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BPSK receiver

Started by rider January 25, 2004
Hi!

For an all digital BPSK receiver, If I place the Equalizer in
baseband, then what could be its exactloaction: Is the following
arrangement acceptable?

 |PASS-BAND|--------------BASE-BAND----------------------|

        fs  ------  fs ------  fs  fb ------  fb -------
 r(nTs)--->|DEMOD|----|  MF  |---/---| EQUZ |---|SLICER |--->
            ------     ------    ^    --- |-     -------  |
               ^                 |        |               |
               |                 |        `-----<---------`
           ------             ------
          |  CR  |           | STR  |
           ------             ------

Where: 
r(nTs)= Received pass-band Digital Samples at freq fs=1/Ts
fs = Sampling Frequency
fb = BPSK SYMBOL rate (bit rate) , fs >= 2.fb , fb=1/Tb , Tb=
symbol/bit rate
DMOD= Mix with recovered carrier and LPF, operating freq=fs
CR= COSTAS LOOP CARRIER RECOVERY
MF=Root Raised Cosine Filter, operating at fs
STR= Symbol Timing recovery
EQUZ= Symbol Spaced Equalizer, operating at fb
SLICER= Hard decisions +/-1 V

1) Is this arrangement acceptable as one possible form of a BPSK
receiver?

2) Since Equalizer is operating at Bit/Symbol rate fb, it is called a
Symbol rate equalizer. What if I want to use a FSE (Fractionally
Spaced Equalizer)? Say Tb/2 spaced(that Equalizer will give output at
rate fb=1/Tb). Where should i place it? Before the MF ? What would the
position of STR in that case(The output of FSE would already have
deimated to fb, so how to implement the STR)? Do i have to move STR in
pass-band?

Regards
Rider

rider wrote:
> > Hi! > > For an all digital BPSK receiver, If I place the Equalizer in > baseband,
There is no practical need in EQ for BPSK.
> then what could be its exactloaction: Is the following > arrangement acceptable? > > |PASS-BAND|--------------BASE-BAND----------------------| > > fs ------ fs ------ fs fb ------ fb ------- > r(nTs)--->|DEMOD|----| MF |---/---| EQUZ |---|SLICER |---> > ------ ------ ^ --- |- ------- | > ^ | | | > | | `-----<---------` > ------ ------ > | CR | | STR | > ------ ------ > > Where: > r(nTs)= Received pass-band Digital Samples at freq fs=1/Ts > fs = Sampling Frequency > fb = BPSK SYMBOL rate (bit rate) , fs >= 2.fb , fb=1/Tb , Tb= > symbol/bit rate > DMOD= Mix with recovered carrier and LPF, operating freq=fs > CR= COSTAS LOOP CARRIER RECOVERY > MF=Root Raised Cosine Filter, operating at fs > STR= Symbol Timing recovery > EQUZ= Symbol Spaced Equalizer, operating at fb > SLICER= Hard decisions +/-1 V
Since your BPSK is RRC filtered prior to transmission, you should remove RRC filter from the receiver or use square root filters both in receiver and transmitter.
> 1) Is this arrangement acceptable as one possible form of a BPSK > receiver?
Why not? This arrangement is as good as any other.
> 2) Since Equalizer is operating at Bit/Symbol rate fb, it is called a > Symbol rate equalizer. What if I want to use a FSE (Fractionally > Spaced Equalizer)? Say Tb/2 spaced(that Equalizer will give output at > rate fb=1/Tb). Where should i place it? Before the MF ?
Anywhere. It does not matter.
> What would the > position of STR in that case(The output of FSE would already have > deimated to fb, so how to implement the STR)?
In this case if the EQ is N-spaced then the STR should recover N*Bitrate.
> Do i have to move STR in > pass-band?
You don't have to. That is just the matter of convenience.
> Regards > Rider
Vladimir Vassilevsky DSP Consultant http://www.abvolt.com
> > > > |PASS-BAND|--------------BASE-BAND----------------------| > > > > fs ------ fs ------ fs fb ------ fb ------- > > r(nTs)--->|DEMOD|----| MF |---/---| EQUZ |---|SLICER |---> > > ------ ------ ^ --- |- ------- | > > ^ | | | > > | | `-----<---------` > > ------ ------ > > | CR | | STR | > > ------ ------ > > > >
Hi Vladimir! Thanks for your reply. If I do use the above arrangement for my receiver, What kind of STR technique should i use? I have heard of various techniques, like Early/Late gate, Gardner etc. As far as i understood, Gardner algorithm requires STR in passband(i.e. it tries to lock to fs not fb) and it requires 2 samples per symbol ( i have fs=8fb) and some interpolator. 1)Is Gardner algortihm applicable in base-band as shown in my figure? 2)If i can use Gardner algorithm as above, the I already have fs=8fb, so do i need to interpolate? 3)Where can i find a suitable resource for Gardner algorithm? Regards Rider Thanks

rider wrote:
> > > > > > > |PASS-BAND|--------------BASE-BAND----------------------| > > > > > > fs ------ fs ------ fs fb ------ fb ------- > > > r(nTs)--->|DEMOD|----| MF |---/---| EQUZ |---|SLICER |---> > > > ------ ------ ^ --- |- ------- | > > > ^ | | | > > > | | `-----<---------` > > > ------ ------ > > > | CR | | STR | > > > ------ ------ > > > > > > > > Hi Vladimir! > Thanks for your reply. If I do use the above arrangement for my > receiver, What kind of STR technique should i use? I have heard of > various techniques, like Early/Late gate, Gardner etc.
I would use the simplest zero crossing STR for BPSK. Here is a good book on CR/STR algorithms: "Digital Communication Receivers : Synchronization, Channel Estimation" Heinrich Meyr; Marc Moeneclaey; Stefan Fechtel Vladimir Vassilevsky DSP Consultant http://www.abvolt.com