# Can someone explain this relation between QPSK and BPSK.

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#### scream_er

##### Member level 5
1) The mathematical analysis shows that QPSK can be used either to double the data rate compared with a BPSK system while maintaining the same bandwidth of the signal.

I believe in this statement QPSK can have double the data rate since each symbol is coded by two bits in QPSK compared to one bit per symbol in BPSK. I would like to know how they both can have the same bandwidth when I am sending more bits in QPSK compared to BPSK.

2)To maintain the data-rate of BPSK but halving the bandwidth needed for QPSK. In this latter case, the BER of QPSK is exactly the same as the BER of BPSK.

In this statement I believe both QPSK and BPSK have same BER because the data rate is same. And again, how can you same data rate by halving the bandwidth of QPSK.

Thanks

#### Marwa Majed

##### Newbie level 5
Hi
I think it is related to phase shift in BPSK the the phase between two bit is 180 so you just can send two bit but in qpsk the phase is 90 so you can send four in same bandwidth.I don't know if success to reach you my idea

#### amiral

##### Newbie level 1
Hi
I think it is related to phase shift in BPSK the the phase between two bit is 180 so you just can send two bit but in qpsk the phase is 90 so you can send four in same bandwidth.I don't know if success to reach you my idea

Hi,
The BER rates of BPSK and QPSK cannot be the same. BPSK has a better BER ratio compare to QPSK.
Because in BPSK phase shift /difference is 180. So it has more possibilty to estimate the correct signal or bit.
in QPSK the signals are more similar to each other (90 phase shift). it is more difficult to correctly estimate the transmitted signal.

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