Spread Spectrum Multi-carrier Multiple Access Research Articles

Multi-Access Techniques Comparison for Remote Lab System

Remote lab systems are one of the essential requirements for an increased academic productivity in the modern digital world. These systems support and facilitate effective migration from face-to-face classroom education to online education. Digital technology applications and processes are required to easily build a remote lab system. With the availability of multiple access techniques, users can comfortably share laboratory equipment among themselves. The sharing of resources using the remote lab system is highly required for a smooth deployment and implementation of online education. This paper therefore proposed and tested some techniques that combine Code Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) in remote lab systems. The tested techniques are Multi-Carrier Direct Sequence CDMA (MC-DS-CDMA), Multi-Tone CDMA (MT-CDMA), Multi-Carrier CDMA (MC-CDMA), and Spread Spectrum Multi-Carrier Multiple Access (SS-MC-MA). The first step proposed in this work had to do with the setting of the comparison criteria. At the second step, the solutions cited previously in the real equipment was tested and the best option that met the criteria was selected for the eLab system since the performance technique varies with the laboratory equipment characteristic. The four techniques that were tested demonstrated high performance in telecommunications and online laboratory systems. The implementation of these techniques will benefit universities in several ways, which include reduction of remote lab cost and optimization of sharing of online resources among users. This will further provide students with conducive learning environment by addressing the challenges of reservation and time slot limit. It is therefore recommended that MC-CDMA should be integrated into remote lab system.

Range improvement of UWB systems using adaptive multicarrier spread‐spectrum and MIMO techniques

AbstractIn this paper, we investigate the use of multiple‐input multiple‐output (MIMO) techniques with a linear precoded orthogonal frequency division multiplexing (LP‐OFDM) waveform, known as spread‐spectrum multicarrier multiple access (SS‐MC‐MA), for high data rate ultra‐wideband (UWB) systems. This scheme is an evolution of the well‐known multiband OFDM (MB‐OFDM) solution supported by the WiMedia Alliance. The aim of this paper is to improve the UWB system range while not significantly increasing the system complexity compared to the WiMedia solution. Firstly, an analytical study of the single‐input single‐output (SISO) LP‐OFDM waveform allows to find how to efficiently precode an OFDM signal in order to improve the system range. To go further and exploit the available spatial diversity, a MIMO component is added and a new geometric statistic MIMO channel model is considered. Secondly, a global MIMO LP‐OFDM system study is led to highlight the benefits of combining MIMO and LP techniques for UWB applications. The analytical and simulation results show that the joint use of MIMO and LP‐OFDM leads to a significant range improvement compared to the WiMedia solution. Copyright © 2008 John Wiley & Sons, Ltd.

An analysis of three multiple access techniques for the uplink of future cellular mobile systems

AbstractOrthogonal frequency division multiple access (OFDMA) is today a part of many standard specifications. Despite its attractive features, it has two main drawbacks when employed in the uplink: the high peak‐to‐average power ratio (PAPR) inherited from orthogonal frequency division multiplexing (OFDM) and the inherent frequency diversity loss. The loss of frequency diversity can be alleviated by precoding. Variants of precoded OFDMA include spread spectrum multi‐carrier multiple access (SS‐MC‐MA) and the frequency‐domain implementation of single‐carrier (SC) FDMA. The latter transforms OFDMA into a SC transmission system, avoiding the PAPR problem. This paper analyses OFDMA, several variants of precoded OFDMA and SC‐FDMA in its frequency‐domain implementations and compares them for uplink transmission. Copyright © 2008 John Wiley & Sons, Ltd.

Loading algorithms for adaptive SS‐MC‐MA systems over wireline channels: comparison with DMT

AbstractIn this paper, we propose to combine adaptive loading principles with the spread‐spectrum multicarrier multiple access (SS‐MC‐MA) scheme. Such an approach has particular interests in the context of powerline communications (PLC), where the transmitter has not only to exploit robust transmission techniques, but has also to adapt the waveform to the channel response. We introduce finite‐granularity loading algorithms that dynamically handle the configuration of the system under power spectral density constraints. The presented algorithms assign subcarriers, spreading codes, bits and energy to each user in order to maximise either the data rate or the noise margin at a given target symbol error rate. These algorithms can actually be viewed as a widening of the classical waterfilling approach in the case of an hybrid spread‐spectrum multicarrier system. Simulation results of the new scheme are presented for different measured PLC channels and are compared with those of the classical discrete multitone modulation (DMT) approach. It is shown that the adaptive SS‐MC‐MA scheme performs significantly better than DMT, due to its natural energy gathering capability. Adaptive SS‐MC‐MA then leads to a more efficient bits and energies distribution and constitutes a simple solution to reduce the quantification loss induced by the use of finite order modulations. Copyright © 2006 AEIT.

