Showing papers on "Co-channel interference published in 2007"

Understanding and mitigating the impact of RF interference on 802.11 networks

Abstract: We study the impact on 802.11 networks of RF interference from devices such as Zigbee and cordless phones that increasingly crowd the 2.4GHz ISM band, and from devices such as wireless camera jammers and non-compliant 802.11 devices that seek to disrupt 802.11 operation. Our experiments show that commodity 802.11 equipment is surprisingly vulnerable to certain patterns of weak or narrow-band interference. This enables us to disrupt a link with an interfering signal whose power is 1000 times weaker than the victim's 802.11 signals, or to shut down a multiple AP, multiple channel managed network at a location with a single radio interferer. We identify several factors that lead to these vulnerabilities, ranging from MAC layer driver implementation strategies to PHY layer radio frequency implementation strategies. Our results further show that these factors are not overcome by simply changing 802.11 operational parameters (such as CCA threshold, rate and packet size) with the exception of frequency shifts. This leads us to explore rapid channel hopping as a strategy to withstand RF interference. We prototype a channel hopping design using PRISM NICs, and find that it can sustain throughput at levels of RF interference well above that needed to disrupt unmodified links, and at a reasonable cost in terms of switching overheads.

Adaptive Power Loading for OFDM-Based Cognitive Radio Systems

Abstract: Cognitive radio (CR) technology is an innovative radio design philosophy which aims to increase spectrum utilization by exploiting unused spectrum in dynamically changing environments. Orthogonal frequency division multiplexing (OFDM) is a potential modulation technique for CR networks' air interface. In this paper, we study and explore optimal power loading algorithm for an OFDM-based CR system and the rate in each subcarrier is adjusted according to the power. As such the downlink capacity of the CR user is maximized while the interference introduced to the primary user remains within a tolerable range. We also propose two suboptimal loading algorithms that have less complexity. The performance of optimal and suboptimal schemes are compared with the performance of classical power loading algorithms that are used for conventional OFDM-based systems e.g., water-filling and uniform power but variable rate loading schemes. Presented numerical results show that for a given interference threshold the proposed optimal scheme allows the CR users to transmit more power in order to achieve higher transmission rate than the classical loading algorithms. These results also show that the proposed suboptimal schemes offer a performance close to the optimal scheme. Finally, we study the effect of subcarrier nulling mechanism on the performance of different algorithms under consideration.

Joint Beamforming and Scheduling in Cognitive Radio Networks

Abstract: Cognitive radio has been recently proposed as a promising technology to improve the spectrum utilization. In this paper, we consider the coexistence between a large number of cognitive radio users and a licensed user in order to enhance the spectrum efficiency. With the deployment of M antennas at the cognitive base station, an efficient transmit beamforming technique combined with user selection is proposed to maximize the downlink throughput and satisfy the signal-to-interference- and-noise ratio (SINR) constraint as well as limit interference to the primary user. In the proposed user selection algorithm, cognitive users who are nearly orthogonal to the primary user are first pre-selected so as to minimize the mutual interference. Then, M best cognitive users who are nearly mutual orthogonal to each other are scheduled from those pre-selected cognitive users. Simulation results show that our proposed method is able to achieve high sum-rate throughput, with affordable complexity. Moreover, our proposed technique with equal power allocation suffers a negligible performance loss compared to the one with the optimal power allocation.

Robust joint interference detection and decoding for OFDM-based cognitive radio systems with unknown interference

Abstract: Cognitive radio technology facilitates spectrum reuse and alleviates spectrum crunch. One fundamental problem in cognitive radio is to avoid the interference caused by other communication systems sharing the same frequency band. However, spectrum sensing cannot guarantee accurate detection of the interference in many practical situations. Hence, it is crucial to design robust receivers to combat the in-band interference. In this paper, we first present a simple pilot aided interference detection method. To combat the residual interference that cannot be detected by the interference detector, we further propose a robust joint interference detection and decoding scheme. By exploiting the code structure in interference detection, the proposed scheme can successfully detect most of the interfered symbols without requiring the knowledge of the interference distribution. Our simulation results show that, even without any prior knowledge of the interference distribution, the proposed joint interference detection and decoding scheme is able to achieve a performance close to that of the maximum likelihood decoder with the full knowledge of the interference distribution

Mimo precoding in the presence of co-channel interference

Abstract: Methods and systems for communicating in a wireless network include mitigating co-channel interference (CCI) for precoded multiple-input multiple-output (MIMO) systems and incorporating the effect of CCI mitigation on channel characteristics in the design of channel state information (CSI) feedback mechanisms. Various embodiments and variants are also disclosed.

