23. Radio Transmission: Modulation

 

23. Radio Transmission: Modulation



17.12.2018

Radio Transmission: Modulation – Video: Register for Course


Radio Transmission: Modulation – Transcript:

Radio Transmission Modulation

  • Now, after several operations on the bits (encoding, puncturing, interleaving) we move to modulation of the symbols to be fed to the antenna.
  • Sine wave parameters that can be modified to carry bits include – Amplitude, frequecy, phase
  • The first type of modulation – the simplest one is AM. In this case we just take the bits and modify the amplitudę of our carrier wave according to the data – i.e. 0 means smaller amplitudę, 1 means higher amp (A2). We have to set the symbol duration – to make the receiver know where the next symbol occur – as due to noise and interference, the signal will be varying all the time. In this case – there are 2 amplitudę levels to represent individual bit value each – so if we have 6 bits we have 6 symbols
  • The first possibility to improve the throughput of the system is to use more amp levels where each level represent 2 bits – thus we can transmit all the 6 bits over 3 symbols – s the improvement in throughput is 2x. On the other hand more amplitudę levels makes it more susceptible to errors – i.e. the same noise sample will make it more possible to pass the decision treshold in the wrong direction
  • So in order to decrease the erroreneous characteristic of AM modulation, its better to use constant amplitudę and change other parameters of the sine waves – ie. Frequency -frequency modulation (higher frequency bit 1, lower freq – bit 0) or phase modulation – where each bit is using the same sine wave freq and amplitudę, but the beginning of the symbol starts at 0 or 180degrees, and by this, we can alter the wave to represent the bit stream

Radio Transmission Modulation Mapping

  • The same 3 cases can be seen on the I/Q plane – i.e. inphase is representing a sine wave starting at 0degree and with certain amplitudę, while quadrature phase is a shift of sinewave to 90deg making it cosine – so, we can say that on I axis we transmit a symbol using sine, and on Q axis –cosine
  • AM correspond to using sinewave with 2 amplitudę levels, so we have amp =1, phase =0. and I component only
  • When we shift to 4 amp levels with AM, we still capture the data transmission by different I values, but with smaller distances between them
  • PM in turn uses max amp value, but the I value is negative, to represent phase of 180deg – this is also called BPSK – binary phase shift keeing

Radio Transmission Mapping 2

  • When we compbine both am and pm we get QAM where the information is carried by both the amplitudę and the phase (but freq is constant)
  • So e.g. we use amp = 1 and phase of 45deg that requires both components to be transmitted ie.. Sine and cosine – the other points on the IQ plane represent different set of bits – in this case we have 16 points (also called 16QAM), that means we can assign 4 bits each –2^4 = 16

So the overal scheme is as follows – we pair (or quadruple in this case) the consecutive bits from the stream – then assign amplitudę and phase accroding to th the mapping scheme, and thus thecarrier amp and phase

Radio Transmission Modulation Mapping 3

  • Here we show different mapping schemes in which each individual point (symbol), can carry different number of bits 1 in BPSK, to up to 8 in 256QAM (2^8) – max mod scheme in LTE e.g. is 1024 at this point in time
  • What you can see on this diagram is that the maximum amplitudę is not changed, so that when putting more and more points to this scheme, we get smaller and smaller distances between each points – thus its easier and easier to make a mistake by receiving the symbol – reading the wrong point – thus in order to receive the highest possible modulation scheme – the signal quality should be very good, on the other hand if we have bad channel we shold switch to lower order modulation (as this is the name – modulation order) thus allowing more noise to still not making the receiver to be wrong.

Radio Transmission RF Modulation

  • Now we have those In phase and Q components – i.e. amp and phase. This goes through the RF modulation, to put the signal on the carrier wave, e.g. 5.8GHz as in our example
  • For this we have analogue components, like local oscillator, phase shifter, RF mixer and power amplifier
  • The local oscillator generates a cosine wave of the Fc, while phase shifter makes the second branch, with sine wave.
  • The inphase and quadrature compoments are multipled with the relevant waves (sine and cosine) – also called „upconverting”
  • Then they are combined and radiated through the antenna
  • So in the air we have both components – sine and cosine, that is received by the receiver, which has a mirrorred set of elements – the local oscillator mixes the signal with the same carrier frequency sine and cosine to get the „baseband signal”, and then the LPF, cuts off the higher frequency components, left outs of the „downconverting” operation.

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