Adaptive frequency hopping in HF environments

Advanced Trends in Wireless Communications, 2011

To improve the performance of short-range wireless communications, channel quality must be improved by avoiding interference and multi-path fading. Frequency hopping spread spectrum (FHSS) is a transmission technique where the carrier hops from frequency to frequency. For frequency hopping a mechanism must be designed so that the data can be transmitted in a clear channel and avoid congested channels. Adaptive frequency hopping is a system which is used to improve immunity toward frequency interference by avoiding using congested frequency channels in hopping sequence. Mathematical modelling is used to simulate and analyze the performance improvement by using frequency hopping spread spectrum with popular modulation schemes, and also the hopping channel situations are investigated. In this chapter the focus is to improve wireless communication performance by adaptive frequency hopping which is implemented by selecting sets of communication channels and adaptively hopping sender's and receiver's frequency channels and determining the channel numbers with less interference. Also the work investigates whether the selected channels are congested or clear then a list of good channels can be generated and in practice to use detected frequency channels as hopping sequence to improve the performance of communication and finally the quality of service. The Fourier transform mathematical modules are used to convert signals from time domain to frequency domain and vice versa. The mathematical modules are applied to represent the frequency and simulate them in MATLAB and as result the simulated spectrums are analysed. Then a simple two-state Gilbert-Elliot Channel Model (Gilbert, 1960; Elliott, 1963) in which a two-state Markov chain with states named "Good" and "Bad" is used to check if the channels are congested or clear in case of interference. Finally, a solution to improve the performance of wireless communications by choosing and using "Good" channels as the next frequency hopping sequence channel is proposed. 2. Review of related theories 2.1 Spread spectrum Spread spectrum is a digital modulation technology and a technique based on principals of spreading a signal among many frequencies to prevent interference and signal detection. As

Int'l J. of Communications, Network and System Sciences, 2010

To improve the performance of short-range wireless communications, channel quality must be improved by avoiding interference and multi-path fading. Frequency hopping spread spectrum (FHSS) is a transmission technique where the carrier hops from frequency to frequency. For frequency hopping a mechanism must be designed so that the data can be transmitted in a clear channel and avoid congested channels. Adaptive frequency hopping is a system which is used to improve immunity toward frequency interference by avoiding using congested frequency channels in hopping sequence. In this paper mathematical modelling is used to simulate and analyze the performance improvement by using FHSS with popular modulation schemes, and also the hopping channel situations are investigated.

2017 8th International Conference on the Network of the Future (NOF)

There are different techniques used by Machineto-machine (M2M) communications technologies to mitigate collision problem and data loss. One of these techniques is Frequency Hopping (FH), which is used by Weightless-N technology with a special random channel selection algorithm. In such a system, the probability of a message collision mainly depends on the randomisation algorithm used to access channels. This paper provides a novel randomisation algorithm for the channel selection process of the Weightless-N system. The new proposed algorithm is based on a uniform randomisation distribution and called a Uniform Randomisation Channel Selection Technique (URCST). This new algorithm provides a better system performance and lower probability of collision. In addition, it is faster and easier than the Mersenne Twister algorithm.

4th National Conference of Telecommunication Technology, 2003. NCTT 2003 Proceedings., 2003

Multipath fading in HF channels causes time-delay spread that limits the maximum symbol rate, and frequency selective fading that attenuates some frequency bands compared to others. Multi-carrier modulation techniques, Multi-carrier ASK (MCASK) and Multi-camer PSK (MCPSK), can be used to ensure reliability of transmission and high data transmission rate. The bit-error rate (BER) performance is compared between single frequency modulation such as Amplitude Shiil-Keying (ASK) or Phase Shiil-Keying (PSK) and them ulti-carrier modulation in the presence o f additive white Gaussian noise and a combination of multipath fading and additive white Gaussian noise. On the average, the BER performance is better for the multi-carrier modulation compared to the single carrier modulation under multipath fading conditions.

1997 IEEE 47th Vehicular Technology Conference. Technology in Motion, 1997

Modeling of the mobile radio channel is considered. Starting from a physical model, a frequency domain description of the channel is obtained. It is then shown that the effect of fading and dispersion can be entirely modeled using frequency transform techniques. Due to the bandlimitation of the simulated stochastic processes, the channel impulse response (CIR) model works on discrete time samples in both time and delay direction. Besides its computational efficiency an advantage of the technique is that the channel center frequency and the normalization unit (ie. the sample duration Ts) for the physical delay spread remain as generic parameters. The generic property of the model is its major advantage especially in the evaluation of new, unspecified scenarios. Due to the block-oriented processing, the fading-bandwidth-delayspread product is limited for this modeling technique. Applications to typical mobile radio data transmission scenarios conclude the paper.

