility of full duplex communications using software radios. In this paper, we address capacity comparisons when the total amount of analog radio hardware is bounded. Under this constraint, it is not immediately clear if one should use these radios to perform full-duplex self-interference cancellation or use the radios to give additional MIMO multiplexing advantage. We find that repurposing radios for cancellation, instead of using all of them for half-duplex over-the-air transmission, can be beneficial since the resulting full-duplex system performs better in some practical SNR regimes and almost always outperforms half duplex in symmetric degrees-of-freedom (large SNR regime).

Intersymbol and co-channel interference in diffusion-based molecular communication

Abstract: Molecular Communication (MC) is a bio-inspired paradigm where information is exchanged by the release, the propagation and the reception of molecules. The objective of this paper is to analyze the effects of interference in the most general type of MC system, i.e., the diffusion of molecules in a fluidic medium. The study of the InterSymbol Interference (ISI) and Co-Channel Interference (CCI) is conducted through the analysis of the propagation of signals in a diffusion-based channel. An in-depth analysis of the attenuation and the dispersion of signals due to molecule diffusion allows to derive simple closed-form formulas for both ISI and CCI. In this paper, two different modulation schemes, namely, the baseband modulation and the diffusion wave modulation are considered for the release of molecules in the diffusion-based MC and are compared in terms of interference. It is determined that the diffusion wave modulation scheme shows lower interference values than the baseband modulation scheme. Moreover, it is revealed that the higher is the frequency of the modulating diffusion wave, the lower are the effects of the ISI and the CCI on the communication channel. The obtained analytical results are compared and validated by numerical simulation results.

Simultaneous transmit and sense for cognitive radios using full-duplex: A first study

Abstract: In two-tier cognitive radio networks, spectrum sensing is important to minimize the interference to the primary users. With half-duplex radios, spectrum sensing has to be performed periodically between transmissions, leading to spectral inefficiencies. In this paper, we study the achievable rate gain and transmission range increase for in-band full-duplex transmissions using directional multi-reconfigurable antennas. The aim is to enable simultaneous transmission and sensing in cognitive radio networks. Our results show that directionality of multi-reconfigurable antennas can increase both the range and rate of full-duplex transmissions over omni-directional antenna based full-duplex transmissions.

Implementation and demonstration of receiver-coordinated distributed transmit beamforming across an ad-hoc radio network

Abstract: Distributed transmit beamforming using an ad-hoc network of 10 RF transmitters was demonstrated using radio nodes developed from off-the-shelf components and modules. A time-slotted protocol allowed carrier phases from each transmitter to be measured at a receiver and fed back to the transmitters where Kalman filters were used to predict the offset phases and frequencies. Offsets were digitally compensated for during beamforming intervals. Beamforming gain within 0.1dB of ideal was demonstrated across 1 km at 910MHz. This is the first report (to our knowledge) of a successful outdoor RF distributed transmit beamforming experiment using independent clocks at this scale.

Analysis and Resolution of RF Interference to Radars Operating in the Band 2700-2900 MHz from Broadband Communication Transmitters

Abstract: This report describes the methodology and results of an investigation into the source, mechanism, and solutions for radiofrequency (RF) interference to WSR-88D Next-Generation Weather Radars (NEXRADs). It shows that the interference source is nearby base stations transmitters in the Broadband Radio Service (BRS) and the Educational Broadband Service (EBS) and that their out-of-band (OOB) emissions can cause interference on NEXRAD receiver frequencies. The methodology for determining interference power levels and mitigation solutions is described. Several technical solutions that can mitigate the problem are shown to be effective. Trade-offs between effectiveness, difficulty, and costs of various solutions are described, but it is shown that there is always at least one effective technical solution. The report shows that careful planning and coordination between communication system service providers and Federal agencies operating nearby radars is important in the implementation of these interference-mitigation techniques. A number of the report’s interference mitigation options have already been implemented in several United States cities served by a BRS/EBS licensee, at licensee WiMAX stations where NEXRAD radar operations are located nearby. As of the date of this report’s release, interference from the licensee’s WiMAX links to NEXRAD receivers in those markets has been successfully mitigated using the techniques described herein.

