On Sun, 22 Nov 2015 09:53:00 -0600, "Sharan123" <99077@DSPRelated> wrote:>Dear Glen, > >>Consider a popular baseband data transmission method, ethernet. >> >>Usual 10 megabit/s ethernet is manchester coded, such that it >>needs 10MHz of bandwidth. If you send a stream of all 0's or >>all 1's, the frequency on the wire is 10MHz. A stream of >>alternating 1's and 0's is 5MHz, and other bit patterns are >>in between. Note that it doesn't go down to 0Hz, and since >>it is transformer coupled can't go down to 0Hz, but is still >>considered baseband. > >Thanks. The above explanation helps a lot. > >>An AM radio station has about 10kHz bandwidth, >>that is, 5kHz above and below the frequency on the dial. > >Are you saying that the frequency of a specific AM signal that is >transmitted can vary by +-5KHz. But being AM modulation, I assumed that >carrier frequency should not vary (in theory at least). > >>> That also brings to one more question - what type of modulation is >>applied >>> at the RF frequency in mobile communication? >> >>Which mobile communication? AM radio uses AM/DSB (double side band). > >I was mainly interested to know the modulation used in case of cell phone >communication.Which standard? There have been a variety of modulation types and techniques used for cellular communications. The GSM standard used GMSK, CDMA used spread-spectrum PSK, LTE uses OFDM with PSK subcarriers. In other words, there are multiple methods used in cellular communications.>>You might try the ARRL handbook, which a library might have, for >>a lot of information on modulation methods and bandwidth. > >Sure. Thanks ... > >--------------------------------------- >Posted through http://www.DSPRelated.comEric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Baseband Bandwidth
Started by ●November 21, 2015
Reply by ●November 22, 20152015-11-22
Reply by ●November 22, 20152015-11-22
"Sharan123" <99077@DSPRelated> writes:>>Do not confuse the terms "data rate" and "bandwidth". While they are >>related, they are definitely not the same thing. >> >>Take a look at the block diagram in Figure 1 here: >>http://www.digitalsignallabs.com/tutorial.pdf. The "data rate" is the >>rate at the output of the data source. However, the bandwidth is the >>bandwidth of the signal m(t) (at the output of the modulator). > >>The bandwidth of the signal depends on several things other than the >>data rate, e.g., modulation scheme, type and amount of coding, access >>type (e.g., CDMA spreads the bandwidth very wide). >> >>Finally, it doesn't matter whether the signal is baseand or carrier: >>the same bandwidth and data rate applies either way. > > Dear Randy, > > If I may ask, in case of modern communications (like mobile), we have data > source, baseband bandwidth and then RF carrier. > I understand that data source sources data limited by max. data rate. > I can understand that RF carrier occupies certain frequency range as the > RF carrier frequency is modulated with user data. > > Where I get a little confused is how the baseband bandwidth is used. > Is there a frequency modulation due to which frequency variations > happen?Hi Sharan, Yes. A digital communication signal is commonly formed at complex baseband (I/Q) and then modulated up to the carrier near the transmitter using an I/Q (or "quadrature") modulator. The I/Q modulator conceptually translates a complex baseband signal of bandwidth B from -B/2 to +B/2 to a real signal at carrier frequency fc in a bandwidth from (fc - B/2) to (fc + B/2). Note that since the output is real, the spectrum is symmetric (specifically, Hermitian symmetric), and so there is also the same information (reflected in frequency) at -(fc + B/2) to -(fc - B/2) Note that I was surprised to find the Wikipedia article on quadrature modulation (https://en.wikipedia.org/wiki/Quadrature_modulation) very "weak" - I wouldn't use it or rely on it.> Also, I have one more conceptual question - in mobile communication, > spectrum is scarce and every operator is given a frequency band to > operate. My question is, why not use some sort of amplitude modulation, in > which case you need just one frequency per operator and not frequency > band?You seem to be assuming that amplitude modulation requires little or no bandwidth. That is absolutely untrue. Once you begin to amplitude modulate a sine wave, you spread its spectrum out from a infinitely thin line to something much wider. You can use linear system theory to see this.> That also brings to one more question - what type of modulation is applied > at the RF frequency in mobile communication?As Eric said, there are a whole bunch, depending on the network. GSM is GMSK, or Gaussian MSK, a form of frequency-shift keying, which is also in turn a form of FM (frequency modulation). GSM/EDGE switches to a type of PSK (phase-shift keying) modulation (3pi/8-shifted 8 PSK). Etc.> PS: sorry for some basic questions but I don't have much background into > wireless communication ...That's perfectly fine - just realize that your depth of understanding will vary proportionally to the amount of time you spend studying the subject (just like about every other engineering subject). I also want to mention that this subject material is not simple. It takes most of us a few years of studying in thick textbooks to "get it," so don't expect much from a few usenet posts. But I do want to encourage you to seek out and study more if the topic interests you. And by all means, all questions along these lines are welcome here. -- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Reply by ●November 22, 20152015-11-22
