Realistic Channel Model Research Articles

On Performance of Cooperative Communication Systems with Spatial Random Relays

Cooperative communications can significantly enhance the performance of wireless systems by exploiting the inherent spatial diversity gains. In this work, a decode-and-forward cooperative cellular system with spatial random users is investigated. Under a realistic channel model including path loss, shadowing and multipath fading, the users are selected as relays depending on their instantaneous SNR so that error propagation caused by detection errors at the relays can be mitigated. The locations of users are modeled as random point processes. Based on the theory of point processes, an analytic approach to system performance is developed to accommodate the random location of users as well as the underlying channel. The locations of the selected relays are investigated and the outage performance averaging over all possible user locations is analyzed. The accuracy of the analytical results is verified by Monte-Carlo simulations. Various issues, such as power allocation, are also investigated using the numerical results.

Orthogonal Signaling-based Sensing Data Reporting for Cooperative Spectrum Sensing in Cognitive Radio

Cognitive radio (CR) features opportunistic access to spectrum when licensed users (LU) are not operating. To avoid interference to LU, cognitive users (CU) need to perform spectrum sensing. Because of local shadowing, fading, or limited sensing capability, it is suggested that multiple CUs cooperate to detect LU. In cooperative spectrum sensing, CUs should exchange their sensing data with minimum bandwidth and delay. In this paper, we introduce a novel method to efficiently report sensing data to the central node in an infrastructured OFDM-based CR network. All CUs simultaneously report their sensing data over unique and orthogonal signals on locally available subcarriers. By detecting the signals, the central node can determine subcarrier availability for each CU. Implementation challenges are identified and then their solutions are suggested. The proposed method is evaluated through simulation on a realistic channel model. The results show that the proposed method is feasible and efficient.

차량간 안전 통신에서 짧은 지연시간을 보장하는 비콘 스케줄링 알고리즘

The safety applications based on the IEEE 802.11p, periodically transmit the safety-related information to all surrounding vehicles with high reliability and a strict timeline. However, due to the high vehicle mobility, dynamic network topology and limited network resource, the fixed beacon scheduling scheme excess delay and packet loss due to the channel contention and network congestion. With this motivation, we propose a novel beacon scheduling algorithm referred to as spatial-aware(SA) beacon scheduling based on the spatial context information, dynamically rescheduling the beaconing rate like a TDMA channel access scheme. The proposed SA beacon scheduling algorithm was evaluated using different highway traffic scenarios with both a realistic channel model and 802.11p model in our simulation. The simulation results showed that the performance of our proposed algorithm was better than the fixed scheduling in terms of throughput, channel access delay, and channel load. Also, our proposed algorithm is satisfy the requirements of vehicular safety application.

Low complexity RAKE receiver for TH-based multiuser UWB system with realistic UWB indoor channel

We have considered multiuser communication scenario with ultra wideband (UWB) communication. UWB device is transmitting the data by time hopping (TH)-based system, TH-PPM, TH-PAM and DS-UWB. The UWB device is transmitting data over multipath rich channel model recommended by IEEE 802.15.3a working group. This channel model is based on Saleh-Valenzuela model. For better performance at receiver side, adaptive equaliser is required; this increases the complexity of receiver. Here we have considered the RAKE receiver for TH-based UWB system. This RAKE receiver structure is not a conventional RAKE, but it is pre-RAKE structure. In this case, we have evaluated performance of system by considering the RAKE receiver at the transmitter side. Here we have assumed the condition that channel sounding is available before transmission for quasi static UWB channel. Based on this knowledge RAKE fingers are selected for pre-RAKE. UWB channel is multipath rich channel so full RAKE adds more complexity in receiver and hence put limitation on effective data rate. Here we have considered low branch RAKE receiver. Performance of the system model is evaluated with realistic UWB channel model.

