Introduction to CDMA

Using the enhanced data rates for GSM evolution (EDGE) system with cyclic frequency hopping as an example, we apply a Kalman-filter power control method based on interference tracking to packet voice service in wireless networks. Our results show that the power-control method significantly improves the spectral efficiency by enabling the 1/3 frequency reuse while maintaining a stringent requirement of 2% packet loss probability for voice service. Specifically, for allocated spectrum of 1.8, 3.6 and 5.4 MHz, the 1/3 reuse with the Kalman power control can yield 102.5%, 49.5% and 32.5% improvement in spectral efficiency, respectively, over the 3/9 reuse (regardless of whether or not power control is used). We also compare the performance of the Kalman method with a traditional signalto-interference-ratio method and a control method that is based on the last interference measurement. We find that appropriate selection of power for the first packet of each talk spurt and the filtering function for noisy measurements are crucial in providing high system capacity for packet voice service. For the EDGE system, we also identify a need for shortening the power update period, which is 480 ms in the specifications.

Based on chaotic map, a new method for generating chaotic frequency-hopping (FH) sequences is proposed. The real-valued trajectory' of chaotic system is first encoded to binary sequences by means of a quantization function, and then we can obtain the chaotic FH sequences by reshaping bits of these binary sequences. Theory analysis and performance experiment results show that this sequence is Bernoulli sequence and its Hamming correlation is shown to be Poisson distributed. It is comparable to other FH sequences on the properties of uniform distribution, Hamming correlation and linear complexity when they have the same number of frequency slots and the same period, but its map iterative number has reduced largely. Consequently, more FH sequences can be generated by this method and they are suitable for FH code-division multiple-access systems.

In this paper the bit error probability of the SFH/MFSK system in presence of wideband and tone interference in a fading environment is evaluated. Simultaneous license-free operation of tone-like applications, Frequency Hopping (FH), and Direct Sequence (DS) based LAN clusters is assumed to be the prime interference facing the subject FH wireless LAN (of the IEEE 802.11 standard). The wideband DS interference is considered to have a sinc2() shape Power Spectral Density (PSD) centered at the middle of FH signal band and tone interferers are assumed to be spread over the whole FH band of the intended signal. Assumptions of slow non-selective fading along with slow hopping justify a quasi-static treatment. Results shows that the FH system is more sensitive to tone interference and fading and less sensitive to wideband interference variations. The paper sheds light on the performance of wireless LAN operation in the unlicensed free band where many other LANs may exist.

A new series of frequency hopping (FH) sequences, designated as n-order prime sequences, are constructed based on the multiplication of extension Galois fields GF(P n ) with P an odd prime and integer n≥2. The result provides more sequences, longer period and more families compared with prime sequences. And after partition, FH sequences in the same group show ideal Hamming auto- and cross-correlation properties.

The past decades of development of underwater acoustic communications (acomms) have produced many advances in physical layer algorithms, in hardware and firmware capabilities tailored to the underwater environment, and in concepts for networked sensors. In general, most deployed systems and most conceptual networks have assumed a strictly half duplex channel wherein all links must be shared over a bandwidth-constrained channel. This has led to an almost universal insistence on avoiding collisions among receptions, which might be achieved by TDMA schemes or by adapting other network concepts derived from (RF) mobile communications technology. For long duration deployment of underwater systems, any requirement for tight TDMA access forces the modem developer to use power-hungry high accuracy clocks, thus quickly draining precious battery resources. While networked acknowledgements at the speed of light make perfect sense, the latency of the acoustic channel demands excessive consumption of time to enable the network simply to know that a message has ultimately been received. There are many studies which attempt to devise minimally intrusive ARQ approaches, but ultimately acknowledgements in some form are necessary. In this paper we revisit some older technology, combine it with new capabilities in hardware, and add some signal processing insights to support discussion of a back channel providing asynchronous, multi-access command and control of a network, with supervised use of a second acoustic channel for high data rate, priority, and/or large data volume transfer. The two contending modulation schemes for asynchronous signalling have long been frequency hopping and direct sequence (DS), both of which are (typically) non-coherently processed spread spectrum methods. For a given bandwidth and data rate in additive white Gaussian (AWGN) the performance of the two is virtually identically. However, in a dynamic, multipath-rich environment, the well-recognized requirement for power control for DS signalling, given the constraint of channel latency, makes FH signalling the preferred choice. Justification for this is found throughout the development of the (NATO) JANUS underwater protocol, which itself is a frequency hopping scheme. In the context of power control, it is possible to transfer the burden of interference mitigation from the multiple transmitters to the receiver at a single modem by judicious spectral whiting and clipping of too loud interference. This not only compensates for variable strength transmissions, but for external sources of interference. Our anecdotal experience shows that a single modem operating this way in an 8 kHz band at a center frequency of 39 kHz can readily accommodate asynchronous reception of at least five messages from irregularly-spaced transmitters. Hardware is now available to support integration of two or more modems into a single system, especially to provide dual frequency acomms. From the hardware/system perspective the main constraint is providing sufficient spectral isolation between the two channels. In our experience, this requires physical separation and spectral filtering between the transducers to overcome high power sidelobe interference which is at best 50 dB below the level of the primary channel. However, given that these constraints are accommodated, we are positioned to consider development of media access control (MAC) and networking systems that benefit from this full duplex approach. We describe an FH signal as defined for the JANUS application and show how it can be used in multi-access applications. We provide an overview of the physical infrastructure we deploy to accommodate the two frequency bands, and we describe a possible network infrastructure utilizing these capabilities. Our goal is to promote discussion of alternatives to conventional half duplex TDMA and conflict avoidance acomms schemes.

