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I am working on transmitting ECG signal over wireless body area networks. According to IEEE802.15.4 standards , I am using ZigBee transceiver at 2.4 GHZ . Complex baseband equivalent channel model is used.

The steps are as follows :

1.      Signal Compression

2.      Quantization

3.      Coding

4.      OQPSK modulation using the matlab function (oqpskmod)

5.      Fading Channel plus AWGN

6.      Equalization

7.      OQPSK Demodulation using the matlab function (oqpskdemod)

8.      Decoding and Dequantization

*  According to the IEEE 802.15.4 standard, a pulse shaping step is performed in the transmitter after the OQPSK modulation step . 

I didn`t perform this pulse shaping process, and I obtained reasonable results. Is it necessary to perform this step ?  If  yes , How should the receiver be modified ?

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  • $\begingroup$ are you using computer simulation or real air transmission (with DAC) ? $\endgroup$ – AlexTP May 14 '17 at 21:26
  • $\begingroup$ I use Matlab and use Complex baseband equivalent channel model which describe real air transmission @ AlexTP $\endgroup$ – Mohamed Aly May 14 '17 at 21:53
  • $\begingroup$ Thus you can follow what Dan Boschen has answered. $\endgroup$ – AlexTP May 15 '17 at 8:42
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The pulse shaping is only required if you are concerned with spectral efficiency, as the pulse shaping will convert what would otherwise be a Sinc function in frequency to a spectrum that is much closer in overall bandwidth to your symbol rate. Also if the receiver assumes a specific pulse shape, the sensitivity will be optimized by transmitting with the matched pulse shape (as the matched filter in the receiver will then present the optimum SNR. Other than that, it is not necessary.

See my picture below demonstrating the time domain waveforms and frequency domain before and after pulse shaping. Note that the primary motivation to do additional pulse shaping (beyond the rectangular pulse you are otherwise using, which is indeed a pulse shaping in itself) is for spectral efficiency. The pulse shaping is required in IEEE 802.15.4 ultimately due to meeting regulations (such as FCC in the US) to ensure we all use spectrum efficiently and within regulated allocations and not interfere with users in other spectrum allocations. Observe the dramatic change in transmitted spectrum with and without the pulse shaping filter in the example below!

Pulse Shaping

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  • $\begingroup$ Thanks a lot for your reply. I did not also use a matched filter in the receiver, just OQPSK demodulation followed by an equalizer. Is it required? @Dan Boschen $\endgroup$ – Mohamed Aly May 14 '17 at 20:02
  • $\begingroup$ No unless you need to meet regulatory requirements. I am not sure how much over the air transmission and at what power level you are transmitting but that would be the issue. (So you are using a matched filter, it's just your receiver matching is a rectangular shape as well). $\endgroup$ – Dan Boschen May 14 '17 at 20:22

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