Impact of the radio channel modelling on the performance of VANET communication protocols

The expected traffic safety and efficiency benefits that can be achieved through the development and deployment of vehicular ad-hoc networks has attracted a significant interest from the networking research community that is currently working on novel vehicular communication protocols. The time-critical nature of vehicular applications and their reliability constraints require a careful protocol design and dimensioning. To this aim, adequate and accurate models should be employed in any research study. One of the critical aspects of any wireless communications system is the radio channel propagation. This is particularly the case in vehicular networks due to their low antenna heights, the fast topology changes and the reliability and latency constraints of traffic safety applications. Despite the research efforts to model the vehicle-to-vehicle communications channel, many networking studies are currently simplifying and even neglecting the radio channel effects on the performance and operation of their protocols. As this work demonstrates, it is critical that realistic and accurate channel models are employed to adequately understand, design and optimize novel vehicular communications and networking protocols.

Orthogonal signaling-based queue status investigation method in IEEE 802.11

IEEE 802.11 specifies four different medium access control (MAC) protocols to coordinate multiple access in a wireless local area network (WLAN). Since several tens of stations can operate in a WLAN, the performance of MAC protocols is important for overall network efficiency. It has been observed that the IEEE 802.11 MAC protocols can be improved by knowing which station has a non-empty queue, i.e., queue status. The point coordination function (PCF) can use this information to avoid polling a station that has no pending data. The HCF controlled channel access can adapt polling parameters based on queue status information, especially when scheduling a bursty and variable bit-rate traffic. Previously suggested methods are rather limited in terms of accuracy and efficiency. In this paper, we propose a novel method to investigate the queue status of multiple stations by exploiting orthogonal signaling. With synchronous transmission of orthogonal codes and symbol level signal processing, the method allows all of the associated stations to report their queue status at the same time. Challenges that can arise in the implementation of the proposed method are identified, and their solutions are suggested. The feasibility of detecting orthogonal signals is thoroughly tested on a realistic channel model. To demonstrate the performance improvement of a MAC protocol, we applied the proposed method to PCF. Both analysis and simulation show that the modified PCF significantly outperforms not only the original PCF but also other previously suggested PCF enhancements.

Exploiting Channel Angular Domain Information for Precoder Design in Distributed Antenna Systems

In this paper, the benefit of linear precoding along with distributed orthogonal space time block codes (OSTBCs) is exploited to improve the performance of distributed antenna system (DAS). In particular, we propose a novel precoder design method based on channel angular domain parameters, i.e., average angle of arrival (AOA), angular spread (AS), normalized antenna spacing (NAS) and Ricean K-factor, instead of the conventional channel correlation matrix which has been widely used to characterize MIMO channel. Built on the distributed characteristics of DAS, a more realistic non-Kronecker channel model is established, and then the optimal precoder is designed as a function of the above angular domain parameters. Asymptotic analysis reveals that, the optimal precoder acts as a combination of channel precoder and power allocator. Furthermore, the impact of the above angular domain parameters on system performance, including diversity order and coding advantage, is investigated in detail. Specifically, we find that better gain can be achieved with larger K-factor. Finally, our theoretical claims are confirmed by the numerical results.

MIMO Downlink Scheduling with Non-Perfect Channel State Knowledge

Downlink scheduling schemes are well-known and widely investigated under the assumption that the channel state is perfectly known to the scheduler. In the multiuser MIMO (broadcast) case, downlink scheduling in the presence of non-perfect channel state information (CSI) is only scantly treated. In this paper we provide a general framework that addresses the problem systematically. Also, we illuminate the key role played by the channel state prediction error: our scheme treats in a fundamentally different way users with small channel prediction error (predictable users) and users with large channel prediction error (non-predictable users), and can be interpreted as a near-optimal opportunistic time-sharing strategy between MIMO downlink beamforming to predictable users and space-time coding to non-predictable users. Our results, based on a realistic MIMO channel model used in 3GPP standardization, show that the proposed algorithms can significantly outperform a conventional mismatched scheduling scheme that treats the available CSI as if it was perfect.

