To support multimedia services with different discrete bit-rate requirements, families of multilength optical codes, such as the carrier-hopping prime code (CHPC), extended CHPC, and multiwavelength optical orthogonal code, were recently constructed for wavelength-time optical code-division multiple-access (O-CDMA). In this paper, the performances of these multilength optical codes in a multimedia O-CDMA system with a variable-weight operation are analyzed. Our study shows that short-length codes generate stronger interference than long-length codes. This supports services prioritization in O-CDMA. Our study also shows that code weight is a more important factor than code length in determining code performance.

In this letter, a homogeneous multi-radio channel-hopping (CH) rendezvous algorithm that is backward compatible with any number of radios for cognitive radio networks is proposed. This algorithm is applicable to CH sequences that have uniform time-to-rendezvous (TTR) patterns and also achieves improved (exact) maximum-TTR, which is a linear function of a number of available channels instead of quadratic functions in heterogeneous multi-radio algorithms.

We propose a modified bipolar-bipolar code for wavelength-time optical code-division multiple access. The new code uses Gold sequences for time spreading, in place of Barker sequences in our previously proposed bipolar-bipolar code. The new code has a cardinality of N+1 times greater than the original code, because there exist N+2 Gold sequences, but only one Barker sequence, for a given code length N. Since the new code is modified from the original code, there is no increase in hardware complexity. The average performance of the new code is analyzed and found very close to that of the original code. In other words, the new code can support greater cardinality without worsening average performance or increasing hardware complexity

In this paper, one family of identification-(ID)-based and one family of non-ID-based asynchronous channel-hopping (CH) sequences are constructed for supporting cognitive-radio wireless networks (CRWNs) in a homogeneous channel setting. The novelty lies in the use of two-dimensional (2-D) algebraic algorithms to properly arrange the linear- and quadratic-congruence sequences in the prime field; channel rendezvous are guaranteed to occur among all the elements (i.e., licensed channels) in at least one of the columns between any two CH matrices. As a result, the new asynchronous CH sequences always have uniform time-to-rendezvous (TTR) structures and satisfy the desirable design criteria of short periods, full degree-of-rendezvous (DoR), and even channel use (ECU). The studies show that the new constructions have the best balance between the full DoR, ECU, short period, small TTR mean and variance, and shortest maximum-time-to-rendezvous (MTTR) and maximum-first-time-to-rendezvous (MFTTR) in their respective categories of constructions. A short period can save memory in small sensors and mobile devices and are suitable for the Internet-of-Things and device-to-device communications. A short MTTR/MFTTR and a small TTR mean/variance can improve the rendezvous frequency and shorten the latency, thus enhancing and stabilizing the throughput.

In this paper, a family of channel-hopping (CH) sequences, so-called multi-MTTR asynchronous-asymmetric prime sequences (MAAPSs), for cognitive radio networks (CRNs) is designed and analyzed. They have known and enlarged cardinality (for the benefit of user identification), full degree of rendezvous, and pairwise shift invariant. For the first time, the MAAPSs can be partitioned into multiple subsets of CH sequences with adjustable maximum time-to-rendezvous (MTTR). This unique property supports a trade-off between cardinality and MTTR in scalable CRNs. With a channel-contention-resolution mechanism in use, the rendezvous-success (RS) rate and variance of the MAAPSs in a practical CRN are analyzed. With short MTTR and smaller RS-rate variance, the MAAPSs can support quicker, more stable and uniform rendezvous processes than their counterparts. Finally, the analytical models of two combined metrics of TTR and RS rate, so-called maximum and average time-to-rendezvous-success, are formulated and validated with computer simulation.

We propose a "synchronous" wavelength-time optical code-division multiple access (CDMA) scheme using 2D Walsh-based bipolar-bipolar code, in which Walsh code sequences are used for time spreading, in place of Barker sequences used in our previously proposed ldquoasynchronousrdquo bipolar-bipolar code. The new synchronous coding scheme supports larger cardinality and better performance than the asynchronous scheme because Walsh code has a larger cardinality than Barker sequences and has a zero cross-correlational property at the expense of system synchronization. One potential application is future CDMA-based passive optical networks, in which synchronous downstream traffic uses the Walsh-based bipolar-bipolar code and asynchronous upstream traffic uses the Barker-based counterpart. Since the new code is modified from the original code, the same hardware can be used for both directions of traffic.