Hello, I am studying implementation of a BPSK modulator and demodulator for radio applications and I am very confused. From what I have read, in the transmitter section, the binary data is given to a BPSK mapper, which maps to bits to one symbol ( 0 =cos(wt), 1=-cos(wt) ). But I had read we are doing pulse shaping because we cannot transmit square pulses through the channel, but we are not sending square waves anyway so where does pulse shaping come into picture? --------------------------------------- Posted through http://www.DSPRelated.com
Confused with digital modulations
Started by ●February 8, 2016
Reply by ●February 8, 20162016-02-08
On 8.2.16 14:46, mohitdaksh wrote:> Hello, > > I am studying implementation of a BPSK modulator and demodulator for radio > applications and I am very confused. > > From what I have read, in the transmitter section, the binary data is > given to a BPSK mapper, which maps to bits to one symbol ( 0 =cos(wt), > 1=-cos(wt) ). > > But I had read we are doing pulse shaping because we cannot transmit > square pulses through the channel, but we are not sending square waves > anyway so where does pulse shaping come into picture?You change the bits so that a 0 becomes -1 and an 1 becomes +1, shape the stream of (-1, +1) and multiply the carrier (cos(wt)) with the shaped modulating signal. -- -TV
Reply by ●February 8, 20162016-02-08
On Mon, 08 Feb 2016 06:46:29 -0600, "mohitdaksh" <111830@DSPRelated> wrote:>Hello, > >I am studying implementation of a BPSK modulator and demodulator for radio >applications and I am very confused. > >From what I have read, in the transmitter section, the binary data is >given to a BPSK mapper, which maps to bits to one symbol ( 0 =cos(wt), >1=-cos(wt) ). > >But I had read we are doing pulse shaping because we cannot transmit >square pulses through the channel, but we are not sending square waves >anyway so where does pulse shaping come into picture?There are several different ways of looking at this, one of which Tauno already alluded to, where you first creat the NRZ bit stream with +/-1 values, filter it, and then mix it up to IF/RF with an oscillator. From that perspective it's easy to see that the spectrum of the NRZ bit stream has a sinx/x shape (the transform of the rectangular pulses). It is essentially the same thing if you look at it the way you mentioned, where a 1 is -cos(wt) and a 0 is cos(wt). In that case you have a tone (the cos(wt)), multiplied by the NRZ bit stream, still with +/-1 values. The resulting spectrum is then the convolution of the tone and the sinx/x shape of the NRZ stream. So either perspective gets you to exactly the same place. The pulse shaping filter provides a number of benefits, one being that the transmit spectrum is then better contained to a much narrower region than the original sinx/x shape, so that the spectrum can be used more efficiently by other signals without interference. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●February 10, 20162016-02-10
>On Mon, 08 Feb 2016 06:46:29 -0600, "mohitdaksh" <111830@DSPRelated> >wrote: > >>Hello, >> >>I am studying implementation of a BPSK modulator and demodulator forradio>>applications and I am very confused. >> >>From what I have read, in the transmitter section, the binary data is >>given to a BPSK mapper, which maps to bits to one symbol ( 0 =cos(wt), >>1=-cos(wt) ). >> >>But I had read we are doing pulse shaping because we cannot transmit >>square pulses through the channel, but we are not sending square waves >>anyway so where does pulse shaping come into picture? > >There are several different ways of looking at this, one of which >Tauno already alluded to, where you first creat the NRZ bit stream >with +/-1 values, filter it, and then mix it up to IF/RF with an >oscillator. > >From that perspective it's easy to see that the spectrum of the NRZ >bit stream has a sinx/x shape (the transform of the rectangular >pulses). > >It is essentially the same thing if you look at it the way you >mentioned, where a 1 is -cos(wt) and a 0 is cos(wt). In that case >you have a tone (the cos(wt)), multiplied by the NRZ bit stream, still >with +/-1 values. The resulting spectrum is then the convolution of >the tone and the sinx/x shape of the NRZ stream. > >So either perspective gets you to exactly the same place. > >The pulse shaping filter provides a number of benefits, one being that >the transmit spectrum is then better contained to a much narrower >region than the original sinx/x shape, so that the spectrum can be >used more efficiently by other signals without interference. > > >Eric Jacobsen >Anchor Hill Communications >http://www.anchorhill.comThanks Eric and Tauno --------------------------------------- Posted through http://www.DSPRelated.com
