Principles of Spread Spectrum Communication
Rahul Chauhan Ver. 1 10 Jan 2002 INDEX

1. Interesting History of Spread Spectrum
   
2. Radio Propagation
   
3. Introduction
   
4. Defining Spread Spectrum
   
5. Advantages of SS Communication
   
6. Concept of Spread Spectrum
   
7. How does Spread Spectrum give secure communication?
   
8. Synchronization
   
9. Types of Spread Spectrum
   
10. Pseudo Random Codes
    
11. Power Control in CDMA Systems
    

Interesting History of Spread Spectrum
CDMA has its roots in pre World War 2 America. In 1940, Hollywood actress turned inventor Hedy Lamer and co-inventor George Antheil, with WW2 looming, co-patented a way for torpedoes to be controlled by sending signals over multiple radio frequencies using random patterns.

Despite all efforts by inventors to advance this technology from experiment to implementation, the US Navy discarded their work as architecturally unfeasible!

The idea which was known as frequency hoping and later as frequency hopping spread spectrum technique remained dormant until 1957 when engineers at the Sylvania Electronic Systems Division in Buffalo, NY took up the idea and the Lamarr-Antheil patent expired, used it to secure communication for the US during 1962 Cuban missile crisis.

After becoming an integral part of government security technology, the US military ,in the mids-80 , declassified what's has now become CDMA technology, a technique based on Spread Spectrum technology.

Radio Propagation
The ideal communication channel model is a communication channel (bandwidth) corrupted with AWGN (i.e. Additive White Gaussian Noise). But in real the radio channel deviates from the ideal model due to the presence of multipath.

AWGN Channel Communication Model 
s(t) is information signal 
s(t) is information signal 
n(t) is AWGN

Wireless communication is evaluated on 2 main points:

1. Median Signal Strength: Minimum power to be radiated so that it is not a source of interference to other users in the radio environment. The interference can be co-channel , adjacent channel
   
2. Signal Variability: In Wireless communication the radio environment is dynamic and hence it results in multipath fading. Multipath fading results due to the fact multiple time-delayed instance of a symbol is received at the receiver.
   

And when there is a relative moment between BS and MS in a multipath we get a Doppler spread. Since the MS receives the same signal at different positions.

Introduction
The major concern in Wireless is digital communication is efficient use of Bandwidth and power. But there are scenarios where it is necessary to sacrifice the efficient use for design considerations. One such scenario is secure communication in hostile environment. This design objective is met using a modulation technique called as Spread Spectrum (SS).
The advantage of using is its ability to reject interference .May it be intentional (some jamming transmission) or unintentional (in this technique signal of one user is interference to another user).

Defining Spread Spectrum
A complete definition to Spread Spectrum I feel is the one given by Haykins. I quote him below.
His definition is in two parts.

1. Spread Spectrum is a means of transmission in which the data sequences occupy a bandwidth in excess of the minimum bandwidth necessary to send it.
   
2. Spread Spectrum is accomplished before transmission through the use of a code that is independent of data sequences .The same code is used at the receiver to despread the received signal so that the original data sequence may be recovered.
   

Advantages of Spread Spectrum

1. Multipath rejection:
   
2. Multipath access: Number of users uses a common channel for communication.
   
3. Random access possibilities: Users can start their transmission at any arbitrary time.
   
4. Privacy due to unknown random codes: The applied codes are - in principle - unknown to a hostile user. This means that it is hardly possible to detect the message of an other user.
   

Concept of Spread Spectrum
When the information bearing signal and a PN sequence is multiplied at a multiplier we obtain the desired modulation. The question is how do we get the increase spectrum? This is a simple Fourier Transform property. Multiplication in time domain is convolution in frequency domain. Hence by multiplying a narrow band information signal and a wide-band code sequence, the multiplied signal will have the spectrum similar to the wide-band PN code sequence.

How Spread Spectrum gives secure communication?
The spread signal gets the characteristics of the PN code sequence. This signal appears noise like to a receiver that has no idea about the spreading code.

Synchronization
The success to a CDMA system is proper synchronization. To despread a spread spectrum signal we need to use the same code used for spreading the signal. The operation takes place in two stages acquisition and tracking. First we have a acquisition or coarse synchronization is done, where we try to align the code in chips away from each other. In tracking or fine synchronization we measure the correlation and bring the receiver code in sync with the transmitted code.

Types of Spread Spectrum
This section shall come up soon. - Rahul

Pseudo Random Codes
This section shall come up soon. - Rahul


Power Control in CDMA Systems
CDMA is an Interference Limited System
CDMA is an interference-limited system. Having made this statement let us dwell on this. The success of the system lies in controlling the total power in the CDMA system.
        In a CDMA environment every MS (i.e. a handset) is a source of noise to the other. At the receiver at the MS sees the radio environment around it as a cumulative addition of information for itself and interference. The interference is information for other MSs plus noise from others sources. Hence if the interference is more, the information signal cannot be retrieved.
        A mobile has a special receiver called a RAKE receiver which can make estimates of multipath fading and retrieve the information for a particular mobile. The simple mathematical steps outline how the receiver works.
SI(t) Information signal for Ith mobile
WI(t) Walsh Code for Ith mobile
R (t) Received signal at RAKE receiver

Hence at any RAKE receiver the received signal will be (assuming no multipath fading)

R (t) = S SI(t)Å WI(t)

Let's us retrieve the signal for user1

W1(t) Å R(t) = W1(t) Å S SI(t)Å WI(t)
= S1(t) + 0
The orthogonal property of Walsh Codes makes zero the noise and retrieves the signal.
        Hence if noise or interference is more it will blind the receiver .So we need power control.


NEAR / FAR Problem
A user close to a cell would saturate the receiver and eliminate all users further away, unless the power is controlled. This is Near /Far problem

CDMA Capacity
The CDMA Capacity is given by the following expression:

Points to Take Note:

1. Power Control prevents Near/Far problem
   
2. Increases CDMA system capacity
   

Hence in the CDMA system we do power control on both the forward and reverse link.
Here we deal the IS-95 A way of handling the power controlling power in a cell.

Forward Link Power Control
The BS uses Closed Loop Power Control on the forward link. The mobile station periodically informs the BS to increase or decrease its power.

Reverse Link Power Control
We have two types here:

1.Open Loop Power Control

The Open Loop Power Control is used during access attempts. It increases the power during each attempt. If you are now thinking that such attempts may disturb the communication by increasing power at the BS receiver. Well this does not happen. Since by what amount the power level should increase during each attempt is informed to the MS by the BS during power on.

2.Closed Loop Power Control

In Closed Loop Power Control there is a feedback procedure. This type of power control is used when the MS is using the traffic channel resources i.e. it is active. The BS is continuously monitoring the reverse link. If its find the quality of the reverse link poor than it will instruct the mobile to increase its power by inserting power control bits in traffic data. This insertion of power bits for power control is called Bit Puncturing. The BS does this every 800 b/s.


Do write to me on how you found this page and any suggestions!! -Rahul Chauhan


Home | CDMA Home | Interesting Links | Your Suggestions
Copyright (c) 2002 Rahul Chauhan
Last Update on 2:25 PM 8/16/2002