Forums

Channel Equalization

Started by rider January 21, 2004
Hi!

I want to add a channel equalizer into my my BPSK receiver. I have a
confusion where to place this module. Either in pass-band or in
base-band? Currently my receiver performs following sequence of
events:

1) The received pass-band signal r(t) is sampled at Fs (8 times the
symbol/bit rate fb) and demodulated using a Costas Loop carrier
recovery and LPF. This LPF filter gives a base-band stream, but still
at Fs.

2)The baseband stream at Fs, is Matched Filtered with a Root-Raised
Cosine filter operating at Fs.(A same RRC filter used to shape the
base-band pulse in the transmitter).

3)The output of the Matched filter is decimated to symbol rate fb,
under the control of a timing recovery loop.

4)These symbols at are fb are used for decisions.

Now where should i place an equalizer (if any?) ..should it be in the
pass-band? i.e. before the costas loop demodulation? or should it be
some where in baseband? like immediately before/after the Matched RRC
filter?

Any help would be highly appreciated.

Rider

rider wrote:
> > Hi! > > I want to add a channel equalizer into my my BPSK receiver.
Generally you don't need any equalization for BPSK.
> I have a > confusion where to place this module. Either in pass-band or in > base-band?
No difference. It is just the matter of convenience. Make sure the equalizer don't interfere with the carrier and timing recovery loops. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
> > Generally you don't need any equalization for BPSK.
It depends on the channel you are looking at and the symbol period (1/fb), not the modulation scheme. If your delay spread is no longer negligible (in comparison to your symbol period) then you can suffer the BER, or use some form of equalisation, or else consider lengthening your symbol period by dropping the throughput or multiplexing your throughput over multiple channels (OFDM?). Granted BPSK shows favourable performance, when compared with other modulations, [e.g. like QPSK,MSK in Chuang's 'The Effects of Time Delay Spread on Portable Radio Communications Channels with Digital Modulation', IEEE] but it still suffers. From Rappaport('Wireless Communications', 1996, p.301), 'an equalizer is usually implemented in baseband or IF in a receiver'. A good read. hope this is of some help.
J_M_1967@hotmail.com (JM) wrote in message news:<eb1fbd07.0401220317.72008e4@posting.google.com>...
> > > > From Rappaport('Wireless Communications', 1996, p.301), 'an equalizer > is usually implemented in baseband or IF in a receiver'. A good read. > hope this is of some help.
Hi! Thans for everyone's replies. For an all digital BPSK receiver, If I place the Equalizer in baseband, then what could be its exact loaction: 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