On Achievable Rates for Interference Relay Channel with Interference Cancelation

Abstract: We consider two interfering source-destination pairs that are assisted by a common relay. The relay simultaneously helps both sources to improve communication rates. We provide coding strategies that combine dirty paper coding, beamforming and interference reduction techniques at the relay and give an achievable rate region for Gaussian interference relay channel. The region shows significant rate gain can be obtained by performing generalized dirty-paper coding at the relay.

On the Performance of IEEE 802.16 OFDMA System Under Different Frequency Reuse and Subcarrier Permutation Patterns

Abstract: In interference-limited wireless cellular systems, interference avoidance and interference averaging are widely adopted to combat co-channel interference. In different types of wireless networks, these two basic frequency planning methods are implemented under variable system deployment strategies. With respect to the IEEE 802.16 OFDMA network, interference avoidance is usually carried out by carefully designing the frequency reuse patterns, while interference averaging is implemented by distributed subcarrier permutation, which can be further divided into multiple operation modes. In this paper, based on the system level simulation of the average signal to interference-plus-noise ratio (SINR) in the 802.16 system, the performance metrics of system throughput and outage probability are estimated. According to the predetermined evaluation criterions, the system performances under variable combination patterns of frequency reuse and subcarrier permutation are evaluated. Finally, the optimal frequency planning strategies, in terms of the frequency reuse pattern and the subcarrier permutation mode, is obtained, which can serve as a reference in practical network deployments.

Effects of reader interference on the RFID interrogation range

Abstract: In this paper, the effects of radio frequency identification (RFID) reader interference are investigated in terms of the interrogation range. In order to evaluate RFID interference quantitatively, the signal-to-interference ratio (SIR) equation is initially derived, and the interrogation-reduction range ratio (IRRR) defined. IRRR is a function of the distance between a desired reader and an interfering reader. Co-channel interference (CCI) and adjacent-channel interference (ACI) instances of IRRR are simulated. Simulation results show that reader-reader distances achieving 0 % IRRR, indicating no interference between the two readers, are 1200 m and 35 m for the CCI and ACI cases, respectively. The IRRR factor is inversely proportional to the reader-reader distance in both cases. The simulation results were also verified by measurement results using an ETRI UHF RFID system. Measurement results were found to be in good agreement with the simulation results. It can be concluded that the present simulation results are reliable and applicable in analyses of more complex interfering problems in actual RFID system deployment instances.

Interference Cancellation: Better Receivers for a New Wireless MAC.

Abstract: We argue that carrier sense in 802.11 and other wireless protocols leads to scheduling decisions that are overly pessimistic and hence waste capacity. As an alternative, we propose interference cancellation, in which simultaneous signals are modeled and decoded together rather than treating all but one as random noise. This method greatly expands the conditions under which overlapping transmissions can be successfully received, even by a single receiver. We demonstrate the practicality of these better receivers via a proof-of-concept experiment with USRP software radios. We argue that supporting concurrent transmissions enables new and more effective wireless MACs in which carrier sense is disabled.

The Effects of Co-channel Interference on Spatial Diversity Techniques

Abstract: This paper investigates the effects of co-channel interference on spatial diversity techniques. By analyzing distributions of post-processing signal to noise plus interference ratio (SINR) after matched filtering, we capture the performance characteristics of spatial diversity techniques in an interference limited environment. Using intuition from the theoretical analysis of a single carrier system, the performance of spatial diversity techniques in an OFDMA system with co-channel interference is also characterized. Our analytical and simulation results show that space time block code (STBC) schemes are more sensitive to co-channel interference than other spatial diversity techniques so their performance gain in a noise-limited environment can be lost in an interference-limited environment.

Interference rejection in telecommunication system

Abstract: An interference suppression scheme is provided for a radio receiver. According to the provided interference suppression scheme, the bandwidth of a received pilot signal and a data signal is divided into a plurality of frequency sub-bands. The pilot signal and the data signal have been transmitted according to single carrier data transmission technology. Interference parameters are calculated (306) for each frequency sub-band separately. Interference suppression (314) may be carried out jointly or separately for each frequency sub-band. After the interference suppression, the frequency sub-bands are combined (322). A filter bank (300) may be used for dividing the total frequency band into sub-bands.

