Wireless Channel Coefficients Research Articles

Copula based analysis of impact of wireless channels correlation on the physical layer security performances in a wireless wiretap channel with artificial noise

AbstractIn this article, a wireless wiretap channel with artificial noise assisted transmission and active eavesdropper is considered, where, wireless channel coefficients are correlated. Aiming at analyzing wireless channels correlation effects on the physical layer security, we exploit copula theory and obtain closed form expressions for the probability of positive secrecy rate, average secrecy rate, and generalized secrecy outage probability. Then, the obtained analytical results are evaluated numerically under positive dependence, independence and negative dependence correlation structures and it is shown that in different scenarios, the effect of correlated fading on the security performance can be beneficial or harmful.

Implementation of a Fast Link Rate Adaptation Algorithm for WLAN Systems

With a target to maximize the throughput, a fast link rate adaptation algorithm for IEEE 802.11a/b/g/n/ac is proposed, which is basically preamble based and can adaptively compensate for the discrepancy between transmitter and receiver radio frequency performances by exploiting the acknowledgment signal. The target system is a 1 × 1 wireless local area network chip with no null data packet or sounding. The algorithm can be supplemented by automatic rate fallback at the initial phase to further expedite rate adaptation. The target system receives wireless channel coefficients and previous packet information, translates them to amended signal-to-noise ratios, and then, via the mean mutual information, selects the modulation and coding scheme with the maximum throughput. Extensive simulation and wireless tests are carried out to demonstrate the validity of the proposed adaptive preamble-based link adaptation in comparison with both the popular automatic rate fallback and ideal link adaptation. The throughput gain of the proposed link adaptation over automatic rate fallback is demonstrated over various packet transmission intervals and Doppler frequencies. The throughput gain of the proposed algorithm over ARF is 46% (15%) for a 1-tap (3-tap) channel over 10 m–250 m (16 m–160 m) normalized Doppler frequencies. Assuming a 3-tap channel and 30 m–50 m normalized Doppler frequencies, the throughput of the proposed algorithm is about 31 Mbps, nearly the same as that of ideal link adaptation, whereas the throughput of ARF is about 24 Mbps, leading to a 30% throughput gain of the proposed algorithm over ARF. The firmware is implemented in C and on Xilinx Zynq 7020 (Xilinx, San Jose, CA, USA) for wireless tests.

Channel Estimation With Pilot Reuse in IQ Imbalanced Massive MIMO

In this paper, we investigate channel estimation with pilot reuse for massive MIMO with in-phase and quadrature-phase imbalances (IQI). Firstly, we obtain an augmented real-valued representation for the received signal by processing the real and imaginary parts individually with relative compensation. Then based on the obtained augmented real-valued representation, we perform minimum mean square error (MMSE) estimation for the effective channel which comprises the IQI matrix and wireless channel coefficients. Pilots are reused to reduce the pilot overhead. A lower bound of the effective channel estimation MSE is obtained. Motivated by the optimal conditions under which the lower bound can be achieved, we propose a pilot schedule algorithm. Finally, a least square based method to obtain the relative IQI compensation coefficients is provided. Numerical results verify the performance of the proposed effective channel estimation with pilot reuse.

Physical Layer Secret Key Generation in Static Environments

Two legitimate parties, referred to as Alice and Bob, wish to generate secret keys from the wireless channel in the presence of an eavesdropper, referred to as Eve, in order to use such keys for encryption and decryption. In general, the secret key rate highly depends on the coherence time of the channel. In particular, a straightforward method of generating secret keys in static environments results in ultra-low rates. In order to resolve this problem, we introduce a low-complexity method called induced randomness. In this method, Alice and Bob independently generate local randomness to be used together with the uniqueness of the wireless channel coefficients in order to enable high-rate secret key generation. In this work, two scenarios are considered: first, when Alice and Bob share a direct communication channel, and second, when Alice and Bob do not have a direct link and communicate through an untrusted relay. After exchanging the induced randomness, post-processing is done by Alice and Bob to generate highly-correlated samples that are used for the key generation. Such samples are then converted into bits, disparities between the sequences generated by Alice and Bob are mitigated, and the resulting sequences are then hashed to compensate for the information leakage to the eavesdropper and to allow consistency checking of the generated key bit sequences. We utilize semantic security measures and information-theoretic inequalities to upper bound the probability of successful eavesdropping attack in terms of the mutual information measures that can be numerically computed. Given certain reasonable system parameters this bound is numerically evaluated to be $2^{-31}$ and $2^{-10.57}$ in the first and the second scenario, respectively.

