This paper is focused on the optimal design and performance analysis of a novel class of no-hit-zone frequency-hopping sequences (NHZ-FHSs) in quasi-synchronous FH multiple-access (QS-FHMA) systems. Although traditional NHZ-FHSs can offer interference-free FHMA performance when the signal arrival delay (τ) does not exceed the width of NHZ (Znh), i.e., |τ|≤Znh, the same performance cannot be guaranteed for |τ|>Znh in which traditional NHZ-FHZs may have large Hamming correlation (which denotes the total number of frequency hits of sequences) and consequently poor error probability performance. Since the strict quasi-synchronization (i.e., |τ|≤Znh) may be hard to maintain at all times in practical FHMA networks (e.g., infrastructureless Ad hoc networks), it is also important to minimize the Hamming correlation for time-shifts outside of the NHZ. Such FHSs are called strong NHZ-FHSs (SNHZ-FHSs) in this paper. We derive a lower bound on the maximum Hamming correlation outside of the NHZ and then present a design algorithm for the optimal SNHZ-FHS set meeting our proposed lower bound with equality. We analyze the bit-error-rate (BER) performance of the FHMA system employing the proposed SNHZ-FHS sets using the average Hamming correlation function. Theoretical analysis and simulation results show that the proposed optimal SNHZ-FHS sets are feasible for practical FHMA networks with relaxed timing requirement and enhanced BER performance.

This letter is concerned with tackling both of the multi-access interference (MAI) and the follower jamming (FJ) in frequency-hopping multiple-access (FHMA) systems via novel FH sequence (FHS) design. We consider low-hit-zone (LHZ) FHS which provides lower MAIs in quasi-synchronous FHMA systems but suffers from FJ attack. We introduce an improved LHZ-FHS with wide-gap (i.e., WG-LHZ-FHS) for the neighboring carrier frequencies so as to combat the MAI and the FJ. We first present a general method to generate new WG-FHS sets and then construct the first known WG-LHZ-FHS sets with optimal periodic Hamming correlation (PHC) properties. Besides, the error-rate performance of the constructed WG-LHZ-FHS set imposed in an FHMA system under FJ is investigated. The numerical analysis and the system simulation results indicate that the proposed WG-LHZ-FHS set provides the better error-rate performance to combat the FJ as well as satisfactory capability of MAI suppression due to its combined merits of WG and LHZ.

Energy harvesting has been recognized as a good paradigm to decrease traditional grid energy expenditure, which caters for 5G visions on the green evolution of cellular networks. However, the harvested energy at each base station is random and intermittent, which imposes a great challenge to reliably satisfy the time-varying users' wireless traffic requirements. What's worse, the perfect probability distributions of harvested energy and wireless traffic are hard to obtain in practical systems, making it difficult to apply the conventional optimization approach to cope with the uncertainties in the energy management problems. Inspired by this fact, we propose a distributionally robust two-stage stochastic optimization framework to minimize the expected total energy cost of mobile network operators (MNOs) in the finite time horizon. To circumvent the computational difficulty of this stochastic optimization problem, we define a novel ambiguity set to capture the uncertainties of the harvested energy and wireless traffic based on their first- and second-order statistics information. By employing the distributionally robust optimization (DRO) methodology, we equivalently transform the original two-stage stochastic programming problem into a computationally tractable second-order cone programming problem. Then, a distributionally robust two-stage energy management algorithm (DRTEMA) is proposed, which has good performance of low complexity and only requires partial information of the stochastic parameters. Rigorous computational complexity analysis and extensive numerical simulations are conducted to show the advantages of the proposed algorithm.

This paper is concerned with the enabling of massive connectivity in a multi-cluster network. In order to tackle significant amount of interference imposed by inter- and intra-cluster users, we consider a frequency-hopping (FH) system where an improved quasi-orthogonal hopping pattern, called optimal strong no-hit-zone FH sequence (SNHZ-FHS) set, is adopted in each cluster. We propose a flexible construction of SNHZ-FHS sets and derive their Hamming correlation values at different access delays. Since the maximum number of supportable users under traditional FH networks is strictly limited by the number of frequency slots, we investigate the design of a novel class of adaptive array beam-forming (BF) receiver in which SNHZ-FHS patterns (i.e., SNHZ-FH/BF) are reused over asynchronous multi-cluster uplink channels. Extensive numerical comparisons show that our proposed SNHZ-FH/BF system provides an effective means for attaining higher user capacity as well as remarkable capability of interference suppression.

Uniformity, good Hamming‐correlation and long period are critical requirements for frequency‐hopping (FH) sequence set. A new class of no‐hit zone FH (NHZ‐FH) sequence set is designed based on chaotic nonlinear map to meet above three demands. Some properties of the proposed sequence set and its performance for frequency‐hopping‐multiple‐access (FHMA) system are investigated. The results show that the proposed NHZ‐FH sequence set can offer more robustness to eavesdropper and lower BER for quasi‐synchronous FHMA system in comparison with other traditional FH sequence sets.

In frequency hopping (FH) sequence design, the wide gap is an important performance indicator. In this paper, the average Hamming correlation of wide gap FH sequence (WG-FH sequence) sets with low hit zone, which has not yet been reported previously, is studied. A lower bound on the average Hamming auto-correlation and the average Hamming cross-correlation of low-hit-zone FH sequence sets is firstly derived. The new lower bound includes the bound for conventional FH sequence sets derived by Peng et al. as a special case. Then a construction of WG-FH sequence sets with multiple low hit zones is presented, which are optimal by the new bound for these low hit zones. Finally, another class of optimal WG-FH sequence sets with multiple low hit zones is presented, which have larger wide gap.