Processing Gain

[islam wagih]

Why when we increase "Spreading Factor", "Processing Gain" increase?

I know that "PG=10 LOG SF" but I want to understand "Practically" why increasing "Spreading Factor" lead to increase in "Processing Gain"?

Kind regards

[babak1349]

Hello,
lets look at the facts in 3G (FDD), The chip rate is always 3.84 Mchip/s and any modulated data should maintain that chip rate
SF factor means each bit of your data will be modulated with SF bits to maintain 3.84 Mchip/s
that's the concept of processing gain, the chip rate divided by data rate
So if you increase SF factor actually you are decreasing your data rate and the processing gain increases, in this case chance of decoding data with lower SIR increases (GP appears when you are demodulating the data, it means you are decoding your data with gain in energy comparing with modulated signal)

cheers

Why when we increase "Spreading Factor", "Processing Gain" increase?

I know that "PG=10 LOG SF" but I want to understand "Practically" why increasing "Spreading Factor" lead to increase in "Processing Gain"?

Kind regards

[islam wagih]

Thank you but I still not understanding !!

[dekili]

Why when we increase "Spreading Factor", "Processing Gain" increase?

I know that "PG=10 LOG SF" but I want to understand "Practically" why increasing "Spreading Factor" lead to increase in "Processing Gain"?

Kind regards

Processing gain is equal to the spread spectrum signal bandwidth (Bss) and base band (Bbb). Multiplying twice the signal with the spread spectrum sequence (M-sequence), signal spreads in spectrum Bss/Bbb times and gets back in original band. On the other hand, deterministic interference is multiplied by the spreading sequence only once so its power is spread to Bss/Bbb wider bandwidth. This means that signal to interference ratio will improve by Bss/Bbb times since the interference power is lowered Bss/Bbb times.
Cheers

[islam wagih]

I begin to understand but anyone can support with more details or support with any detailed document please?

[yellowant2512]

PG refers to the ratio of the final spreading rate to the bit rate (cps/bps) (You should know Diagram of the WCDMA System: Source Coding -> Channe coding/interleving -> Spreading -> Scrambing..., bps is data before spreading and cps is data after spreading)
When increasing "Spreading Factor", the data obtained upon final spreading has higher chip rate.

The spreading rate of WCDMA is: 3.84 Mcps and the processing gain depends on the specific service.
For the 12.2 Kbps voice service, its processing gain is as follows:
10*log10 (3.84 Mcps/12.2 Kbps) =10*log10314.75=25 db
Processing gain makes it possible to obtain the signal-to-noise ratio (Eb/Nt) required for demodulation when the carrier-to-interference ratio (Ec/Io) of the input signals is low. For example, in a typical pedestrian scenario, the Eb/Nt required to demodulate the 12.2 Kbps voice service is 7 dB (the simulation result). Since the processing gain is 25 dB, the acceptable minimum Ec/Io of the input signals is 7-25=-18 dB. That is, the signals can be submerged in the noise.