Sharan, Allow me to follow up a few items below. Randy Yates <yates@digitalsignallabs.com> writes:> "Sharan123" <99077@DSPRelated> writes: > >>>Do not confuse the terms "data rate" and "bandwidth". While they are >>>related, they are definitely not the same thing. >>> >>>Take a look at the block diagram in Figure 1 here: >>>http://www.digitalsignallabs.com/tutorial.pdf. The "data rate" is the >>>rate at the output of the data source. However, the bandwidth is the >>>bandwidth of the signal m(t) (at the output of the modulator). >> >>>The bandwidth of the signal depends on several things other than the >>>data rate, e.g., modulation scheme, type and amount of coding, access >>>type (e.g., CDMA spreads the bandwidth very wide). >>> >>>Finally, it doesn't matter whether the signal is baseand or carrier: >>>the same bandwidth and data rate applies either way. >> >> Dear Randy, >> >> If I may ask, in case of modern communications (like mobile), we have data >> source, baseband bandwidth and then RF carrier. >> I understand that data source sources data limited by max. data rate. >> I can understand that RF carrier occupies certain frequency range as the >> RF carrier frequency is modulated with user data. >> >> Where I get a little confused is how the baseband bandwidth is used. >> Is there a frequency modulation due to which frequency variations >> happen? > > Hi Sharan, > > Yes. A digital communication signal is commonly formed at complex > baseband (I/Q) and then modulated up to the carrier near the transmitter > using an I/Q (or "quadrature") modulator. The I/Q modulator conceptually > translates a complex baseband signal of bandwidth B from -B/2 to +B/2 to > a real signal at carrier frequency fc in a bandwidth from (fc - B/2) to > (fc + B/2). Note that since the output is real, the spectrum is > symmetric (specifically, Hermitian symmetric), and so there is also the > same information (reflected in frequency) at -(fc + B/2) to -(fc - B/2) > > Note that I was surprised to find the Wikipedia article on quadrature > modulation (https://en.wikipedia.org/wiki/Quadrature_modulation) very > "weak" - I wouldn't use it or rely on it. > >> Also, I have one more conceptual question - in mobile communication, >> spectrum is scarce and every operator is given a frequency band to >> operate. My question is, why not use some sort of amplitude modulation, in >> which case you need just one frequency per operator and not frequency >> band? > > You seem to be assuming that amplitude modulation requires little or no > bandwidth. That is absolutely untrue. Once you begin to amplitude > modulate a sine wave, you spread its spectrum out from a infinitely thin > line to something much wider. You can use linear system theory to see > this.The bandwidth of an AM-modulated signal is directly related to the bandwidth of the modulating signal.>> That also brings to one more question - what type of modulation is applied >> at the RF frequency in mobile communication?After rereading I just wanted you to be careful to understand that the modulation (QAM, PSK, MSK, etc.) is not applied "at the RF frequency" (necessarily). It is wherever the signal is created, which is usually complex baseband (but not necessarily, this is an implementation choice).> As Eric said, there are a whole bunch, depending on the network. GSM is > GMSK, or Gaussian MSK, a form of frequency-shift keying, which is also > in turn a form of FM (frequency modulation). GSM/EDGE switches to a type > of PSK (phase-shift keying) modulation (3pi/8-shifted 8 PSK). Etc. > >> PS: sorry for some basic questions but I don't have much background into >> wireless communication ... > > That's perfectly fine - just realize that your depth of understanding > will vary proportionally to the amount of time you spend studying the > subject (just like about every other engineering subject). > > I also want to mention that this subject material is not simple. It > takes most of us a few years of studying in thick textbooks to "get it," > so don't expect much from a few usenet posts. But I do want to encourage > you to seek out and study more if the topic interests you. And by all > means, all questions along these lines are welcome here.By the way, an excellent text for this topic would be: @BOOK{lyonsthird, title = "{Understanding Digital Signal Processing}", edition = "third", author = "{Richard~G.~Lyons}", publisher = "Prentice Hall", year = "2011"} -- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Reply by ●November 23, 20152015-11-23
Sharan123 <99077@dsprelated> wrote: (snip)> Thanks. The above explanation helps a lot.(snip, I wrote)>>An AM radio station has about 10kHz bandwidth, >>that is, 5kHz above and below the frequency on the dial.> Are you saying that the frequency of a specific AM signal that is > transmitted can vary by +-5KHz. But being AM modulation, I assumed that > carrier frequency should not vary (in theory at least).http://www.purplemath.com/modules/idents.htm#product sin(x)sin(y) = (cos(x-y)-cos(x+y))/2 Multiplying a sine wave by another sine creates new frequencies at the sum and different frequencies. It takes a little getting used to, but once you do you will understand much about modulation and spectral bandwidth. Mutliplying a sine by a sum of sine waves generates new frequency components at the sum and different frequencies, which are called the upper and lower sideband, respectively. Since Fourier tells us that any function can be generated as a sum of sinusoids, when used in an AM modulator, you get a whole image of the source shifted up above the carrier, and a mirror image below the carrier. That is, the bandwidth of the modulator output is twice the highest frequency of the modulator input. AM radio stations are space 10kHz to allow for modulation up to 5kHz. (It might be that they can go over 5kHz if there isn't a close frequency station nearby.) -- glen