Adaptive spread-spectrum multicarrier multiple-access over wirelines

In this paper, we investigate the dynamic resource allocation adapted to spread-spectrum multicarrier multiple-access (SS-MC-MA) systems in a multiuser power line communication (PLC) context. The developed adaptive system is valid for uplink, downlink, as well as for indoor and outdoor communications. The studied SS-MC-MA system is based on classical multicarrier modulation like digital multitone (DMT), combined with a spread-spectrum (SS) component used to multiplex several information symbols of a given user over the same subcarriers. The multiple-access task is carried out using a frequency-division multiple-access (FDMA) approach so that each user is assigned one or more subcarrier sets. The number of subcarriers in each set is given by the spreading code length as in classical SS-MC-MA systems usually studied in the wireless context. We derive herein a new loading algorithm that dynamically handles the system configuration in order to maximize the data throughput. The algorithm consists in an adaptive subcarrier, code, bit, and energy assignment algorithm. Power-spectral density constraint due to spectral mask specifications is considered, as well as finite-order modulations. In that case, it is shown that SS-MC-MA combined with the proposed loading algorithm achieves higher throughput than DMT in a multiuser PLC context. Because of the finite granularity of the modulations, some residual energy is indeed wasted on each subcarrier of the DMT spectrum. The combining of a spreading component with DMT allows to merge these amounts of energy so that one or more additional bits can be transmitted in each subcarrier subset leading to significant throughput gain. Simulations have been run over measured PLC channel responses and highlight that the proposed system is all the more interesting than the signal-to-noise ratio is low.

MIMO Frequency Hopping Spread Spectrum Multi-Carrier Multiple Access: A Novel Uplink System for B3G Cellular Networks

In this paper, we propose and give the performance of a novel uplink system based on the combination of multi-carrier (MC), code division multiple access (CDMA) and multiple input multiple output (MIMO) techniques. First, we describe the interests of spread-spectrum multi-carrier multiple access (SS-MC-MA) scheme for uplink, especially compared to MC-CDMA. Classically, with SS-MC-MA, each user spreads its data symbols on a specific subset of adjacent or multiplexed subcarriers, to facilitate the channel estimation and reduce complexity at the reception. In order to compensate for the lack of frequency diversity of SS-MC-MA with adjacent subcarriers, we first combine it with an orthogonal space-time block code (STBC) and demonstrate the resulting spatial diversity gain. Then, we propose to allocate the subsets to the different users by applying a frequency hopping pattern (FH). In that case, each user benefits from the frequency diversity linked to the total bandwidth as with the multiplexed subcarriers solution, while keeping the advantages of the adjacent subcarriers solution. The gain provided by the use of the frequency hopping is stressed on. Finally, the performance of this scheme is evaluated over realistic MIMO channel with channel turbo coding for systems offering asymptotic spectrum efficiency of 1, 2, 3 and 4.5 bit/s/Hz. Thus, the efficiency of the novel proposed STBC FH SS-MC-MA system as a very promising multiple access and modulation scheme for the uplink of the future wideband wireless networks is successfully demonstrated.

SS-MC-MA Systems with Pilot Symbol Aided Channel Estimation in the Asynchronous Uplink

Spread spectrum multi-carrier multiple access (SS-MC-MA) systems exploit in a similar manner as MC-CDMA systems the advantages of the diversity gain due to the spread spectrum technique and the high bandwidth efficiency due to the orthogonal multi-carrier modulation. However, the main difference is that SS-MC-MA systems use FDMA for the user separation and therefore are less vulnerable to multiple access interference, and thus are especially of interest for the uplink of mobile radio systems. They allow asynchronism between the mobile users. In this paper we investigate the effects of ISI and ICI due to asynchronism in the uplink of an SS-MC-MA system on the performance of pilot symbol aided channel estimation. Moreover, we consider the suitability of power boosting of the pilot symbols to improve the link performance.

Performance Effects of the Uplink Asynchronism in a Spread Spectrum Multi‐Carrier Multiple Access System

AbstractIn this paper we investigate the effects of the asynchronism of user signals in the uplink of a spread spectrum multi‐carrier multiple access (SS‐MC‐MA) system. Different propagation delays due to different distances between the mobile transmitters and their base station cause asynchronous arrivals of the uplink signals in addition to multipath propagation. The proposed uplink SS‐MC‐MA scheme uses for synchronization only the frame structure received on the synchronous downlink and requires no additional synchronization measures. A guard interval that is smaller than the sum of the maximum time offset between the users and the maximum excess delay of the frequency selective multipath channel is used. It minimizes the loss in bandwidth efficiency by allowing residual interference. The residual interference is minimized by proper positioning of the detection interval in the receiver. It is shown that with this approach the guard interval can be reduced by about 50%. The performance of the proposed uplink scheme is compared to an uplink SS‐MC‐MA scheme with perfect synchronization.