IEEE Journal on Selected Areas in Communications, 1990

The interleaving span of coded frequency-hopped (FH) systems is often constrained to be smaller than the decoder memory length, i.e., nonideal interleaving is performed. An upper bound on the performance of a Viterbi decoder of a convolutional code with nonideal interleaving is presented. Also, a soft decision diversity combining technique is introduced, and the performance of combined convolutional and diversity coding subject to worst case partial band noise jamming is investigated. Optimization of the FH system performance subject to constraints of allowed delay and synthesizer settling time provides the best combination of interleaving span and hopping rate. The FH system considered employs M-ary frequency-shift key (MFSK) modulation and noncoherent demodulation with 2 b soft decision based on Viterhi's ratio-threshold technique.

IEEE Transactions on Wireless Communications, 2002

We evaluate the performance of recently proposed dynamic frequency hopping (DFH) when applied to cellular systems with a limited total bandwidth. We also illustrate a practical implementation for DFH deployment using network-assisted resource allocation (NARA). The performance evaluation is accomplished by system-level simulations of a system with 12 carriers and 1/1 frequency reuse, based on the EDGE-Compact specification. Voice-only circuit-switched operation is assumed. Fading channel, multicell interference, voice activity, and antenna sectorization are modeled. We present the performance of dynamic frequency hopping compared to random frequency hopping and fixed channel assignment by showing distributions of word error rates. Sensitivity to occupancy, Rayleigh fading assumptions, number of carriers, voice activity, and measurement errors are studied. We also compare the uplink and downlink performance. The results indicate that DFH can significantly improve the performance compared to random frequency hopping. For example, at a 2% frame error rate with 90% coverage, the capacity improvement of DFH is almost 100% when compared with fixed channel assignment, and about 50% when compared to random frequency hopping. The amount of improvement for the uplink direction is smaller than the improvement for the downlink direction, especially for higher occupancies.

IEEE Transactions on Information Theory, 1989

IEEE Journal on Selected Areas in Communications, 2000

A wireless LAN standard developed by IEEE committee P802.11 operates in the unlicensed 2.4 GHz ISM band. This band is a very hostile environment due to many unpredictable interference sources, such as microwave ovens, utilizing the same frequency bands. At the same time the allowed signal power density that can be used is limited in order to minimize interference to other users in the same band. In order to avoid these interference and keep low signal power density the standard supports both direct sequence (DS) and frequency hopping (FH) modulation for these applications. The new test beds for multimedia wireless (WLAN's) also use FH modulation. In this paper, we analyze the performance of a modification of the FH multiple frequency shift keying (MFSK) system that includes a multitone multiamplitude MFSK signal which will be designated as MFSK modulation. In this case, in order to meet the transmitted power density limits imposed in ISM band, the signal energy is split into separate tones. This makes the system more vulnerable to noise and fading, but still the overall flow of useful information will be increased. The results demonstrate that under the large range of the signal, channel and interference parameters this system offers better performance.

The International Conference on Electrical Engineering (Print), 2014

Frequency Hopping Spread Spectrum (FHSS) is a technique that wireless devices communicate such that the transmission frequencies are alternated in a predetermined ordered hopping pattern. In tradition, the hopping sequence is controlled by using a pseudo-random code sequence (PN code) known only to both transmitter and receiver. In this paper, the carrier frequency of a frequency shift keying FSK system is interfered by the surrounding environment. Thus, FHSS will be used instead to switch between multiple carriers within the available bandwidth to minimize the interference. Such a system requires that the generated PN code ensures that all carrier frequencies within the hopping pattern should be semiequally used during transmission. Several simulation scenarios were carried out to compare between a single long PN code and an accumulated short one. Finally, the developed FH system is implemented using an AD9910 Direct Digital Synthesizer (DDS) and an ALTERA Cyclone-III FPGA chip. Results proved that, according to the system requirements, an accumulated short PN code offers noticeable residual correlations and provides a higher level of autocorrelation compared to a single long PN code. Results, also, verified that the developed FH system provides lower Bit Error Rate (BER) than the FSK one.