07-07-2012, 12:07 PM
CDMA RAKE RECEIVER
CDMA RAKE RECEIVER.ppt (Size: 458.5 KB / Downloads: 32)
CDMA
CDMA is a form of multiplexing, which allows multiple access.
The technology is used in (UHF) cellular telephone systems.
It uses a “spread-spectrum” technique.
In a CDMA system, the same frequency can be used in every cell.
CDMA can also effectively reject narrow band interference.
CDMA consistently provides better capacity for voice and data communications than other commercial mobile technologies.
CDMA- History & Evolution
The CDMA standard was originally designed by Qualcomm in the U.S.
1993, the first CDMA standard IS-95 was issued;
In 1995, CDMA technology was put into commercialization in Hong Kong and America on large scale;
In April, 2001, China Unicom began to construct CDMA networks—the largest in the world (about 70Million line now);
At present, CDMA commercial networks are established in about 40 countries or area, almost 20% of all users in the world.
CDMA is a military technology first used during World War II
Introduction
Rake receiver is a radio receiver designed to counter the multi path fading.
RAKE receiver is used in CDMAbased systems and can combine multipathcomponents, which are timedelayed versions of the originalsignal transmission. Combinig is done in order to improve thesignal to noise ratio at the receiver.
RAKE receiver attempts tocollect the timeshifted versions of the original signal byproviding a separate correlation receiver for each of the multipath signals.
The basic idea of A RAKE receiver was first proposed by Price and Green. These fellows also filed the RAKE receiver patent in 1956.
Propagation of Tx Signal
Signals sent over the air can take multi-paths to the receiver. It can result in the receiver getting serveral versions of the same signal but at slightly different times.
The combination of the direct and reflected signals most often leads to significant signal loss due to mutual cancellation.
Signals acquire multipath because of following reasons.
Reflection
diffraction can occur at the edge of an impenetrable body or at a
surface with sharp irregularities that is large compared to the wavelength of the radio wave.
Basic idea of Rake receiver
The most common receiver used in most CDMA receivers
is RAKE receiver.
RAKE receiver is basically a maximum ratio combiner that combines differently propagated signal replicas in anoptimalmanner.
RAKE receiver consists of several baseband correlators(fingers)that each process a single multipath component independently.
The outputs of the correlators are then combined to form an improved symbol estimate.