Inter-Channel Crosstalk Cancellation for Nyquist-WDM Superchannel Applications

Abstract: Superchannel WDM systems employ narrow channel spacing to achieve high spectral efficiency and increase channel capacity. Additionally, these systems attempt to avoid inter-channel interference (ICI) and inter-symbol interference (ISI), by creating and maintaining both spectral and temporal orthogonality. This in turn imposes a strong requirement on the spectral amplitude and phase of the received signals. For Nyquist-WDM systems, the temporal shapes are Nyquist pulses, requiring uniform spectral density with flat phase. In practice, these requirements are only partially achieved, resulting in non-ideal Nyquist systems with inter-channel interference (ICI). We propose and demonstrate two joint ICI cancellation methods based on our new “super receiver” architecture, which jointly detects and demodulates multiple subchannels simultaneously. The maximum a posteriori (MAP) algorithm is most readily implemented for systems with channel spacing equal to the baud rate, and the adaptive linear equalizer is effective for all channel spacings. Simulation results show that both joint ICI cancellation schemes outperform conventional linear equalization, approaching the performance of an isolated single channel.

Decontaminating pilots in cognitive massive MIMO networks

Abstract: Cognitive radio has been lately suggested as a promising technology in order to improve spectrum utilization. This paper addresses the problem of channel estimation in an underlay interference-prone cognitive radio setup. We consider a primary and a secondary base station, both with multiple antenna capability and serving multiple users. Although previous studies propose the use of beamforming to handle secondary-caused interference, this cannot be done in practice unless channels are correctly estimated in the first place. However channel estimation itself is plagued by interference (pilot contamination effects). Therefore we propose a method to address channel estimation at the primary system while removing contamination caused by the secondary transmitter. The approach is twofold: (i) We develop a robust channel estimator which makes use of covariance information. We show analytically that the performance of this estimator is identical to the interference free scenario under certain when the number of antennas becomes large under a condition on the distribution of the multipath model. (ii) We build a pilot assignment algorithm which seeks to fulfill this condition. Significant gains are reported.

Effective Interference Cancellation Mechanisms for D2D Communication in Multi-Cell Cellular Networks

Abstract: In a Device-to-Device (D2D) underlaying cellular system, interference cancellation is a key problem when uplink spectrums are shared and this problem becomes more complex in a multi-cell cellular network. In this paper, we propose an effective near-far interference avoidance scheme in a multi-cell cellular system. After monitoring the related control channels of a D2D subsystem and exchanging necessary information among neighboring eNBs, an eNB may stop scheduling the interfering cellular devices or facilitate power control mechanisms for the victim D2D users such that the inter-cell near-far interference can be mitigated. Simulations prove that our mechanism is effective and satisfying performance improvement can be obtained.

Parameterized Cancellation of Partial-Band Partial-Block-Duration Interference for Underwater Acoustic OFDM

Abstract: Despite that underwater acoustic channels are well known to contain various interferences, research on interference mitigation in underwater acoustic communications has been very limited. In this paper, we deal with a wideband orthogonal frequency division multiplexing (OFDM) transmission in the presence of an external interference which occupies partially the signal band and whose time duration is shorter than the OFDM block. We parameterize the unknown interference waveform by a number of parameters assuming prior knowledge of the frequency band and time duration of the interference, and develop an iterative receiver, which couples interference detection via a generalized likelihood-ratio-test (GLRT), interference reconstruction and cancellation, channel estimation, and data detection. In addition to simulation results, we verify the receiver performance using data sets collected from two experiments. In both time-invariant and time-varying channels, the proposed iterative receiver achieves robust performance in the presence of unknown interference.