Reply by ●November 23, 20152015-11-23
>You seem to be assuming that amplitude modulation requires little or no >bandwidth. That is absolutely untrue. Once you begin to amplitude >modulate a sine wave, you spread its spectrum out from a infinitely thin >line to something much wider. You can use linear system theory to see >this.Dear Randy, Yes. I intuitively assumed that amplitude modulation keeps frequency of the carrier intact. I have made similar assumption about frequency modulation. I stand corrected. I am going to try out a Matlab example to see this for myself.>As Eric said, there are a whole bunch, depending on the network. GSM is >GMSK, or Gaussian MSK, a form of frequency-shift keying, which is also >in turn a form of FM (frequency modulation). GSM/EDGE switches to a type >of PSK (phase-shift keying) modulation (3pi/8-shifted 8 PSK). Etc.As you have corrected in the following thread, these are not used at RF stage. So, I am interested to know the type of modulation happens at RF. --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●November 23, 20152015-11-23
>Which standard? There have been a variety of modulation types and >techniques used for cellular communications. The GSM standard used >GMSK, CDMA used spread-spectrum PSK, LTE uses OFDM with PSK >subcarriers. > >In other words, there are multiple methods used in cellular >communications.Dear Eric, I think I have created confusion by being generic. Sorry about that ... If I take LTE itself as an example, I believe that, 1) first the source data is converted into QPSK/16QAM/64QAM constellation points 2) these are then applied OFDM (normally through iFFT) 3) the OFDM signals are then modulated at RF level I would like to know what are the exact terminologies for 1), 2) and 3) above. Are 1) and 2) baseband modulation and 3), RF modulation? Thank you very much ... --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●November 23, 20152015-11-23
On 23.11.2015 3:32, Randy Yates wrote:> By the way, an excellent text for this topic would be: > > @BOOK{lyonsthird, > title = "{Understanding Digital Signal Processing}", > edition = "third", > author = "{Richard~G.~Lyons}", > publisher = "Prentice Hall", > year = "2011"} >Another great text is "Fundamentals of Communication Systems" (2013, 2nd edition) by Proakis and Salehi. It introduces you to the field of digital communications and is very accessible. It's fine as a stepping stone to more specialized books. -- Evgeny.
Reply by ●November 23, 20152015-11-23
>>Which standard? There have been a variety of modulation types and >>techniques used for cellular communications. The GSM standard used >>GMSK, CDMA used spread-spectrum PSK, LTE uses OFDM with PSK >>subcarriers. >> >>In other words, there are multiple methods used in cellular >>communications. > >Dear Eric, > >I think I have created confusion by being generic. Sorry about that ... > >If I take LTE itself as an example, I believe that, > >1) first the source data is converted into QPSK/16QAM/64QAMconstellation>pointsmapping>2) these are then applied OFDM (normally through iFFT)OFDM>3) the OFDM signals are then modulated at RF level >FDM Kaz --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●November 23, 20152015-11-23
"Sharan123" <99077@DSPRelated> writes:> [...] > As you have corrected in the following thread, these are not used at RF > stage. So, I am interested to know the type of modulation happens at > RF.None. -- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Reply by ●November 23, 20152015-11-23
On Mon, 23 Nov 2015 02:15:44 -0600, "Sharan123" <99077@DSPRelated> wrote:>>Which standard? There have been a variety of modulation types and >>techniques used for cellular communications. The GSM standard used >>GMSK, CDMA used spread-spectrum PSK, LTE uses OFDM with PSK >>subcarriers. >> >>In other words, there are multiple methods used in cellular >>communications. > >Dear Eric, > >I think I have created confusion by being generic. Sorry about that ... > >If I take LTE itself as an example, I believe that, > >1) first the source data is converted into QPSK/16QAM/64QAM constellation >points >2) these are then applied OFDM (normally through iFFT) >3) the OFDM signals are then modulated at RF level > >I would like to know what are the exact terminologies for 1), 2) and 3) >above. >Are 1) and 2) baseband modulation and 3), RF modulation? > >Thank you very much ..."Baseband" generally means the signal is at it's lowest frequency for simplest processing. For OFDM and most digital signals this means that the signal spectrum is centered at 0 and the signal spans from -BW/2 to +BW/2. For things like AM this means the signal may run from 0 to BW (or BW/2 for a double-sideband signal). Once the signal is mixed (i.e., translated) up to the frequency at which it is sent over the air, it is at RF. It may go through a two-step or multiple-step conversion to get to the RF frequency, and the intermediate steps are called IF (Intermediate Frequency). Generally the signal will have the same bandwidth at baseband, IF and RF. Things like double-sideband AM are an exception, where the modulated bandwidth is really half of the RF signal bandwidth. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