A Design of Precoding and Equalisation for Broadband MIMO Systems

Abstract: We propose a new approach to precoding and equalisation for frequency selective MIMO channels by applying a recently proposed broadband singular value decomposition to decouple the MIMO channel matrix into independent frequency selective SISO subchannels. In a second step, ISI present in the subchannels is eliminated using methods reported in the literature. The first step helps to remove not only co-channel interference but also eliminates part of the inter-symbol interference with a very small loss in channel power gain. It also helps to use the system resource more effectively, leading to a better performance. The numerical example given in the paper shows that our proposed method can provide better bit error rate performance than that of the benchmark design. Under a quality of service constraint, our design can achieve higher data throughput and mutual information than that of the benchmark design while maintaining a similar error performance.

Non-Intrusive Cognitive Radio Networks Based on Smart Antenna Technology

Abstract: Cognitive radio has recently been identified as a potential relief to spectrum scarcity by improving temporal spectral efficiency. We investigate a flexible non-intrusive cognitive radio network based on smart antenna technologies. The proposed scheme exploits transmit beamforming to enable better spectral sharing between primary users and cognitive (secondary) users. As proposed, the cognitive transmitter equipped with antenna array forms transmit beamforming to keep the interference to primary receiver below a given threshold. By also adopting smart antennas at primary transmitters, we can significantly boost the successful transmission probability of cognitive users; thereby improving the spectrum utilization efficiency of the wireless communication networks.

On Downlink Intercell Interference in a Cellular System

Abstract: The main difference between a non-cellular and a cellular communication system is the intercell interference. Therefore, modelling the intercell interference and analyzing its effects is of particular interest for cellular communication systems. On the one hand, the intercell interference can be modeled by system level simulations. On the other hand, it is also meaningful to assess the intercell interference without performing exhaustive simulations which nonetheless at the same time still capture the major effects that determine the interference. To this end, we consider the intercell interference as a random variable composed of many other random variables. In this paper, we present a semi-analytical method to analyze the downlink intercell interference in a cellular system. Through such an approach a quick and reliable assessment of the downlink intercell interference can be obtained. Our focus is on the methodology and we consider basic access schemes: WCDMA, TDMA, FDMA, and random OFDMA. However, our model can be extended to include other access schemes and other features.

Relay-enhanced Cellular Performance of OFDMA-TDD System for Mobile Wireless Broadband Services

Abstract: Multi-hop relay (MR) and repeater are useful means of improving system throughput and coverage in the cellular mobile packet access system, as carrier-to-interference ratio can be improved with deploying them in the heavily shadowed region. In this paper, we investigate the bandwidth efficiency and the associated service outage performance for the different relay scenarios, using the system level simulation for a cellular OFDMA-TDD system. It is demonstrated that a capacity gain by an optical repeater which has a simple amplify-and-forwarding capability only could be minimal due to co-channel interference from all repeaters in the neighbor cells, even though they are usually useful for warranting the outage performance with a multiple order of combining gain, especially in the destructive area, e.g., edge of sub-cell coverage. Meanwhile, multi-hop relays are shown to increase the average system capacity (almost doubling the system throughput) by fully reusing the frequency in every relay station, while improving the per-user data rate in the cell edges or heavily shadowed areas.

Improved Measurement-Based Frequency Allocation Algorithms for Wireless Networks

Abstract: This paper presents three algorithms that outperform all other published work for allocating a limited number of orthogonal frequency channels to access points (APs) in wireless networks. Unlike other work, we minimize interference seen by both users and APs, we use a physical rather than binary model for interference, and we mitigate the impact of rogue RF interference. Our three algorithms have different mechanisms of switching the channels of APs based on the in- situ interference measured at clients and/or APs. The convergence of the algorithms is proven and characterized. Our algorithms consistently yield high throughput gains irrespective of network topology, the level of AP activity, and the number of controlled APs, rogue interferers, and available channels. We outperform the best published work by 15% and 18% for mean and median user throughputs respectively, and 81%, 168%, and 1011% for 25, 20, and 15 percentiles of user throughputs, respectively.

Characterization of co-channel interference in a wireless communication system

Abstract: Disclosed herein is a method for characterizing interference in a radio communication system including a plurality of radio transceiver stations configured to communicate with user equipments, the method including: measuring, by a user equipment in radio communication with a serving transceiver station, quantities indicative of interference from interfering transceiver stations; sending the measured quantities to the serving transceiver station; associating at least one codebook with the serving transceiver station, the codebook including a number of codewords each representing a respective quantized interference situation in an area covered by the serving transceiver station; and • identifying in the codebook a codeword representative of the quantities measured by the user equipment.

