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I want to do one project and I just want to make sure that I've understood correctly.

Human's voice frequency is around 85-255Hz.

When we talk on phone, carrier is modulated with voice signal in order to shift frequency spectrum to higher frequency for transmission. If we take for example GSM900 which is 876 - 915 MHz paired with 921 - 960 MHz. That means that our speech will be in this GSM900 range after modulation?

Moreover, how much bandwidth user gets in 876 - 915 MHz paired with 921 - 960 MHz range? Any articles about that?

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  • $\begingroup$ Human Voice frequency is not 85-255Hz. $\endgroup$ – arpit jain Aug 21 '18 at 7:36
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For the classical analog wired telephony lines, the human speech bandwidth is taken to be in [300 , 3000] Hz. This range can be extended by different similar applications such as wireless handsets, voice over IP, streaming apps, and GSM.

Most probably GSM also takes human speech range to be in the similar bandwidth, although later mobile phone standards 4G/4G+ etc also support higher quality voice services and hence consider a wider speech bandwidth.

Your understanding of modulation is basically correct for classical analog continuous wave amplitude modulation (AM, DSB-SC, SSB etc) in which a carrier cosine wave amplitude is continuously controlled by the instantaneous amplitude of the message signal which is voice in this case.

However, GSM for cellular phones employs a digital modulation scheme. Hence it's sending bits instead of continuous amplitudes. Note that it sends the bits not as 5V or 0V states (like it's in a digital computer wire), rather it still uses sorts of sine wave packets whose various attributes are used to convey the digital information. Other digital modulation techniques also exist.

Furthermore and most importantly, GSM is not sending the waveform shape of the input speech; rather it transforms the input speech into a compressed form (using LPC type of voice coder) and sends those bits instead.

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  • $\begingroup$ Not sure what you mean by versions G2/G3. The usual terms in telecom are 2G/3G, and GSM is definitely 2G. $\endgroup$ – MSalters Aug 21 '18 at 0:54
  • $\begingroup$ @MSalters Opps yes! Like most people, I also mixed the mobile phone with the GSM thing. So what I meant was of course, the first mobile phones before GSM (2G) was using analog. GSM (beginning by 2G of mobile telephony) was all digital ! Let's correct it, thanx. $\endgroup$ – Fat32 Aug 21 '18 at 1:33
  • $\begingroup$ @Andrew there's an edit for you, thanx for MSalter. $\endgroup$ – Fat32 Aug 21 '18 at 1:35
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GSM since it's first day has been digital (Fat32 is incorrect). That means you can't send analog audio. Luckily, analog audio had already been phased out in the 80s, in telecom.

In digital telephony, audio was digitally sampled at 8 kHz, 8 bits per sample, for a 64 kbit/s signal. If you has an ISDN phone, you got this quality at home, but it certainly was used inside and between the central phone switches.

GSM built on that 64 kbit/s idea, but used audio compression. The first codec used was GSM Full Rate, which achieved 5x compression for a 13 kbit/s net data stream.

GSM furthermore divides every frequency band into 8 slots, which means that phones can only transmit short 5 ms bursts and then have to wait for 7 other bursts from other phone users. Band spacing is a mere 200 kHz; a typical carrier gets 5Mhz blocks of spectrum which means 25 bands and 200 slots total.

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