Efficacy of Multiband OFDM Approach in High Data Rate Ultra WideBand WPAN Physical Layer Standard using Realistic Channel Models

Multiband Orthogonal Frequency Division Multiplexing (MBOFDM) approach using UWB signals with short duration of pulses provide unique advantages in short-range high data rate wireless applications which include easy penetration through obstacles, high precision ranging and low processing power. In this paper a performance study on MB-OFDM is attempted for high data rate ultra wideband (UWB) WPAN physical layer standard according to modulation techniques such as QPSK, 16-QAM, 64-QAM etc. Different multipath components in the UWB channels are characterized by different delays and attenuation. The channel models chosen in simulation of MB-OFDM UWB communication system are different from standard UWB channels considering realistic multipath resolution and operating frequencies. In this research different amplitude fading statistics for UWB channel (IEEE 802.15.3a) are analysed. The BER performance of MB-OFDM UWB on modified UWB channel models are investigated through simulation study. It is observed that QPSK modulation for the MB-OFDM UWB system performs better than 16-QAM and 64-QAM in the CM3 (NLOS) and CM4 (NLOS) channel environment. We show that, at low data rate (55 Mbps) the MB-OFDM system performs better in the CM4 channel environment than in the CM3, CM2 and CM1 channel model due to its inherent frequency diversity. In high data rate (480 Mbps), the BER performance in CM1 channel model is found to be better than in CM2, CM3, CM4 channel model.

Post-Time-Reversed MIMO Ultrawideband Transmission Scheme

A post-time-reversed multiple-input multiple-output ultrawideband (UWB) transmission scheme is proposed, which enhances the energy capture and improves the system robustness against channel estimation error when compared with the pre-time-reversed UWB scheme and the classical (no TR) scheme. The advantages are: an improvement in bit-error-rate; and flexibility in a fast varying or extremely complex environment. Following the theoretical model description, simulation results are reported over two types of channels, the IEEE 802.15.3a proposed channel model, and a realistic office environment channel model. The results demonstrate the promise of the proposed scheme for higher data rates in a harsh environment.

Relay selection and power allocation in cooperative cellular networks

We consider a system with a single base station communicating with multiple users over orthogonal channels while being assisted by multiple relays. Several recent works have suggested that, in such a scenario, selection, i.e., a single relay helping the source, is the best relaying option in terms of the resulting complexity and overhead. However, in a multiuser setting, optimal relay assignment is a combinatorial problem. In this paper, we formulate a related convex optimization problem that provides an extremely tight upper bound on performance and show that selection is, almost always, inherent in the solution. We also provide a heuristic to find a close-to-optimal relay assignment and power allocation across users supported by a single relay. Simulation results using realistic channel models demonstrate the efficacy of the proposed schemes, but also raise the question as to whether the gains from relaying are worth the additional costs.

Blind linear equalization of PPM signals using third-order moments

A novel blind equalization strategy for pulse position modulation (PPM) based on maximizing the third-order moment of the equalizer output is presented. Compared to traditional fourth-order (e.g. kurtosis-based) methods, third-order moments give faster convergence and are less sensitive to noise. This work demonstrates that the intersymbol interference that plagues typical indoor ultra wideband (UWB) channels can be combatted using a third-moment maximizing blind equalizer which could therefore provide a cold start-up for a decision-directed scheme. Adaptation is shown to be asymptotically globally convergent with increasing time-hopped PPM frame length. Simulation experiments compare the performance to standard fourth-order schemes in practical settings, taking into account the conditions of time-hopping,multiple-access interference and a realistic UWB channel model.

