simple-minded non-OFDM signal design question

OK, now that I've got my OFDM question off my chest, here's a related
question.

If one used a normal single-carrier signal in place of OFDM, you'd
have a relatively huge bandwidth to deal with, which means your
single-carrier data rate is going to be many times higher than the
rate across any one carrier of a comparable OFDM signal.

Well, given all that freakin' bandwidth, it seems that there must be
SOME way to fight frequency-selective multipath that works comparably
to the OFDM approach. It also seems pretty obvious that this is going
to be some "extra" stuff in the receiver, for example, a whitening
filter, or equalization, or some-such.

So I guess I have two questions: 1) Is it possible to design
single-carrier systems (including the receiver) which are just as
immune to multipath as multi-carrier systems? and 2) Would it be safe
to say that the receiver design for single-carrier systems is more
complex compared with a comparable OFDM receiver?
-- 
%  Randy Yates                  % "Maybe one day I'll feel her cold embrace,
%% Fuquay-Varina, NC            %                    and kiss her interface, 
%%% 919-577-9882                %            til then, I'll leave her alone."
%%%% <yates@ieee.org>           %        'Yours Truly, 2095', *Time*, ELO   
http://home.earthlink.net/~yatescr

Randy Yates <yates@ieee.org> writes:
> [...]
> Is it possible to design single-carrier systems (including the
> receiver) which are just as immune to multipath as multi-carrier
> systems?

I should also clarify that I meant the two systems should use the same
average transmit power and bandwidth.
-- 
%  Randy Yates                  % "Though you ride on the wheels of tomorrow,
%% Fuquay-Varina, NC            %  you still wander the fields of your
%%% 919-577-9882                %  sorrow."
%%%% <yates@ieee.org>           % '21st Century Man', *Time*, ELO
http://home.earthlink.net/~yatescr

Randy Yates <yates@ieee.org> writes:
> [...]

OK, let me tip my hand. 

There are some folks/circles that are ragingly debating
whether the COFDM used in DVB/T is better than the US's
8VSB. I'm wondering if there is any real difference IF
the end-to-end systems are designed "properly."
-- 
%  Randy Yates                  % "My Shangri-la has gone away, fading like 
%% Fuquay-Varina, NC            %  the Beatles on 'Hey Jude'" 
%%% 919-577-9882                %  
%%%% <yates@ieee.org>           % 'Shangri-La', *A New World Record*, ELO
http://home.earthlink.net/~yatescr

Randy,

I guess if, for instance, the SNR is really bad in some bands, one
would have to devise quite a complicated scheme to encode the signal
for a single-carrier system to compensate for this, making sure some
symbol transitions do not happen (if it's possible at all. I wonder if
there's been any work in this direction). At least that was my
impression when I experimented with "self-made" OFDM for some weird
channel. Though I cannot call myself an expert in OFDM, and there could
be other issues...

Dmitry.

Randy Yates wrote:

> OK, now that I've got my OFDM question off my chest, here's a related
> question.
> 
> If one used a normal single-carrier signal in place of OFDM, you'd
> have a relatively huge bandwidth to deal with, which means your
> single-carrier data rate is going to be many times higher than the
> rate across any one carrier of a comparable OFDM signal.
> 
> Well, given all that freakin' bandwidth, it seems that there must be
> SOME way to fight frequency-selective multipath that works comparably
> to the OFDM approach. It also seems pretty obvious that this is going
> to be some "extra" stuff in the receiver, for example, a whitening
> filter, or equalization, or some-such.


Strictly speaking, it does not matter how are you going to utilize the 
bandwidth. All kinds of modulations are equivalent. It is just the 
matter of convenience which one to prefer.

> 
> So I guess I have two questions: 1) Is it possible to design
> single-carrier systems (including the receiver) which are just as
> immune to multipath as multi-carrier systems?

Certainly. Although it will require a huge amount of computation.


  and 2) Would it be safe
> to say that the receiver design for single-carrier systems is more
> complex compared with a comparable OFDM receiver?

