Sub-channel Bandwidth Research Articles

Energy Efficient Subchannel and Power Allocation in Cooperative VLC Systems

This letter studies the energy efficiency (EE) optimization of cooperative visible light communication (VLC) systems while considering the inter-cell interference and the lineof-sight blockage problems. Specifically, the optimization problem of subchannel and power allocation to maximize EE under transmit power budgets and users' minimum rate constraints is considered. The formulated problem turns out to be a difficult nonlinear fractional program for which a low-complexity iterative solution based on fractional programming theory and the quadratic transform approach is proposed. Extensive simulations are conducted to show the efficacy of the proposed scheme over conventional approaches. In addition, the outage analysis and impacts of varying the transmit power and the subchannel bandwidth on the EE performance are investigated.

Rate maximizing OFDM parameter optimization on HF channels

In this work, we propose an algorithm which maximizes the bit rate of a given wideband OFDM (Orthogonal Frequency-Division Multiplexing) system operating in the HF (High Frequency) band. The proposed algorithm is built on the work given in the literature for estimating the electron densities in the ionosphere and estimating the propagation path of signals throughout the ionosphere. Algorithm uses the outputs of the ray tracing programs in order to calculate the channel parameters for using to optimize the parameters of the HF communication system. Proposed algorithm helps to determine the capacity of the subchannels, transmitter power, number of subchannels, carrier frequency, subchannel bandwidth and cyclic prefix for the OFDM in order to maximize the bit rate. Carrier frequencies produced by the proposed algorithm are compared with the frequencies suggested by the VOACAP (Voice of America Coverage Analysis Program) program.

A Frequency-Diversity System With Diversity Encoder and OFDM Modulation

In this work, a diversity encoder maps vectors of N modulated symbols to vectors of N diversity symbols using an orthogonal matrix. Each modulated symbol appears in N diversity symbols. An Orthogonal-Frequency-Division-Multiplexing (OFDM) modulator converts the diversity vectors to OFDM symbols. The number of subcarriers in the OFDM symbol equals N. Each diversity symbol modulates one subcarrier; therefore, each modulated symbol is transmitted by N orthogonal subcarriers. The diversity bandwidth of the modulated symbols is N times the subchannel bandwidth. The modulated symbols experience independent fading gains on each subchannel since the diversity bandwidth (OFDM bandwidth) is chosen to be multiple of the coherence bandwidth of the transmission channel. In the receiver, an OFDM demodulator converts back the OFDM symbols to vectors of diversity symbols. The diversity detector maps the diversity vectors to vectors of modulated symbols. The noise samples at the output of the diversity detector are uncorrelated. Maximum likelihood and linear decorrelator detectors are used as diversity detectors. The transmitter and the receiver of this work are implemented using Field-Programable-Gate-Array (FPGA) technology. The performance of the implemented system is the same as the performance of the N channels diversity system with the Maximal-Ratio-Combiner (MRC) receiver.

Telecommunication Systems with Multiple Access Based on Data Factorial Coding

The paper outlines methods for constructing telecommunication systems based on the principle of time division of channels and using factorial code in order to maintain the cyclic phase of receiver/transmitter and ensure information security. These systems provide integrated data protection (against errors in communication channel and attempts of unauthorized access to information), as well as the ability to allocate system resources (bandwidth) between its subscribers. The purpose of this work is to develop and research the approach of building telecommunication systems with multiple access to a common channel for collective use, based on the property of factorial codes to keep clock and frame synchronism with a working signal. The basic idea is to choose the permutations to transfer information with a single set of properties or attributes. This provides opportunities of dividing channel into subchannels, bandwidth sharing on transmission accuracy, and dividing plurality of network users into closed groups. The channel resource sharing procedure based on factorial coding is described. Examples of multiplex cycle organization and the order of occupying/releasing multiplex cycle time channels by users are given. The system load and the subchannel bandwidth are estimated.