Outage Probability Analysis for MRC in η-μ Fading Channels with Co-Channel Interference

Abstract: Exact closed-form expressions are obtained for the outage probability of maximal ratio combining in η-μ fading channels with antenna correlation and co-channel interference The scenario considered in this work assumes the joint presence of background white Gaussian noise and independent Rayleigh-faded interferers with arbitrary powers Outage probability results are obtained through an appropriate generalization of the moment-generating function of the η-μ fading distribution, for which new closed-form expressions are provided

Performance Analysis of Fixed Gain MIMO Relay Systems in the Presence of Co-Channel Interference

Abstract: The performance of a dual-hop fixed gain amplify-and-forward multiple-input multiple-output relay network is analyzed. Both the relay station and the mobile station are subjected to multiple co-channel interferers. An exact closed-form expression is derived for the outage probability which is validated by the Monte Carlo simulations. Furthermore, to render insights into the effect of multi-antenna and interference on the network performance, the asymptotic behavior of the outage probability is also investigated. Our results show that the diversity order is determined by the minimum of the antenna numbers at the base station and the mobile station and the interference degrade the performance by reducing the coding gain.

An overview of Cognitive Radio for satellite communications

Abstract: The present scarcity of frequency spectrum allocated to radio communications has motivated the search for technologies able to alleviate it by adapting the transmission to changing environmental and network-usage conditions. Cognitive Radio is one of the most promising among these technologies. Three paradigms for its application have emerged. With “interweaving,” the secondary (unlicensed) users are able to occupy the portions of the spectrum left temporarily free by the primary (licensed) users. In the “underlay” paradigm, the secondary transmitter overlaps in frequency with the primary user, after making sure that the interference level it causes is below a given threshold. The “overlay” paradigm uses the secondary user's knowledge of the primary users transmission scheme and of the channel to choose a transmission scheme that causes an acceptable amount of interference. In this paper we briefly review the basic concepts underlying Cognitive Radio, and examine their applications to satellite communications.

An Interference Coordination Scheme for Device-to-Device Multicast in Cellular Networks

Abstract: In this paper a multicast concept for Device-to-Device (D2D) communication underlaying a cellular infrastructure has been investigated. To improve system throughput and increase resource utilization, a novel interference coordination scheme is proposed. The proposed scheme includes two steps. First, in order to mitigate the interference from D2D transmission to cellular communication, we propose an efficient power control scheme to obtain an upper bound of D2D transmitter power. Next, based on the upper bound, two resource block (RB) allocation rules aiming to control the interference from cellular networks (CNs) to receivers in D2D multicast group are proposed. Simulation results show that by applying the proposed scheme, the performance of the hybrid system can be significantly improved compared to the conventional ways.

Mitigation of Impulsive Frequency-Selective Interference in OFDM Based Systems

Abstract: In this paper, an algorithm for mitigating impulsive interference in OFDM based systems is presented. It improves the conventional blanking nonlinearity approach for interference mitigation, which typically distorts the entire received signal, by combining the blanked and the original signal. The algorithm uses a Neyman-Pearson like testing procedure to detect interference at individual sub-carriers. Provided interference is detected, the blanked and the original received signals are then optimally combined such as to maximize the signal-to-interference-and-noise ratio. The algorithm does not require any prior knowledge about the impulsive interference and only marginally increases computational complexity as compared to the conventional blanking nonlinearity approach. Numerical results demonstrate the superior performance of the proposed scheme.

Performance Evaluation and Optimization of Dual-Hop Communication over Nakagami-m Fading Channels in the Presence of Co-Channel Interferences

Abstract: The performance of dual-hop communication over independent and non-identical Nakagami-m fading channels in the presence of co-channel interference is investigated. Specifically, an upper-bound of the equivalent signal-to-interference-plus-noise ratio (SINR) at the destination is introduced. Then, the cumulative distribution function (CDF) and the probability density function (PDF) of the upper bounded SINR are obtained. Moreover, simple, yet general, asymptotic expression for the error probability is presented and discussed. Besides, with the goal to minimize the asymptotic average error probability of the system under study, we investigate three optimization schemes, namely, adaptive power allocation under fixed location for the relay, optimal relay positioning with fixed power allocation, and joint optimization of the power allocation and relay location under transmit power constraint. Results show that optimum power allocation brings only coding gain, while the optimum relay location scheme yields diversity gains as well.