Yes.
The OFDM advantage is in the low symbol rate and the simple equalization 
in the frequecy domain. This allows for relatively simple receivers.



Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

Vladimir Vassilevsky <antispam_bogus@hotmail.com> writes:
> [...]
> Randy Yates wrote:
>> So I guess I have two questions: 1) Is it possible to design
>> single-carrier systems (including the receiver) which are just as
>> immune to multipath as multi-carrier systems?
>
> Certainly. Although it will require a huge amount of computation.

Hi Vladimir,

Why? What sort of things would you have to do to a single-carrier
signal to "get out the multipath?"
-- 
%  Randy Yates                  % "I met someone who looks alot like you,
%% Fuquay-Varina, NC            %             she does the things you do, 
%%% 919-577-9882                %                     but she is an IBM."
%%%% <yates@ieee.org>           %        'Yours Truly, 2095', *Time*, ELO   
http://home.earthlink.net/~yatescr

A method of dealing with frequency-selective fading multipath in a
single carrier system is:
direct-sequence spread-spectrum (DS-SpSp). A very popular application
in cellular communications is in DS-CDMA, used in the original CDMA
(IS-95), and now in WCDMA (UMTS).

The idea is to use a higher symbol-rate, as you say:
"single-carrier data rate is going to be many times higher than the
rate across any one carrier of a comparable OFDM signal"

but not all the bandwidth is used to increase the symbol-rate. Rather,
a large multiple of the bandwidth is used by "fattening" the signal
spectrum with a known spreading sequence. The known spreading sequence
multiplies the high-rate symbol-rate.

In DS-CDMA (DS-SpSp), the fading multipath is combated by resolving the
multipath components separately for each of the stronger multipaths. In
OFDM this is not necessary if the cyclic-prefix is longer than the
delay-spread (the most-delayed strong multipath).

When everything is considered, a DS-CDMA (DS-SpSp) receiver has
relatively similar overall complexity compared to an OFDM receiver.
Each of the 2 methods has strengths and weaknesses, and the system
complexity is not low in either of them, if high performance ("almost
optimum") is required.

You will find DS-CDMA (DS-SpSp) described in almost every modern
communication book because of its popularity in the cellular world.

k_gideon@hotmail.com writes:

> A method of dealing with frequency-selective fading multipath in a
> single carrier system is:
> direct-sequence spread-spectrum (DS-SpSp). A very popular application
> in cellular communications is in DS-CDMA, used in the original CDMA
> (IS-95), and now in WCDMA (UMTS).
>
> The idea is to use a higher symbol-rate, as you say:
> "single-carrier data rate is going to be many times higher than the
> rate across any one carrier of a comparable OFDM signal"
>
> but not all the bandwidth is used to increase the symbol-rate. Rather,
> a large multiple of the bandwidth is used by "fattening" the signal
> spectrum with a known spreading sequence. The known spreading sequence
> multiplies the high-rate symbol-rate.
>
> In DS-CDMA (DS-SpSp), the fading multipath is combated by resolving the
> multipath components separately for each of the stronger multipaths. In
> OFDM this is not necessary if the cyclic-prefix is longer than the
> delay-spread (the most-delayed strong multipath).
>
> When everything is considered, a DS-CDMA (DS-SpSp) receiver has
> relatively similar overall complexity compared to an OFDM receiver.
> Each of the 2 methods has strengths and weaknesses, and the system
> complexity is not low in either of them, if high performance ("almost
> optimum") is required.
>
> You will find DS-CDMA (DS-SpSp) described in almost every modern
> communication book because of its popularity in the cellular world.

Hi k_gideon,

Thank you for discussing this with me. 