Scheduling Policies for Federated Learning in Wireless Networks

Motivated by the increasing computational capacity of wireless user equipments (UEs), e.g., smart phones, tablets, or vehicles, as well as the increasing concerns about sharing private data, a new machine learning model has emerged, namely federated learning (FL), that allows a decoupling of data acquisition and computation at the central unit. Unlike centralized learning taking place in a data center, FL usually operates in a wireless edge network where the communication medium is resource-constrained and unreliable. Due to limited bandwidth, only a portion of UEs can be scheduled for updates at each iteration. Due to the shared nature of the wireless medium, transmissions are subjected to interference and are not guaranteed. The performance of FL system in such a setting is not well understood. In this paper, an analytical model is developed to characterize the performance of FL in wireless networks. Particularly, tractable expressions are derived for the convergence rate of FL in a wireless setting, accounting for effects from both scheduling schemes and inter-cell interference. Using the developed analysis, the effectiveness of three different scheduling policies, i.e., random scheduling (RS), round robin (RR), and proportional fair (PF), are compared in terms of FL convergence rate. It is shown that running FL with PF outperforms RS and RR if the network is operating under a high signal-to-interference-plus-noise ratio (SINR) threshold, while RR is more preferable when the SINR threshold is low. Moreover, the FL convergence rate decreases rapidly as the SINR threshold increases, thus confirming the importance of compression and quantization of the update parameters. The analysis also reveals a trade-off between the number of scheduled UEs and subchannel bandwidth under a fixed amount of available spectrum.

Throughput Optimization of Multi-BSS IEEE 802.11 Networks with Universal Frequency Reuse

For IEEE 802.11 networks with multiple basic service sets (BSSs), most studies have focused on how to allocate different frequency sub-channels to BSSs for minimizing the co-channel interference. With the significant increase of the sub-channel bandwidth, however, it becomes increasingly important to study the network performance with universal frequency reuse. In this paper, we focus on an uplink $M$ -BSS IEEE 802.11 network, where all the BSSs share the frequency band rather than operate at different sub-channels. By dividing the nodes in each BSS into multiple groups according to the set of access points (APs) they can be heard by, the steady-state points of $M$ BSSs in saturated conditions are obtained as the functions of the number of nodes in each group and the initial backoff window size of nodes of each BSS. The maximum network throughput is further characterized by optimally choosing the initial backoff window sizes of all the nodes and shown to be closely dependent on the percentage of nodes that can be heard by multiple APs. The comparison with orthogonal frequency division reveals that although the maximum network throughput is degraded due to interference among BSSs, a higher network data rate can still be achieved by universal frequency reuse, which makes it a preferable option for multi-BSS IEEE 802.11 networks.

Access granularity control of multichannel random access in next-generation wireless LANs

As the next-generation Wireless LANs (WLANs) will provide the ubiquitous high-data-rate network coverage, the traditional contention-based Medium Access Control (MAC) scheme may be unable to fulfill the requirement of efficient channel access. To address this problem, several research works have proposed the random access systems combined with Orthogonal Frequency Division Multiplex Access (OFDMA) technology. Access granularity control is a crucial issue in this combination. Specifically, this issue focuses on how to tune the subchannel bandwidth and the number of accessible subchannels towards the maximum channel utilization. This paper analyzes access granularity control in an OFDMA system that adopts a multichannel Carrier Sensing Multiple Access (CSMA) MAC and resolves contention by the frequency-domain backoff. The theoretical analysis verifies the significance of access granularity control. In addition, the simulation experiments demonstrate that the proposed dynamic access granularity control algorithms notably outperform the traditional ones that divide channel band statically and adjust the number of accessible subchannels empirically.