System-Level Modeling and Evaluation of Interference Suppression Receivers in LTE System

Abstract: In this paper we study the performance of linear interference suppression receivers which estimate the interference covariance either from the data symbols or from the reference symbols. For numerical radio system-level simulation purposes, receiver algorithm models are proposed which take into account practical estimation losses caused by the channel and interference covariance estimation. These models are verified using a multi-link simulator, and finally the performance of the receivers is studied using radio system simulations. The results indicate that the algorithm based on the reference symbols can constantly outperform a receiver that assumes white inter-cell interference. Furthermore the modeling proposed in the paper can be utilized in assessing radio system performance when receivers are equipped with interference suppression receivers.

Self-interference cancellation models for full-duplex wireless communications

Abstract: In this paper, we study two models for self or loopback interference cancellation in full-duplex wireless communications. Both models are based on an underlying Z-channel with side information. We obtain achievable rate regions with suitable coding schemes under both models. Under model 1, where the self-interference channel gain is random, we employ training to estimate the unknown gain, and optimize the required training time. Under model 2, where the self-interference gain is exactly known, we show that the capacity of an ideal full-duplex node can be realized even when the side information is low rate and quantized. Our results show that loopback interference, rather than being treated as noise, can be effectively dealt with by suitable coding.

Characterizing Decentralized Wireless Networks with Temporal Correlation in the Low Outage Regime

Abstract: Communication in decentralized wireless networks is limited by interference. Because transmissions typically last for more than a single contention time slot, interference often exhibits a strong statistical dependence over time that results in temporally correlated communication performance. The temporal dependence in interference increases as user mobility decreases and/or the total transmission time increases. We propose a network model that spans the extremes of temporal independence to long-term temporal dependence. Using the proposed model, closed-form single hop communication performance metrics are derived that are asymptotically exact in the low outage regime. The primary contributions are (i) deriving the joint temporal statistics of network interference and showing that it follows a multivariate symmetric alpha stable distribution; (ii) utilizing the joint interference statistics to derive closed-form expressions for local delay, throughput outage probability, and average network throughput; and (iii) using the joint interference statistics to redefine and analyze the network transmission capacity that captures the throughput-delay-reliability tradeoffs in single hop transmissions. Simulation results verify the closed-form expressions derived in this paper and we demonstrate up to 2× gain in network throughput and reliability by optimizing certain parameters of medium access control layer protocol in view of the temporal correlations.

Dirty-Paper Coding for the Gaussian Multiaccess Channel With Conferencing

Abstract: We derive the capacity region of the two-user dirty-paper Gaussian multiaccess channel (MAC) with conferencing encoders. In this MAC, prior to each transmission block, the transmitters can hold a conference in which they can communicate with each other over error-free bit pipes of given capacities. The received signal suffers not only from additive Gaussian noise but also from additive interference, which is known noncausally to the transmitters but not to the receiver. The additive interference is modeled as Gaussian or uniform over a sphere. We show that the interference can be perfectly mitigated, i.e., that the capacity region without interference can also be achieved in its presence. This holds irrespective of whether the transmitters learn the interference before or after the conference. It follows as a corollary that also for the MAC with degraded message sets, the interference can be perfectly mitigated if it is known noncausally to the transmitters. To derive our results, we generalize Costa's single-user writing-on-dirty-paper achievability result to channels with dependent interference and not-necessarily Gaussian noise.

Channel segregation based dynamic channel assignment for WLAN

Abstract: In a frequency reusing wireless local area network (WLAN), the co-channel interference (CCI) environment will change when new access points (APs) join the network. Therefore, the user throughput reduces if the available channels are not properly re-allocated. A newly joined AP is designed to measure the CCI levels on the available channels to select the best channel which experiences the lowest CCI (this is referred to as the conventional channel assignment). However, old APs' channels are not anymore the best since the CCI environment has changed after the new one joined. If each AP periodically measures the CCI environment and reselects the channel experiencing the lowest CCI, the throughput may improve. This is called the dynamic channel assignment (DCA). In this paper, we apply the channel segregation based DCA (CS-DCA) to WLAN. In order to examine the channel distribution pattern obtained by CS-DCA, we introduce three indicators: the autocorrelation of channel distribution pattern indicating the degree of stability, the deviation of channel reusing, and the minimum co-channel AP distance. We confirm by computer simulation that the CS-DCA can autonomously form a channel distribution pattern which minimizes the CCI at each AP and that the CS-DCA improves the signal-to-interference-ratio (SIR) compared to the conventional scheme.

