2.  We use the term modulation to refer to changes made in a carrier -according to the information being sent  Modulation takes two inputs -a carrier -and a signal

5. To distinguish between analog and digital modulation -We use the term shift keying rather than modulation

7.  ASK is the abbreviation of Amplitude Shift Keying  Ask is called On-Off Keying.  The simplest and most common form of operate as a switch.

8.  It is Abbreviated as Frequency shift keying  Values represented by different frequencies (near carrier)  Less susceptible to error than ASK  Typically used up to 1200bps on voice grade lines  High frequency radio  Even higher frequency on LANs using co-ax

9.  Amplitude and frequency modulations require at least one cycle of a carrier wave to send a single bit  unless a special encoding scheme is used  The number of bits sent per unit time can be increased  if the encoding scheme permits multiple bits to be encoded in a single cycle of the carrier  Data communications systems often use techniques that can send more bits  PSK changes the phase of the carrier wave abruptly  each such change is called a phase shift * Phase of carrier signal is shifted to represent data

10. Quadrature amplitude modulation is a combination of ASK and PSK. Note

11.  To send more than one bit at a time  It is possible to send two bits on one wave by defining four different amplitudes.  The same approach can be used for frequency and phase modulation.

13. In practice, the maximum number of bits that can be sent with any one of these techniques is about five bits. The solution is to combine modulation techniques. One popular technique is quadrature amplitude modulation (QAM) involves splitting the signal into eight different phases, and two different amplitude for a total of 16 different possible values, giving us lg(16) or 4 bits per value.

16. *Minimum Shift Keying  MSK is a variant of OQPSK  The rectangular symbol pulse is replaced by a half cycle sinusoidal symbol pulse.  MSK provides a constant envelope signal which has it’s phase continuous at all times including the interbit switching times.  In MSK, also the phase shifts can be detected with an I or Q modulator.  At even numbered symbols, the polarity of I channel conveys the transmitted data and at odd numbered symbols the polarity of Q channel conveys the data.  A phase shift of + 90 degrees represents a data bit equal to 1 and a phase shift of –90 degrees represents a data bit equal to zero

17. It possesses properties such as: constant envelope spectral efficiency good BER performance self-synchronizing capability. MSK PROPERTIES

19. Gaussian Minimum Shift Keying Modulation scheme used in GSM Bandwidth-time product Describes the amount the symbols overlap BT = 0.3 for GSM networks Good spectral efficiency At the expense of some inter-symbol interference (ISI) Data rate: 270.8 kbps

20. GMSK is based on MSK Minimum Shift Keying Linear phase changes Spectrally efficient At baseband, bit transitions are represented by ½ cycle sinusoid

21.  Improved spectral efficiency  Power Spectral Density  Reduced main lobe over MSK  Requires more power to transmit data than many comparable modulation schemes

22. Binary sequence GMSK modulated Signal  ()t  ztt I ()cos()   ztt Q ()sin()  5 1 05101520 0 1 05101520 0 1 t 05101520 1 0 t 05101520 -5 0 t 05101520 0 t t in rd