An OFDM-aware Reservation Random Access Protocol for Interference Mitigation in OFDMA Femtocells

An efficient random access protocol utilizing the frequency diversity of OFDMA is proposed for mitigating femtocell interference in a decentralized cellular network. The proposed OFDM reservation random access (OFDM-RR) protocol builds upon a generalized version of the frequency-domain backoff scheme, which was originally proposed for WiFi networks. In this protocol, each node chooses one of K subchannels randomly and transmits a short reservation request on the corresponding subchannel. At the end of the slot, the node whose subchannel index is the highest wins the contention and immediately transmits on the full channel. A tie on the highest index is treated as a collision and is resolved by repeating the process with only those nodes involved in the collision. The throughput of this protocol is derived and compared with that of the classic reservation Aloha (R-Aloha) with the same total channel bandwidth. Although channelization increases the transmission time of the reservation packet in OFDM-RR (due to the smaller subchannel bandwidth), the collision resolution mechanism, which reduces the collision rate sharply, helps improve the throughput, particularly in the high-load region. For example, while R-Aloha almost collapses (near zero throughput) at an average load of ten requests/slot, the proposed protocol can provide a throughput of more than 55% at the same load using only two subchannels.

An Energy and Cost Efficient WDM/OFDMA PON System: Design and Demonstration

In this paper, we propose an energy and cost efficient WDM/OFDMA PON system, referred to as NEWOPS. Based on a new two-tiered semi-tree architecture, NEWOPS allows a large number of ONUs to be connected to OLT using only a handful of colorless transceivers. Due to the architecture, NEWOPS not only facilitates high spectral efficiency and flexible sub-channel bandwidth allocation, but also reduces the Rayleigh backscattering (RB) problem and achieves high energy and cost efficiency. Experimental results demonstrate that, taking only 5-GHz bandwidth for each wavelength, NEWOPS successfully achieves 20-Gb/s downstream and 10-Gb/s upstream OFDM-xQAM transmissions via the same fiber trunk. We depict that, to support 40 ONUs for example, NEWOPS achieves energy efficiency that is three to six times higher than that of a typical 10-Gb/s WDM/OFDM PON.

Channel Equalization for Multi-Antenna FBMC/OQAM Receivers

In this paper, the problem of channel equalization in filter bank multicarrier (FBMC) transmission based on the offset quadrature-amplitude modulation (OQAM) subcarrier modulation is addressed. Finite impulse response (FIR) per-subchannel equalizers are derived based on the frequency sampling (FS) approach, both for the single-input multiple-output (SIMO) receive diversity and the multiple-input multiple-output (MIMO) spatially multiplexed FBMC/OQAM systems. The FS design consists of computing the equalizer in the frequency domain at a number of frequency points within a subchannel bandwidth, and based on this, the coefficients of subcarrier-wise equalizers are derived. We evaluate the error rate performance and computational complexity of the proposed scheme for both antenna configurations and compare them with the SIMO/MIMO OFDM equalizers. The results obtained confirm the effectiveness of the proposed technique with channels that exhibit significant frequency selectivity at the subchannel level and show a performance comparable with the optimum minimum mean-square-error equalizer, despite a significantly lower computational complexity. The possibility of tolerating significant subchannel frequency selectivity gives more freedom in the multicarrier system parameterization. For example, it is possible to use significantly wider subcarrier spacing than what is feasible in OFDM, thus relieving various critical design constraints.

Multipath Characterization of Indoor Power-Line Networks

The time- and frequency-varying behavior of an indoor power-line network is the result of variable impedance loads connected to its termination points. In fact, any signal transmitted through such a communications network is subject to time-varying multipath fading. In this paper, an analytical calculation method is presented, which can be used to determine the multipath components of any point-to-point channel in the indoor power-line environment. The method calculates all transmission characteristics of the network and, therefore, it can be exploited in the process of designing proper transmission algorithms for optimizing system performance. The proposed method is applied to an example network to demonstrate its usefulness in explaining the network's time-dependent behavior and in estimating channel parameters, such as subchannel bandwidth, multipath delay spread, fading conditions, etc.