Large-scale multiple-input multiple-output (MIMO) communication systems can bring substantial improvement in spectral efficiency and/or energy efficiency, due to the excessive degrees-of-freedom and huge array gain. However, large-scale MIMO is expected to deploy lower-cost radio frequency (RF) components, which are particularly prone to hardware impairments. Unfortunately, compensation schemes are not able to remove the impact of hardware impairments completely, such that a certain amount of residual impairments always exists. In this paper, we investigate the impact of residual transmit RF impairments (RTRI) on the spectral and energy efficiency of training-based point-to-point large-scale MIMO systems, and seek to determine the optimal training length and number of antennas which maximize the energy efficiency. We derive deterministic equivalents of the signal-to-noise-and-interference ratio (SINR) with zero-forcing (ZF) receivers, as well as the corresponding spectral and energy efficiency, which are shown to be accurate even for small number of antennas. Through an iterative sequential optimization, we find that the optimal training length of systems with RTRI can be smaller compared to ideal hardware systems in the moderate SNR regime, while larger in the high SNR regime. Moreover, it is observed that RTRI can significantly decrease the optimal number of transmit and receive antennas.

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.

In this paper, we derive a new bound on maximum nontrivial Hamming correlation of frequency hopping (FH) sequences from the Singleton bound in error correcting code literature, and we discuss the relation between the new bound and the known ones on FH sequences. Further, we construct two classes of FH sequences from punctured Reed–Solomon codes and one class of FH sequences from polynomial functions, which meet the new bound.

This paper makes preliminary steps at exploring a novel application of cognitive radios (CRs) for reliable satellite communications. We propose the use of dynamically adjusted frequency hopping (FH) sequences for satellite transmissions. Such sequences are more robust against smart eavesdropping and targeted interference than fixed FH sequences. In our approach, FH sequence is adjusted according to the outcome of out-of-band sensing, carried out by a CR module that resides in the satellite itself or at the receiving ground station. Our protocol, called OSDFH, relies on exploiting the spectrum sensing capabilities of CRs for proactive detection of channel quality. We analyze the characteristics of the proposed OSDFH using a finite state Markov chain (FSMC) framework. Level crossing rate (LCR) analysis is used to determine the transition probabilities of the Markov chain. These probabilities are then used to measure the "channel stability," a metric that reflects the freshness of sensed channel interference. We use simulations to study the effects of different system parameters on the performance of our proposed protocol.

Spectral efficiency is always a key factor to be improved and optimized along mobile communication networks evolving generation by generation. 5G enabling technologies must take spectral efficiency into consideration. In this paper, we show the performance of three key 5G technologies in sense of spectral efficiency improvement. Sparse code multiple access, polar codes, and filtered orthogonal frequency-division multiplexing are novel multiple access technology, channel coding scheme, and waveform, respectively. The combination of them is implemented in a 5G field trial testbed by NTT DOCOMO and Huawei for the first time. According to the field test results, we achieve over 100% spectral efficiency improvement comparison with baseline, where orthogonal frequency-division multiple access and turbo coding as LTE are used.

