Ec 2401 wireless communication unit 3
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The document summarizes various digital modulation and demodulation schemes used in wireless communication systems. It describes the structure of a basic wireless communication link and then provides details about modulation formats such as BPSK, DPSK, QPSK, OQPSK, and π/4 QPSK. It explains the key aspects of each scheme such as symbol mapping, transmitter and receiver operations, and their advantages over other schemes in terms of spectral efficiency and robustness to noise and fading channels.
![Types of Modulation formats
• Binary Phase Shift Keying
[BPSK]
• Differential Phase Shift
Keying [DPSK]
• Quadrature Phase Shift
Keying [QPSK]
• Offset – Quadratrure
Phase Shift Keying
[OQPSK]
• Pi / 4 Quatrature Phase
Shift Keying [pi / 4 -
QPSK]
• Binary Frequency Shift
Keying [BFSK]
• Minimum Shift Keying
[MSK]
• Gaussian Minimum Shift
Keying [GMSK]](https://image.slidesharecdn.com/vstho8ptwvfny9lkcufq-signature-1a54dbf6b9107cf4725eb896f30d0069f8ca16fc0693247580a62f09bb5c127d-poli-170906152748/85/Ec-2401-wireless-communication-unit-3-12-320.jpg)
![Quadrature Phase Shift Keying - QPSK
• It has twice the bandwidth efficiency of the
BPSK, since two bits are transferred in a single
symbol.
• The phase of the signal will take one of the
four equally spaced values such as
0,pi/2, pi, 3pi/2 [or] pi/4, 3pi/4, 5pi/4, 7pi/4.
• The QPSK symbol is given by:](https://image.slidesharecdn.com/vstho8ptwvfny9lkcufq-signature-1a54dbf6b9107cf4725eb896f30d0069f8ca16fc0693247580a62f09bb5c127d-poli-170906152748/85/Ec-2401-wireless-communication-unit-3-28-320.jpg)
Ec 2401 wireless communication unit 3
- 1. UNIT – 3 WIRELESS TRANSCEIVERS
- 2. SYLLABUS • Structure of a wireless communication link • Modulation and demodulation schemes – BPSK – DQPSK – QPSK – Pi / 4 QPSK – OQPSK – BFSK – MSK – GMSK • Power spectrum and error performance in fading channels
- 4. Structure of a wireless communication link
- 5. Block Explanation • Information source: – Provides the source signal – Can be either analog or digital • Source coder • They are used to reduce the redundancy of the source messages. In order to improve the bit rate. • Original message bits are converted to symbols • Ex. Zero padding
- 6. • Channel coder – This process adds the additional bits in order to protect data against transmission errors. – Ex. Error detection codes, header and trailer bits, Reed Solomon codes, CRC Codes etc., • Modulator – This converts the input bit stream suitable for transmission. – Converts the low frequency signals to high frequency signals • Channel – Provides the electrical connection between the transmitter and receiver. – The various channels used are pairs of wires, co axial cables, optical fibers or radio channels
- 7. • Diversity Combiner: – A normal receiver will receive multiple signals from various antennas. All signals will be combined here. • Equalizers – Mainly they are used to reduce the ISI and dispersion in the signal caused by the channels • Demodulator – They are the reverse process of the modulation. – They extracts the message signal from the modulated signal.
- 8. • Channel decoder – Used to reconstruct the original wave form from the encoded signal . – Inverse algorithm of the encoder is used to reconstruct the original message bits • Source decoder – They convert the symbols to message bits • Data sink – These devices converts the waveform to analog signals and they are fed to the respective devices.
- 11. Introduction • Deciding the modulation and demodulation formats is very important in deciding the following parameters. • Spectral efficiency: – It must be as high as possible • Adjacent channel interference: – It must be small • Sensitivity with respect to noise: – This must be small • Robustness with respect to delay and dispersion: – This must be as large as possible because channel itself introduces the delay and dispersion. • Easy to generate waveforms: – The hardware requirements should be easy to implement and must satisfy the practical needs.
