Gold sequences have been proposed by Gold in 1967 and 1968

Prepared byNehal Shyal

M.E(EC)-1st semester.Roll no:110010741007ADIT,2011-2013 batch

Gold sequences have been proposed by Gold in 1967 and 1968.

A Gold code, also known as Gold sequence, is a type of binary sequence, used in telecommunication (CDMA).

Gold code generation is one of the technique to generate more sequences out of a pair of m-sequences giving many more different sequences to have multiple users.

Gold sequences constructed by the XOR of two m-sequences with the same clocking.

Codes have well-defined cross correlation properties.

m-sequences, the cross correlation only takes on three possible values, namely -1, -t or t-2. Two such sequences are called preferred sequences. Where t is defined as,

t = 2(m+1)/2 + 1, if m is odd, &

t = 2(m+2)/2 + 1, if m is even.

The steps that were followed to generate the gold code are :

Step1:Two PN sequences PN1 and PN2 are produced, each of 20 bit long by XORing specific following a prescribed pattern and shifting each of them continuously.

Step2: Each output bit from the two sequences were XORed and stored in an array called GOLD.

In following Figure two shift registers generate the two m-sequences and these are then bitwise XORed

PN1

PN2

XOR

Modulation of the Message signal with GOLD Code:

The basic idea behind modulation was that we have to replace each 0 in the 20 bit message signal with the Original Gold Code and each 1 in the message signal with the Inverted 20 bit Gold Code thus converting the 20 bit message signal into a 400 (20 x 20) bit spreaded signal.

if messege bit is ‘0’=original Gold code

if messege bit is ‘1’=inverted Gold code

Message signal : 1 0 1 0 1 1 0 0 1 0 1 1 1 1 0 0 1 1 0 1 Gold Code: 1 0 0 0 1 1 1 1 0 0 0 0 0 0 0 1 1 0 1 0 Inverted Gold Code:0 1 1 1 0 0 0 0 1 1 1 1 1 1 1 0 0 1 0 1Spreaded Signal: for 1st bit of message signal0 1 1 1 0 0 0 0 1 1 1 1 1 1 1 0 0 1 0 1

->Repeat this process for all remaining 19 bits.

Block Diagram of Receiver

At receiver we are simply demodulating the received spreaded signal with the gold code to produce the original message signal.

Steps for Demodulation are:

Since both the original message signal as well as the gold code are 20 bit long, we xor each successive 20 bit of the received spreaded signal with the 20 bit gold code and store the result in an array.

Since each 20 bit block of the message signal consists of either the gold code or the inverted gold code, XORing with the gold code will give us a result either 0 or 1.

If the result is 0, then we store 0 in an array called despreaded signal and if the result is 1, we store 1.

So, if received sequence of 20 bit = Gold CodeRecovered bit =0

if received sequence of 20 bit =inverted Gold CodeRecovered bit =1

The process is repeated for each set of 20 bit till all the 400 bits of the received signal are covered.

The array despreaded signal ultimately gives us the original message signal.

Get the more numbers of sequences.

• In Gold code, suppose we have two m-sequences each having , n=5

• By combining two of these sequences, we can obtain up to 2n − 1 = 31 plus the two m-sequences themselves, generate 33 sequences (each one 2n − 1 length) that can be used to spread different input messages (different users CDMA).

• So,many users can use same frequency in CDMA with less interference

Only simple circuitry needed to generate large number of unique codes.

In Spread-spectrum communication systems• IS-95• WCDMA &• UTRA-TDD standards as the scrambling code.

Scrambling is accomplished by the addition of components to the original signal or the changing of some important component of the original signal in order to make extraction of the original signal difficult. Examples of the latter might include removing or changing vertical or horizontal sync pulses in television signals; televisions will not be able to display a picture from such a signal.

Some modern scramblers are actually encryption devices.

In Satellite systems• GPS• NASA’s Tracking & Data Relay Satellite

Systems(TDRSS)

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