16 research outputs found
Performance Analysis of MC-CDMA for Rayleigh fading Channel
For wireless communication system multipath fading is a common problem specially in urban areas where a large number of buildings reflects the radio signals which results in interference amongst the reflected signals which causes the multipath fading effect since its selective by nature some spectrum at some specific location cancels out hence the receive signal losses some part of their information this abruptly increases the BER of communication system in slight movement of receiver, this paper specially analyzes the BER performance under Rayleigh fading channel conditions of MC-CDMA (Multicarrier Code Division Multiple Access) in presence of AWGN (Additive White Gaussian Noise) for different number of subcarrier, different number of users, and different path gains system analysis is performed by simulating the MC-CDMA using MATLAB program, and finally the paper also presents a comparison between simulated results. Keywords: MC-CDMA (Multicarrier Code Division Multiple Access), AWGN (Additive White Gaussian Noise), Rayleigh fading
Implementation of multi carrier-code division multiple access-frequency division multiple access with beyond 4G specifications
Hybrid code division multiple access techniques present the open door for the future of
code division multiple access and wireless communications. Multicarrier CDMA is the
most popular type of hybrid CDMA because of its robustness against multipath fading
channels and flexible multiple access capability. MC-CDMA is a predictable technique
for future high data rate wireless communication systems according to these appealed
properties. The main drawback of MC-CDMA is the power level in uplink, i.e. the ratio
of peak power to the average power is high and leads to high instantaneous power which
is required in transmission of mobile station. However, there are many researchers
working towards reducing the level of the transmitted power. This research presents new
method of peak to average power ratio (PAPR) reduction. The proposed method is
making use of the characteristics of uplink for current 4th Generation (single carrier
frequency division multiple access) which has low PAPR into current MC-CDMA
system to reproduce a new MC-CDMA system (MC-CDMA-FDMA) with low PAPR
and keep all the characteristics of the basic MC-CDMA system. MC-CDMA-FDMA
reduced the level of power from 10 dB to 2 dB in case of 64 FFT size and Walsh
Hadamard code is used in spreading block. In addition bit error rate has been reduced
from 96x10-5
bps to 82x10-5
bps comparing to SC-FDMA bit error rate. The proposed
system also has high flexibility to deal with modern communication systems with
minimum required hardware at the base station through optimization of FFT size. The
simulation results show that MC-CDMA-FDMA system will be a good candidate for
beyond 4th Generation for mobile communication
OFDMA-Based Medium Access Control for Next-Generation WLANs
Existing medium access control (MAC) schemes for wireless local area networks (WLANs) have been shown to lack scalability in crowded networks and can suffer from widely varying delays rendering them unsuited to delay sensitive applications, such as voice and video communications. These deficiencies are mainly due to the use of random multiple access techniques in the MAC layer. The design of these techniques is highly linked to the choice of the underlying physical (PHY) layer technology. The advent of new PHY schemes that are based on orthogonal frequency division multiple access (OFDMA) provides new opportunities for devising more efficient MAC protocols. We propose a new adaptive MAC design based on OFDMA technology. The design uses OFDMA to reduce collision during transmission request phases and makes channel access more predictable. To improve throughput, we combine the OFDMA access with a carrier sense multiple access (CSMA) scheme. Data transmission opportunities are assigned through an access point that can schedule traffic streams in both time and frequency (subchannels) domains. We demonstrate the effectiveness of the proposed MAC and compare it to existing mechanisms through simulation and by deriving an analytical model for the operation of the MAC in saturation mode
Packet CDMA communication without preamble
Code-Division Multiple-Access (CDMA) is one of the leading digital wireless communication methods currently employed throughout the world. Third generation (3G) and future wireless CDMA systems are required to provide services to a large number of users where each user sends data burst only occasionally. The preferred approach is packet based CDMA so that many users share the same physical channel simultaneously. In CDMA, each user is assigned a pseudo-random (PN) code sequence. PN codephase synchronization between received signals and a locally generated replica by the receiver is one of the fundamental requirements for successful implementation of any CDMA technique. The customary approach is to start each CDMA packet with a synchronization preamble which consists of PN code without data modulation. Packets with preambles impose overheads for communications in CDMA systems especially for short packets such as mouse-clicks or ATM packets of a few hundred bits. Thus, it becomes desirable to perform PN codephase synchronization using the information-bearing signal without a preamble.
This work uses a segmented matched filter (SMF) which is capable of acquiring PN codephase in the presence of data modulation. Hence the preamble can be eliminated, reducing the system overhead. Filter segmentation is also shown to increase the tolerance to Doppler shift and local carrier frequency offset. Computer simulations in MATLAB® were carried out to determine various performance measures of the acquisition system. Substantial improvement in probability of correct codephase detection in the presence of multiple-access interference and data modulation is obtained by accumulating matched filter samples over several code cycles prior to making the codephase decision. Correct detection probabilities exceeding 99% are indicated from simulations with 25 co-users and 10 kHz carrier frequency offset or Doppler shift by accumulating five or more PN code cycles, using maximum selection detection criterion. Analysis and simulation also shows that cyclic accumulation can improve packet throughput by 50% and by as much as 100% under conditions of high offered traffic and Doppler shift for both fixed capacity and infinite capacity systems
Space time transceiver design over multipath fading channels
Imperial Users onl