Interference Tolerant Agile Cognitive Radio: Maximize Channel Capacity of Cognitive Radio

Abstract: In this article we propose a novel Dynamic Spectrum Access (DSA) paradigm to combine the benefits of UWB transmission and cognitive radio technology to optimize the efficiency of wireless spectrum usage and maximize the channel capacity Specifically, this work presents a multi-carrier platform not only transmits over unused spectrum "holes", but transmits over underused spectrum blocks as well, while minimizing interference to existing primary narrowband transmissions It is also shown that current cognitive radio and UWB transmission are two special cases of the proposed general paradigm As a direct result, this paradigm offers a high degree of flexibility of spectrum use for next generation wireless communication systems

Design of next-generation cdma using orthogonal complementary codes and offset stacked spreading

Abstract: This article presents an innovative code-division multiple access system architecture that is based on orthogonal complementary spreading codes and time-frequency domain spreading. The architecture has several advantages compared to conventional CDMA systems. Specifically, it offers multiple-access-interference-free operation in AWGN channels, reduces co-channel interference significantly, and has the potential for higher capacity and spectral efficiency than conventional CDMA systems. This is accomplished by using an "offset stacked" spreading modulation technique followed by quadrature amplitude modulation, which optimizes performance in a fading environment. This new spreading modulation scheme also simplifies the rate matching algorithms relevant for multimedia services and IP-based applications.

Wireless repeater apparatus for canceling interference signal

Abstract: There is provided a micro integrated wireless repeater apparatus for canceling an interference signal, including: a receiving means for receiving a repetition signal; an analog interference cancellation means for generating an interference cancellation signal according to a control signal and removing an interference signal from the repetition signal received from the receiving means; a digital interference cancellation means for canceling a residual interference signal remaining in a repetition signal obtained by canceling an interference signal component by the analog interference cancellation means; a control means for controlling the analog interference cancellation means by transmitting the control signal to the analog interference cancellation means according to control information received from the digital interference cancellation means; and a transmitting means for transmitting a repetition signal obtained by canceling a residual interference signal component by the digital interference cancellation means.

Signal transmission in a wireless communication system

Abstract: Interference management is provided through use of a user-based interference control and/or a network-based interference control. For user-based interference control, the terminals are informed of the inter-sector interference observed by the neighbor sectors and can adjust their transmit powers accordingly so that the inter-sector interference is maintained within acceptable levels. For network-based interference control, each sector is informed of the inter-sector interference observed by the neighbor sectors and regulates data transmissions for its terminals such that the inter-sector interference is maintained within acceptable levels. Each system may utilize only user-based interference control, or only network-based interference control, or both.

Uplink Intercell Interference and Capacity in 3G LTE systems

Abstract: In this paper, we study the inter-cell interference in the uplink of 3G LTE systems. The multiple access is based on SC-FDMA. This is an orthogonal access (no intra-cell interference). However, inter-cell interference remains as there is no orthogonality between cells. We then model the inter-cell interference by calculating the distribution of the number of collisions, taking into account the impact of power control. We then use this model to calculate the user capacity of the system, by mean of a Markovian analysis. We finally use the developed model to evaluate and compare different interference mitigation schemes.

Asynchronous Co-channel Interference Suppression in MIMO OFDM Systems

Abstract: We present algorithms to suppress the asynchronous co-channel interference (CCI) in MIMO OFDM systems, which is becoming the dominant limiting factor in the performance of the emerging high-density WLANs. The key challenge is that the cyclic prefix of the interference signal does not line up with that of the intended signal due to asynchronous transmission in WLAN. Therefore, the orthogonality among the different tones of the interference signal is destroyed and conventional frequency domain minimum mean square error (MMSE) cancelation techniques that measure the interference channel response for each tone can not work effectively. To suppress the asynchronous interference, we design an efficient estimator to measure the interference spatial covariance matrix using Cholesky decomposition and low-pass smoothing. Both a MMSE and a maximum a posteriori (MAP) receiver are derived based on the estimated interference statistics. Simulation results demonstrate the effectiveness of our solution.

Radio communication system, and communication method thereof

Abstract: PROBLEM TO BE SOLVED: To perform cognitive radio communication without any mutual interference between Primary and Secondary systems. SOLUTION: An access point AP of the Secondary system estimates an amount of interference given to a base station BS by the access point AP of the Secondary system, based on a downlink signal in a frequency band f1 transmitted from the base station BS of the Primary system 100. When the amount of interference is larger than a threshold, the frequency band f1 is set to be an operating frequency band. When the given amount of interference is smaller than a threshold, a frequency band f2 is set to be an operating frequency band. Scheduling information, which is transmitted from the base station BS of the Primary system 100 to subscriber stations SS#1-SS#3, is acquired. Based on the scheduling information and the amount of interference, interference given to the Primary system 100 and interference from the Primary system 100 are avoided. COPYRIGHT: (C)2009,JPO&INPIT