I should have been more specific and asked "without spreading, how can
one protect a single-carrier signal against multipath." Yes, I know
about DSSS. Sorry for my imprecision. 
-- 
%  Randy Yates                  % "Bird, on the wing,
%% Fuquay-Varina, NC            %   goes floating by
%%% 919-577-9882                %   but there's a teardrop in his eye..."
%%%% <yates@ieee.org>           % 'One Summer Dream', *Face The Music*, ELO
http://home.earthlink.net/~yatescr

Randy,

You have brought up an interesting topic for discussion here.  The way
i look at it, there are generally 3 methods for mitigating the
frequency selective fading in a single carrier system.

1) Precoding - if you know what the channel snapshots looks like in
time, then you pre-distort for this in the transmitter.
2) Equalisation - use a suitable adaptive technique to track and
mitigate the channel frequency selectivity.
3) Single Carrier Cyclic Prefix - this is really a subset of (2) above,
whereby you add a cyclic prefix to a single carrier system and perform
the 'simple' equalisation in the freq domain.  This is similar to the
OFDM scenario with a single sub-carrier, however, you modulate the
carrier with a time domain information signal.

So to answer your question, it boils down to you either protect the
single carrier by sticking in a prefix a la OFDM, or you estimate (via
training sequence) and compensate  at either the transmitter or
reciever.  I think that's all of the general methods...

If the real crux of your question is that of comparing OFDM and single
carrier (all things being equal) then the answer is for uncoded systems
you can get situations whereby single carrier outperforms OFDM.
However, when you add coding into the equation, then OFDM comes out as
the winner.   The comparison between OFDM and DS-CDMA is a long and
labourious one and not one i want to go into.

col


Randy Yates wrote:
> k_gideon@hotmail.com writes:
>
> > A method of dealing with frequency-selective fading multipath in a
> > single carrier system is:
> > direct-sequence spread-spectrum (DS-SpSp). A very popular application
> > in cellular communications is in DS-CDMA, used in the original CDMA
> > (IS-95), and now in WCDMA (UMTS).
> >
> > The idea is to use a higher symbol-rate, as you say:
> > "single-carrier data rate is going to be many times higher than the
> > rate across any one carrier of a comparable OFDM signal"
> >
> > but not all the bandwidth is used to increase the symbol-rate. Rather,
> > a large multiple of the bandwidth is used by "fattening" the signal
> > spectrum with a known spreading sequence. The known spreading sequence
> > multiplies the high-rate symbol-rate.
> >
> > In DS-CDMA (DS-SpSp), the fading multipath is combated by resolving the
> > multipath components separately for each of the stronger multipaths. In
> > OFDM this is not necessary if the cyclic-prefix is longer than the
> > delay-spread (the most-delayed strong multipath).
> >
> > When everything is considered, a DS-CDMA (DS-SpSp) receiver has
> > relatively similar overall complexity compared to an OFDM receiver.
> > Each of the 2 methods has strengths and weaknesses, and the system
> > complexity is not low in either of them, if high performance ("almost
> > optimum") is required.
> >
> > You will find DS-CDMA (DS-SpSp) described in almost every modern
> > communication book because of its popularity in the cellular world.
>
> Hi k_gideon,
>
> Thank you for discussing this with me.
>
> I should have been more specific and asked "without spreading, how can
> one protect a single-carrier signal against multipath." Yes, I know
> about DSSS. Sorry for my imprecision.
> --
> %  Randy Yates                  % "Bird, on the wing,
> %% Fuquay-Varina, NC            %   goes floating by
> %%% 919-577-9882                %   but there's a teardrop in his eye..."
> %%%% <yates@ieee.org>           % 'One Summer Dream', *Face The Music*, ELO
> http://home.earthlink.net/~yatescr

Randy Yates wrote:


>>>So I guess I have two questions: 1) Is it possible to design
>>>single-carrier systems (including the receiver) which are just as
>>>immune to multipath as multi-carrier systems?
>>
>>Certainly. Although it will require a huge amount of computation.
> 
> Why? What sort of things would you have to do to a single-carrier
> signal to "get out the multipath?"

The single carrier receiver will have to use an adaptive model of the 
channel and apply some sort of the maximum likelihood algorithm for the 
optimal demodulation. This is a heavy computational task.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com