Deterministic capacity of information flow in molecular nanonetworks

Molecular communication enables nanomachines to exchange information with each other by emitting molecules to their surrounding environment. Molecular nanonetworks are envisioned as a number of nanomachines that are deployed in an environment to share specific molecular information such as odor, flavor, or any chemical state. In this paper, using the stochastic model of molecular reactions in biochemical systems, a realistic channel model is first introduced for molecular communication. Then, based on the realistic channel model, we introduce a deterministic capacity expression for point-to-point, broadcast, and multiple-access molecular channels. We also investigate information flow capacity in a molecular nanonetwork for the realization of efficient communication and networking techniques for frontier nanonetwork applications. The results reveal that molecular point-to-point, broadcast, and multiple-access channels are feasible with a satisfactorily high molecular communication rate, which allows molecular information flow in nanonetworks. Furthermore, the derived molecular channel model with input-dependent noise term also reveals that unlike a traditional Gaussian communication channel, achievable capacity is affected by both lower and upper bounds of the channel input in molecular communication channels.

Cooperative MIMO channel models: A survey

Cooperative multiple-input multiple-output technology allows a wireless network to coordinate among distributed antennas and achieve considerable performance gains similar to those provided by conventional MIMO systems. It promises significant improvements in spectral efficiency and network coverage and is a major candidate technology in various standard proposals for the fourth-generation wireless communication systems. For the design and accurate performance assessment of cooperative MIMO systems, realistic cooperative MIMO channel models are indispensable. This article provides an overview of the state of the art in cooperative MIMO channel modeling. We show that although the existing standardized point-to-point MIMO channel models can be applied to a certain extent to model cooperative MIMO channels, many new challenges remain in cooperative MIMO channel modeling, such as how to model mobile-to-mobile channels, and how to characterize the heterogeneity and correlation of multiple links at the system level appropriately.

Congestion Awareness Multi-Hop Broadcasting for Safety Message Dissemination in VANET

The safety applications for cooperative driving in VANETs, typically require the dissemination of safety-related information to all vehicles with high reliability and a strict timeline. However, due to the high vehicle mobility, dynamic traffic density, and a self-organized network, Safety message dissemination has a special challenge to efficiently use the limited network resources to satisfy its requirements. With this motivation, we propose a novel broadcasting protocol referred to as congestion awareness multi-hop broadcasting (CAMB) based loosely on a TDMA-like transmission scheduling scheme. The proposed protocol was evaluated using different traffic scenarios within both a realistic channel model and an 802.11p PHY/MAC model in our simulation. The simulation results showed that the performance of our CAMB protocol was better than those of the existing broadcasting protocols in terms of channel access delay, packet delivery ratio, end-to-end delay, and network overhead.

CRBAR: Cooperative relay-based auto rate MAC for multirate wireless networks

Cooperative communication is becoming a promising technology for wireless networks by exploiting two key characteristics of the wireless medium: broadcast nature and spatial diversity. To integrate cooperation into practical systems, efficient protocols are needed across the entire protocol stack. This paper presents a novel cooperative relay-based auto rate medium access control (MAC) protocol (CRBAR) for multirate wireless networks, in which low-rate stations can be adaptively assisted by high-rate stations. Different from state-of-the-art table-based proactive relay selection schemes, a reactive relay selection scheme is developed to adapt to dynamical channel variation and network topology in which the relay candidates adaptively select themselves as the relay nodes and determine the relay scheme and transmission rates based on the instantaneous channel measurements. In terms of relay schemes, cooperative relaying can be employed whenever possible in addition to simple relaying. The receiver can improve its capability to decode the original frame by combining the information from both the sender and the relay. We have developed analytical models of different schemes for a simple scenario and carried out further simulations under a realistic fading channel and mobility model. Both analysis and simulation show that CRBAR significantly outperforms the table-based proactive relay selection scheme and direct rate adaptation scheme.

Flow uniformity optimization for large size planar solid oxide fuel cells with U-type parallel channel designs

The U-type parallel channels for large size planar solid oxide fuel cell (pSOFC) are systematically optimized with the computational fluid dynamics (CFD) method. The CFD calculations are based on realistic 3D gas channel models and typical pSOFC working parameters. The optimized geometric parameters include the height of interconnect ribs, aspect ratios of the inlet-header and outlet-header cross-sections, the sum of inlet- and outlet-header widths and the ratio of the outlet-header width to the inlet-header width ( α). Detailed CFD calculations show that a suitable α and a relatively large header width are necessary for the flow uniformity of both air and fuel in large size pSOFCs. In particular, α is demonstrated to be a key parameter for the flow uniformity of large size pSOFCs with U-type parallel channel designs and a proper choice of α is of critical engineering importance. The physical origin for the importance of α on the flow distribution is analyzed.

