Let only that little be left of me

whereby I may name thee my all.

Let only that little be left of my will

whereby I may feel thee on every side,

and come to thee in everything,

and offer to thee my love every moment.

Let only that little be left of me

whereby I may never hide thee.

Let only that little of my fetters be left

whereby I am bound with thy will,

and thy purpose is carried out in my life—and that is the fetter of thy love.

# Rabindranath Tagore

## Code Division Multiple Access

3G? What is 3G? On what Technology it is based on? 3G is the third generation in cellular technology, which purely depends on CDMA200 though 2G also uses CDMA IS-95, the CDMA used in 3G is much more sophisticated than CDMA in 2G. So now the question is what is CDMA? Unlike FDMA (frequency division multiple access) and TDMA (time division multiple access), CDMA is much more sophisticated modulation!! oh sorry its not a modulation, its a technique used for multiple access at the same time using codes. Its completely based on spread spectrum technology, this can be done using either Direct Sequence Spread Spectrum (DSSS) or Frequency Hopping Spread Spectrum (FHSS). FHSS is mainly used in military defence and its a complex animal at the first. So, here I deal with DSCDMA which is Direct Sequence Code Division Multiple Access.

**Direct Sequence Code Division Multiple Access:**

In CDMA each user gets unique codes which then using the same code they can retrieve the data coming from the base station. So I guess you have a doubt with the codes? what are codes? okay let me tell you what these codes are, these are pseudo random codes. Confused? well let me comprehend about this. These codes are randomly generated bit streams of 1 and -1, which when looked looks like noise but since its not a noise lets give it a name as pseudo noise. Yes, these are called PN codes. Now lets get into more detailed description of CDMA.

Now say we have a bandwidth (BW) B Hz, then the time taken for each bit to be transmitted will be how much ? yes it is 1/B seconds. Now as we are transmitting a bit in the form of codes (say N sequence code) and to transmit the date at the same bit rate we need to increase the bandwidth as each symbol of the bit should be sent at 1/NB seconds so we need NB Hz bandwidth.

**CDMA transceiver :**

At the transmitter the data is merged with the codes generated using some techniques (will be discussed later) and transmitted to the channel. Now the data at the transmitter looks like a noise i.e., the information of the signal is mingled with the noise floor, so now the problem is how do we retrieve the transmitted data? So, to retrieve the data the receiver must know the code that was generated at the transmission side, once the codes are known the information (data) can be retrieved by performing the corss-correlation using the known code and the received data. Thus you have the transmitted data.

**Generation of codes :**

To keep the overall bit rate same the bit rate of codes has to be increased as a consequence of this the bandwidth will increase, so as these codes are increasing the bandwidth i.e., these codes spread the data over the entire bandwidth, they are called as spreading codes and each bit in code is called as chip and the bit rate of chips (codes) is called chip rate.

The spreading factor is defined as the ratio of Chip rate to the Symbol rate(data rate).

The codes used in the CDMA are :

- PN Codes
- Walsch codes
- Gold codes and etc..

Here I discuss the above first two codes, they are PN codes and Walsch Codes.

**PN codes:**

PN codes as said before they are pseudo-random noise codes, generated using a -1 and 1’s in NRZ form, 0 and 1’s in polar form. These PN codes are generated using Linear Feedback Shift Register (LFSR). A typical LFSR is shown below.

The number of 0’s and 1’s thus generated in PN code must always be 1. The PN codes has very good auto-correlation but not that good cross-correlation i.e., the correlation is not equal to zero at different lags.

**Walsch codes:**

Walsch codes are generated using Hadamard Matrix. These codes generated are orthogonal to each other. They have bad auto-correlation and good cross-correlation, so many users can be allowed at the same time. But because of mutlipath interference, there would be a delay in the signal, as a consequence of this it looses its orthogonality and can no longer be used. In that case PN codes are preferred to Walsche codes.

Walsch code are generated as follows,

let H1= [1]

then H2=[ H1 H1 ]

[ H1 H1bar].

for futher details on walsch codes and on CDMA follow the links given below…

## Felicitation!!

This day

is a memorable day in my life. My friend Abhinov Balagoni has placed in Idea under management profile after long waiting. So, I would like to felicitate him on this occasion on my blog. Hope he will be having a great future ahead.

## Noise Characterization

Finally! received awgn (not white exactly, it is colored as its a band-limited noise) by sdr and performed the auto-correlation of noise, which gave outlandish results though. To study the characteristic of colored noise, random noise samples are generated and passed through an elliptic filter (low pass) which then was subjected to auto-correlation. The auto-co variance of generated band-width noise was much like a sinc. The study the signal characteristics asine wave of frequency say 100 kHz was sent from one sdr and was received by the other sdr and performed the auto-correlation on the received signal. The results were erroneous couple of times and also don’t hold together this was because of mismatching of sampling rates perhaps. And it was really surprising to see the exact base-band (sine) signal when it was properly sampled.

## Autocorrelation!

Today (Dec 12, 2012), I have done autocorrelation of noise received by SDR, the beauty of autocorrelation lies in extracting the signal from the additive white guassian noise (AWGN) which has autocorrelation zero except at zero lag in theory atleast !!!

## First day!

This is the first time I ever started blogging since my birth, though I have tried to start blogging a couple of times before; but it ended up as a disaster.