Ultra wideband channel coefficient measurements for detecting methane gas in a multipath environment

In this paper, an investigation was carried out into the effect of a non-explosive methane gas mixture on various ultra wide band channel coefficients. Using simplified apparatus consisting of a rubberised tube (6 mm diameter) connected for significant periods to either a (pressurised) source of mixed methane and nitrogen or alternatively to a nitrogen only source. The tube containing either gas feed was wrapped using two complete turns around a 160 mm diameter PVC pipe. Several flows of the mixed gas, containing 2.57% methane, were introduced and flowed through the pipe before being sealed in the tube. Removal of any methane in the tube was achieved by flushing it with the pure nitrogen source, initially and in-between samples of the mixed source. A Vector Network Analyser was connected using two identical directional antennas with the wrapped section of tubing placed between the two antennas and the wireless channel coefficients were measured over the ultra wide band frequency span of 0.3–8 GHz. Magnitude differences were taken between the baseline condition (tube flushed out with nitrogen) and alternatively containing the methane mixture. It was found that consistent and repeatable experiments produced the same trend of differences over the same frequency span. This system based on this approach could thus be used as a simple sensor to detect the accumulation of methane gas in an environment well before it becomes explosive at around a concentration of 5%.

Secret Key Establishment Using Temporally and Spatially Correlated Wireless Channel Coefficients

When implementing data encryption and decryption in a symmetric cryptosystem, secure distribution of the secret key to legitimate nodes can be a challenge. In this paper, we consider establishing secret keys using the common wireless channel, with particular emphasis on the spatial and temporal correlations of the channel coefficients. Specifically, we investigate the influence of channel correlation on the bound of the key size generated from the common channel using a simple single-input single-output channel model, and we verify the existence of a sampling approach able to generate a key using the minimum possible sampling window. We also explore decorrelation of the channel coefficients in a multiple-input multiple-output channel, and we use a statistical independence test to demonstrate that this process cannot be separated into spatial and temporal decorrelation processes. The insights gained from these studies assist in the development of a practical key generation protocol based on a published channel coefficient quantization method and incorporating flexible quantization levels, transmission of the correlation eigenvector matrix, and LDPC coding to improve key agreement in an authenticated public channel. Finally, we present simulations with real channel measurements that solidify the fundamental conclusions.

Spatial Link Scheduling for SCDMA Multihop Cellular Networks: A Cross Layer Framework

of error based Spatial Code Division Multiple Access scheduling algorithm is presented in this paper to systematically reuse the orthogonal CDMA codes in a given cell for Multihop Cellular Network. We assign and reuse the CDMA codes to peer-to-peer links such that the probability of error in all scheduled links are below certain threshold. The proposed scheduling algorithm PoE-LinkSchedule involves two phases. In the first phase we present a scheduling metric Probability of Error (PoE) as a function of first and second order statistics of wireless channel coefficients between nodes. The second phase presents a graph theoretical as well as PoE based centralized scheduling algorithm. For a graph of network with n number of nodes, U number of links and ? thickness, the proposed scheduling algorithm has computational complexity of O(Unlogn + Un?) as opposed to O(U U ) in the case of exhaustive search algorithm. The performance of the proposed algorithm is evaluated in terms of spatial reuse and end-to-end throughput. We show that the proposed algorithm has considerably higher end-to-end throughput and higher spatial reuse compared to existing link scheduling algorithms.