The usable satellite spectrum is becoming scarce due to static spectrum allocation policies. Cognitive radio approaches have already demonstrated their potential towards spectral efficiency for providing more spectrum access opportunities to secondary user (SU) with sufficient protection to licensed primary user (PU). Hence, recent scientific literature has been focused on the tradeoff between spectrum reuse and PU protection within narrowband spectrum sensing (SS) in terrestrial wireless sensing networks. However, those narrowband SS techniques investigated in the context of terrestrial CR may not be applicable for detecting wideband satellite signals. In this paper, we mainly investigate the problem of joint designing sensing time and hard fusion scheme to maximize SU spectral efficiency in the scenario of low earth orbit (LEO) mobile satellite services based on wideband spectrum sensing. Compressed detection model is established to prove that there indeed exists one optimal sensing time achieving maximal spectral efficiency. Moreover, we propose novel wideband cooperative spectrum sensing (CSS) framework where each SU reporting duration can be utilized for its following SU sensing. The sensing performance benefits from the novel CSS framework because the equivalent sensing time is extended by making full use of reporting slot. Furthermore, in respect of time-varying channel, the spatiotemporal CSS (ST-CSS) is presented to attain space and time diversity gain simultaneously under hard decision fusion rule. Computer simulations show that the optimal sensing settings algorithm of joint optimization of sensing time, hard fusion rule and scheduling strategy achieves significant improvement in spectral efficiency. Additionally, the novel ST-CSS scheme performs much higher spectral efficiency than that of general CSS framework.

Frequency hopping spread spectrum and direct sequence spread spectrum are two main spread coding technologies. Frequency hopping sequences are needed in FH-CDMA systems. In this paper, a construction of optimal sets of frequency hopping sequences is presented via cyclotomy. The construction is based on the set-theoretic characterization of an optimal set of FH sequences. As a consequence, new optimal sets of FH sequences are obtained.

Visible Light Communication (VLC) is among one-of-the top emerging next-generation technology and a key-enabler for the future networks, utilizing the visible light spectrum to achieve nominal data rates of Gbps. The compatibility of VLC with the existing infrastructure proves to be a major thrust in this domain, but suffers from signal fading and considerable interference provided by the nearby illuminants. Thus, a cooperative gain stage becomes essential to boost the signal present in the Non-line-of-sight (NLOS) with minimum power overheads. This paper proposes an optimal solution for the spectral efficiency of the VLC link: by introducing relay-based adaptive power-sharing schemes for improvement in signal strength, under the constraints of LED linear radiation region and threshold power consumption. DCO-OFDM modulation technique has been used to achieve minimum inter-symbol interference owing to its circular convolution properties. The Analytic analysis, as well as simulation results, are presented to show the effect of the scheme over an uncontrolled relay link. Later, a generalized model is proposed, and analytic methodology is adopted to determine optimal power-sharing coefficients. As a Figure of merit, we observe 18.29 % improvement in spectral efficiency at 13 dB of signal to noise ratio (SNR) when compared to an unoptimized collaborative link.

Minimum Shift Keying (MSK) is considered to be a spectrally efficient modulation scheme, in comparison with other families of constant envelope modulation. However, its main sidelobe can be of some worry in digital data transmission, particularly over nonlinear channels. Therefore, it is practically of interest to search for efficient pulse shaping schemes, largely proposed either through baseband pulse shaping and/or filtering processing. In this paper a new pulse shaping method based on Walsh functions is proposed to be used in MSK-type signaling. A methodology is proposed for designing the optimum pulse shape; and this attains improvement in spectral efficiency (i.e., bps/Hz) over straight MSK, for the same channel bandwidth. The fractional out-of-band power is used to compare the behavior of the system under the new proposed technique. As well, error rate performance in such system is used in the evaluation of this work. Results reveal that good improvement can be attained in spectral efficiency at the expense of error rate performance when these functions are used.

As the demand for high-speed, low-latency, and energy-efficient mobile communications continues to surge with the proliferation of IoT, AR/VR, and ultra-reliable applications, traditional Distributed Radio Access Network (D-RAN) architectures face critical limitations. Cloud Radio Access Network (C-RAN) emerges as a promising alternative that centralizes baseband processing to improve scalability, resource utilization, and operational flexibility. This paper presents a comprehensive evaluation of C-RAN architecture, focusing on structural models, fronthaul technologies, and cloud-based service logic. A detailed mathematical modeling framework is developed to assess key performance indicators, including latency, spectral efficiency, energy efficiency, and fronthaul capacity. Extensive results demonstrate that C-RAN achieves up to 45% gains in energy efficiency, a 35% improvement in spectral efficiency, and latency reductions of over 40% compared to D-RAN. Additional results reveal enhanced handover success rates, better BBU pool utili-zation, and increased reliability, with packet loss rates reduced to under 0.5%. Despite increased fronthaul bandwidth requirements, optical solutions such as DWDM and PON mitigate the bottleneck effectively. The findings confirm that C-RAN offers a robust, scalable, and cost-efficient solution for 5G and future mobile networks, enabling dynamic resource allocation, advanced interference management, and centralized network intelligence. The paper also addresses implementation challenges, including fronthaul provisioning and security, and outlines future research directions such as virtualization, AI-driven orchestration, and edge-cloud integration to fully harness the potential of C-RAN in ultra-dense and heterogeneous network environments.