Comparison of underlay and overlay spectrum sharing strategies in MISO cognitive channels

Abstract: We consider an extension of the cognitive radio channel model in which the secondary transmitter has to obtain (“learn”) the primary message in a first phase rather than having non-causal knowledge of it. We propose an achievable rate region that combines elements of decode-and-forward relaying with coding for the pure cognitive radio channel model. Moreover, we find the choice of parameters that maximize the secondary rate under a primary rate constraint. Finally, we compare numerically the performance of our system to that of an underlay scheme that combines beamforming, rate splitting, and successive decoding. We observe that although the overlay design provides higher rates, the losses due to the first phase are quite severe. In fact, for the considered scenarios, cleverly designed underlay schemes can provide comparable performance.

Performance comparison of fixed and moving relays under co-channel interference

Abstract: In this paper, we study and compare the outage probability (OP) of a vehicular user of dual-hop moving relay node (MRN) assisted transmission, dual-hop transmission assisted by a fixed relay node (FRN), as well as of the baseline single-hop direct transmission under of co-channel interference. For an accurate comparison, we numerically optimize the FRN position which minimizes the average vehicular user OP. When vehicular penetration loss is moderate to high, MRN assisted transmission is shown to greatly outperform transmission assisted by an FRN as well as direct transmission. Hence the use of MRNs is very promising for improving the quality-of-service of vehicular users for future mobile communication systems.

Macro-femto inter-cell interference mitigation for 3GPP LTE-A downlink

Abstract: In LTE-A or Rel 10+, heterogeneous network deployments are supported to improve the user experienced link performance However, such deployments give rise to new interference problems In this study, macro-femto-cell interference scenarios for LTE-A downlink are investigated We focus on the critical interference scenario of colliding cell-specific reference signals (CRSs) As conventional receivers with channel estimation based on the interfered CRSs fail to work properly, we present advanced interference mitigation schemes for user equipment (UE), such as the received-power dependent interference cancellation (IC), decision-directed channel estimation and IC-assisted channel estimation Numerical simulation results showed that the proposed schemes are able to significantly reduce the inter-cell interference so that high throughput can be ensured even under the harsh macro-femto interferences

Impact of Imperfect Channel Estimation and Co-Channel Interference on Regenerative Cooperative Networks

Abstract: In this work, we analyze the performance of decode-and-forward cooperative diversity networks with imperfect channel estimation and co-channel interference. Orthogonal relaying is considered and all relays that correctly decode the source message in the broadcasting phase participate in the relaying phase. First, the output signal-to-interference-plus-noise ratio (SINR) at the destination is derived. Then, considering error and outage as the performance metrics, we obtain exact closed-form expressions for the error and outage probabilities. Simple and general asymptotic expression for the error probability, which explicitly shows the coding and the diversity gains, is also derived and discussed.

Techniques to improve the radio co-existence of wireless signals

Abstract: Techniques are described that can be used to perform one or more of the following actions in order to reduce signal interference between WLAN and BlueTooth radios that are proximate to one another. One action is to selectively reduce a filter bandwidth in a WLAN radio applied to a received WLAN radio signal to reduce BlueTooth signal interference. An additional or alternative action is to selectively indicate a WLAN radio channel bandwidth larger than a channel bandwidth used for the WLAN radio so that the BlueTooth radio avoids transmitting over the indicated channel bandwidth. An additional or alternative action is to selectively reduce BlueTooth transmitter power in response to antenna isolation between BT and WLAN radio being less than a first threshold in order to reduce the likelihood of WLAN radio front end saturation.