On the Need for Knowledge of the Phase in Exploiting Known Primary Transmissions

Abstract: Recently Tarokh and others have raised the possibility that a cognitive radio might know the interference signal being transmitted by a strong primary user in a non-causal way and use this knowledge to increase its data rates. However, there is a subtle difference between knowing the signal transmitted by the primary and the actual interference experienced at our receiver since there is a wireless channel between these two points. We show that even an unknown phase results in a substantial decrease in the data rates that can be achieved, and thus there is a need to feedback interference channel estimates to the cognitive transmitter. We give a scheme that uses appropriate "training" to obtain such estimates and quantify this scheme's required overhead as a function of the relevant coherence time and interference power.

Closed loop adaptive clock RFI mitigation

Abstract: A method according to one embodiment for mitigating radio frequency interference by identifying system clocks, identifying active radio channels, measuring clock harmonics in or near the active radio channels, determining potential interference occurring if the clocks were moved to new fundamental frequencies, and shifting clock fundamental frequencies to reduce interference to the active radio channels based on existing interference and the potential interference of a plurality of new fundamental frequencies. Of course, many alternatives, variations, and modifications are possible without departing from this embodiment.

Co-Channel Interference Mitigation in MIMO-OFDM System

Abstract: OFDMA (orthogonal frequency division multiple access) has emerged as a promising physical layer technology for 4G wireless networks. Along with the requirement for higher system capacity, how to improve cell edge performance to provide better QoS (quality of service) support has become a pressing issue. Considering the channel characteristic in wireless communication, new interference avoidance based algorithms, which consider both time domain and frequency domains, are proposed to mitigate the influence of frequency selective channel on cell edge user in MIMO-OFDM system. Meanwhile, in order to further improve the system spectral efficiency, an algorithm that integrates beamforming is put forward. Simulation results show that the proposed algorithms can improve the throughput of cell edge user effectively as well as the overall system throughput.

Capacity of MIMO channels in the presence of co-channel interference

Abstract: This paper presents analytical results on the capacity of multiple-input-multiple-output (MIMO) fading channels in the presence of co-channel interference (CCI). We consider the scenario in which the desired and CCI users are all subject to Rayleigh fading. We assume that channel realizations of both the desired and CCI users are known at the receiver. Moreover, we consider the case where the transmitter does not have any CSI and as such equal-power allocation among transmit antennas is used. Given this setup, we derive the moment generating function (MGF) and the mean of the mutual information (MI). We then study the complementary cumulative distribution function of the MI using a Gaussian approximation. Finally, we present and discuss numerical examples to illustrate the mathematical formalism and to show the effect of various parameters on the capacity of MIMO channels in the presence of CCI. Copyright © 2006 John Wiley & Sons, Ltd.

Compensation of non-reciprocal interference in adaptive MIMO-OFDM cellular systems

Abstract: In a time-division-duplex communication system, the channel knowledge can be obtained at the transmitter side due to channel reciprocity and it can be used to increase the spectral efficiency of a multiple-input multiple-output (MIMO) communications. However, the interference structure between transmission directions does not necessarily correlate. The obtained quality of service at the receiver may differ significantly from the desired one if the transmission parameters are assigned based on the reverse link measurements only. In this paper, the performance of an orthogonal frequency division multiplexing (OFDM) cellular system with adaptive MIMO transmission is studied in the presence of non-reciprocal inter-cell interference when the downlink interference structure is known at the receiver and only limited feedback information about the interference is available at the transmitter. The results are compared to those with perfectly known interference structure per each sub-carrier. The system level impact of realistic interference non-reciprocity scenarios is studied via network simulations. Linear minimum mean squared error (MMSE) filter is applied at the receiver to suppress the impact of structured inter-cell interference together with a simple and bandwidth efficient closed-loop compensation algorithm. Both link and system level simulation results show that the proposed compensation algorithm with a simple scalar power offset feedback combined with interference suppression at the receiver results in nearly the same performance as the ideal case

Satellite interference canceling

Abstract: A satellite interference canceling system cancels the interference between two or more signals received by a satellite receiver. The signals can be two signals experiencing cross polarization interference or signals that experience interference from other satellite signals. Gain and phase are applied to the received signals and then combined with the other signals to result in cancellation by subtraction. The gain and phase values needed to cancel the interference are derived from measurements of carrier-to-noise ratio (C/N) as an indication of the interference level. The C/N can be measured in the set-top box indoor unit. Coherency restoration is performed in receivers that downconvert the signals before performing interference cancellation.