- 12. Types of Modulation formats • Binary Phase Shift Keying [BPSK] • Differential Phase Shift Keying [DPSK] • Quadrature Phase Shift Keying [QPSK] • Offset – Quadratrure Phase Shift Keying [OQPSK] • Pi / 4 Quatrature Phase Shift Keying [pi / 4 - QPSK] • Binary Frequency Shift Keying [BFSK] • Minimum Shift Keying [MSK] • Gaussian Minimum Shift Keying [GMSK]
- 13. Binary Phase Shift Keying - BPSK
- 14. BPSK • In bpsk , the phase of the constant amplitude carrier is shifted between 2 values according to the possible signals • Since it is binary we have only 2 symbols “1” and “0” • The transmitted signal is given by
- 15. Wave form of BPSK
- 16. • In general the BPSK message signal is given by • Constellation diagram of BPSK: – It is the graphical representation to compute the bit error probabilities
- 17. • The complex representation of the BPSK Signals are given by • Power spectral density of the BPSK is given by
- 18. PSD of BPSK
- 19. Demodulation in BPSK • Demodulation in Rx – Requires reference of Tx signal in order to properly determine phase • carrier must be transmitted along with signal – Called Synchronous or “Coherent” detection • complex & costly Rx circuitry • good BER performance for low SNR → power efficient
- 20. BPSK Receiver
- 21. Drawbacks of BPSK • The major drawbacks of BPSK are – Only one bit is used per symbol, thus higher data rates are not possible – It requires the coherent detection method, which requires the prior knowledge of phase and amplitude of the transmitted signal during detection.
- 22. END of BPSK
- 23. DPSK • DPSK → Differential Phase Shift Keying – Non-coherent Rx can be used • easy & cheap to build • no need for coherent reference signal from Tx – Bit information determined by transition between two phase states • incoming bit = 1 → signal phase stays the same as previous bit • incoming bit = 0 → phase switches state
- 24. Generation of Random Sequence
- 26. Receiver - DPSK
- 27. END of DPSK
- 28. Quadrature Phase Shift Keying - QPSK • It has twice the bandwidth efficiency of the BPSK, since two bits are transferred in a single symbol. • The phase of the signal will take one of the four equally spaced values such as 0,pi/2, pi, 3pi/2 [or] pi/4, 3pi/4, 5pi/4, 7pi/4. • The QPSK symbol is given by:
- 30. Quadrature Phase Shift Keying - QPSK
- 33. Modulation Steps • Unipolar binary sequence are converted to bi polar NRZ sequence • The bit stream M(t) is split in to two bit streams MI(t) and MQ(t) – MI(t) In phase streams (or) Even Stream – MQ(t) Quadrature streams (or) Odd Stream The binary sequences are modulated separately using Ø1(t) and Ø2(t). These 2 signals are now considered as the BPSK Signals and they are BPSK Modulated.
- 34. Splitting up of input binary sequence
- 35. QPSK Transmitter
- 36. QPSK Demodulator
- 37. END OF QPSK
- 38. OFFSET Quadrature Phase Shift Keying - OQPSK • Offset QPSK – The occasional phase shift of π radians can cause the signal envelope to pass through zero for just in instant. – Any kind of hard limiting or nonlinear amplification of the zero-crossings brings back the filtered sidelobes • since the fidelity of the signal at small voltage levels is lost in transmission. – OQPSK ensures there are fewer baseband signal transitions applied to the RF amplifier, • helps eliminate spectrum regrowth after amplification.
- 39. Example above: First symbol (00) at 0º, and the next symbol (11) is at 180º. Notice the signal going through zero at 2 microseconds. This causes problems.
- 40. • Using an offset approach: First symbol (00) at 0º, then an intermediate symbol at (10) at 90º, then the next full symbol (11) at 180º. – The intermediate symbol is used halfway through the symbol period. – It corresponds to allowing the first bit of the symbol to change halfway through the symbol period. – The figure below does have phase changes more often, but no extra transitions through zero. – IS-95 uses OQPSK, so it is one of the major modulation schemes used.
- 42. • In QPSK signaling, the bit transitions of the even and odd bit streams occur at the same time instants. • but in OQPSK signaling, the even and odd bit Streams, mI(t) and mQ(t), are offset in their relative alignment by one bit period (half- symbol period)
- 43. QPSK Vs OQPSK
- 44. • This time alignment will avoid the simultaneous transitions of the message bits on In phase and quadrature bit streams. • In BPSK, the maximum phase shift will be 180o • In OQPSK, the maximum phase shift will be 90o END OF OQPSK
- 45. π/4 QPSK • π/4 QPSK – The π/4 shifted QPSK modulation is a quadrature phase shift keying technique • offers a compromise between OQPSK and QPSK in terms of the allowed maximum phase transitions. – It may be demodulated in a coherent or non coherent fashion. • greatly simplifies receiver design. – In π/4 QPSK, the maximum phase change is limited to ± 135o – in the presence of multipath spread and fading, π/4 QPSK performs better than OQPSK
- 47. I. Differential detection of pi/4 QPSK
- 48. II. IF Differential Detection
- 49. III. FM Discriminator detector