Soft-Switching Hybrid FSO/RF Links Using Short-Length Raptor Codes: Design and Implementation

Free-space optical (FSO) links offer gigabit per second data rates and low system complexity, but suffer from atmospheric loss due to fog and scintillation. Radio-frequency (RF) links have lower data rates, but are relatively insensitive to weather. Hybrid FSO/RF links combine the advantages of both links. Currently, selection or is performed between FSO or RF links depending on feedback from the receiver. This technique is inefficient since only one medium is used at a time. In this paper, we develop a soft-switching scheme for hybrid FSO/RF links using short-length Raptor codes. Raptor encoded packets are sent simultaneously on both links and the code adapts to the conditions on either link with very limited feedback. A set of short-length Raptor codes (k = 16 to 1024) are presented which are amenable to highspeed implementation. A practical Raptor encoder and decoder are implemented in an FPGA and shown to support a 714 Mbps data rate with a 97 mW power consumption and 26360 gate circuit scale. The performance of the switching algorithms is simulated in a realistic channel model based on climate data. For a 1 Gbps FSO link combined with a 96 Mbps WiMAX RF link, an average rate of over 472 Mbps is achieved using the implemented Raptor code while hard-switching techniques achieved 112 Mbps on average.

Pulse antenna permutation and pulse antenna modulation: two novel diversity schemes for achieving very high data-rates with unipolar MIMO-UWB communications

In this paper, we consider the problem of applying the Multiple-Input-Multiple-Output (MIMO) techniques on Impulse-Radio Time-Hopping Ultra-Wideband (IR-TH-UWB) communications. In particular, we propose two novel Space-Time (ST) block codes that are suitable for UWB. The proposed encoded MIMO-UWB schemes present the main advantage of conveying the information only through the positions of the very short unipolar UWB pulses. The constraint of unipolar transmissions keeps the transceiver structures very simple since it imposes no additional constraints on the RF circuitry to control the amplitudes or the phases of the sub-nanosecond UWB pulses. Consider the case where the transmitter is equipped with P antennas and where M PPM modulation positions are available. The first proposed scheme achieves a full transmit diversity order for M ges P while transmitting at the rate of log <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (M) bits Per Channel Use (PCU). The second scheme is fully diverse with any number of antennas and transmits at a rate of M log <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (P)/P bits PCU. The proposed codes permit to achieve different levels of compromise between complexity and performance since scheme 1 necessitates M-dimensional Maximum-Likelihood (ML) decoding while scheme 2 necessitates MP-dimensional decoding. We also present a comprehensive analysis on the enhancement in terms of the data rate achieved at a certain communication distance based on realistic indoor channel models and on an exact system model that takes inter-pulse-interference and intersymbol- interference into consideration.

Collision model for performance analysis of coded transmission in time hopping impulse radio over multipath nakagami-m channels

Time hopping is the strategy employed in impulse radio systems to increase the communication reliability by spreading the multiple access interference (MAI) over numerous time intervals. This results in a non-stationary interference that makes the error probability performance to be dominated by the worst-case. In this paper we employ a collision model analysis for this time-varying interference that is ruled by the system loading in term of the corresponding MAI collision probability. The model proves to be simple and accurate to characterize the decision variable conditioned to the collision events. This provides a reliable framework for evaluating the error probability in multipath faded channels with RAKE receiver either for coded and uncoded systems. In this paper we derive closed form approximations of the bit-error-rate for multipath channels with exponential power delay profile and Nakagami-m fading. Validations by numerical analysis show that the collision model provides an accurate framework to evaluate also performance of realistic channel models.