11-03-2011, 02:28 PM
Presented by;
K.HEMAKISHORE
P.MANASA
K.SAIKUMAR
A.D.V.PRASAD
main ppt1.pptx (Size: 354.12 KB / Downloads: 74)
BER Performance Analysis of Asynchronous DS-CDMA Systems Using BPSK Modulation over Fading Channels
ABSTRACT :
An asynchronous binary DS-CDMA system using
random spreading sequences is considered when communicating
over various fading channels. A unified analysis is provided for
Calculating the exact average Bit Error Rate (BER) expressed in
the form of a single numerical integration based on the CF
approach.
The numerical results obtained from our exact BER analysis are verified by simulation results.
INTRODUCTION :
Code division Multiple Access has been one of the most
successful radio access techniques since the 1990s and
Direct Sequence (DS) CDMA has been integrated into the
third generation mobile systems.
The Bit Error Rate(BER) is one of the most important performance metrics in communication systems and hence it has been extensively
studied in various DS-CDMA systems .
Block Diagram Of CDMA:
CLASSIFICATON OF CDMA:
1. Synchronous CDMA :
CDMA exploits mathematical properties of orthogonality between vectors representing the data strings is called Synchronous CDMA.
2. Asynchronous CDMA :
“Pseudo-random" or "pseudo-noise" (PN) sequences are used in asynchronous CDMA systems. A PN code is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receivers.
Advantages of asynchronous CDMA over other techniques :
1.Efficient Practical utilization of Fixed Frequency Spectrum
2.Flexible Allocation of Resources
3. Asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals.
Direct-sequence spread spectrum:
In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique.
As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated.
The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency.
DS CDMA system using BPSK modulation:
BPSK modulation is used for the information signal.
The carrier signal has a frequency of Wc and amplitude of A=√p where p is the average power of the signal. This gives an energy per bit Eb= A^2= PT.
The length of the spreading code used is Q.
Each information bit has a period of T and each chip has a period of Tc, where T= QTc.
The total number of users in the system is represented by K and each user is identified by subscript k.
All K users are transmitting at the same bit rate.
Multipath Fading :
Multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths.
Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings.
Fading
Fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media .
The presence of reflectors in the environment surrounding a transmitter and receiver create multiple paths that a transmitted signal can traverse.
As a result, the receiver sees the superposition of multiple copies of the transmitted signal, each traversing a different path.
Rayleigh fading :
Rayleigh fading is a statistical model for the effect of a propagation environment on a radio signal, such as that used by wireless devices
Rician fading:
Rician fading is a stochastic model for radio propagation anomaly caused by partial cancellation of a radio signal.
The Ricean distribution is also known as the Nakagami-n distribution.
Hoyt distribution
The Hoyt distribution is also known as the Nakagami-q distribution the Hoyt fading phase ϕ is neither uniformly distributed nor independent of the fading amplitude h.
Nakagami-m fading:
The Nakagami distribution or the Nakagami-m distribution is a probability distribution related to the gamma distribution. It has two parameters: a shape parameter μ and a second parameter controlling spread, ω.