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
simple-minded non-OFDM signal design question
Started by ●September 26, 2006
Reply by ●September 26, 20062006-09-26
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
Reply by ●September 26, 20062006-09-26
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
Reply by ●September 26, 20062006-09-26
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.
Reply by ●September 26, 20062006-09-26
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
Reply by ●September 27, 20062006-09-27
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
Reply by ●September 27, 20062006-09-27
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.
Reply by ●September 27, 20062006-09-27
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
Reply by ●September 27, 20062006-09-27
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
Reply by ●September 27, 20062006-09-27
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