1. Background
(Opening Slide)

2. Triptych Base Graphic (Fade-in)
In this video, we will discuss the transmission of bits, or zeros and ones,…

3. Fade back (Fade-out all but plane, radio channel and ground station)
…across the radio link between the airplane and ground station shown in this picture.

4. Radio Channel
This radio link is also sometimes called a "radio channel".
(Fade-in highlight of radio channel)

5. Plane/Ground/Radio Link
(Fade-out highlight, fade-in circle showing location of modulator block in aircraft)

6. Block1 Pullout (Wipe left modulator block from aircraft)
The radio system in the airplane can be broken into the three basic components shown in this block diagram,
namely, the voice coder, or vocoder, the modulator, and the Radio Frequency converter, or RF converter.
(fade-in circle showing location of modulator block in ground station)

7. Block2 Pullout (Wipe right modulator block from ground station)
The transmitted signal is sent through the radio channel to the receiver, where it passes through another RF converter, a demodulator, and a voice decoder, which reproduces the transmitted bits, or zeros and ones, and recreates the sound of the voice.

8. Circle Modulator (Circle modulator in first block)
In this video, we will focus on the modulator, and explain how it changes the radio wave to communicate the bits to the receiver.

9. Phone/Vocoder
Recall that the bits to be transmitted are generated by a vocoder, in the case of a voice call,…

10. Phone/Vocoder with Bits

11. Computer
…or could be data generated by a computer, such as .

12. Computer with Bits

13. Modulator
The basic idea is that we will change the radio wave according to the specific bits we want to transmit. The change in this radio wave serves as a symbol that corresponds to the specific bits we want to transmit.

14. Modulator Sequence Single 0 Bit In/Wave Out
An artificial example that helps explain how the modulator might change the radio wave is shown here.
(O-bit enters modulator, green wave exits)
Namely, the modulator converts a "zero" to the green signal, or wave,…

15. Modulator Sequence Single 1 Bit In/Wave Out
…and converts a "one" to the orange wave.
(1-bit enters modulator, orange wave exits)

16. Modulator Sequence 0 Bitstream in/Wave out
The radio wave can then change as each bit enters the modulator.
(Bitstream enters module, one bit per second, wave changes accordingly)

17. Modulator Sequence 1 Bitstream in/Wave out
After passing through the radio channel, the signal is processed at the receiver, which reads the changes in the radio wave and converts these back to the original bits.

18. 2 Flag Positions
Perhaps a useful analogy is to think of the modulator as being a flagman on a ship, who sends a zero by raising the flag, and a one by lowering the flag.

19. Modulator Sequence Flagman Up
The different flag positions can be thought of as different "symbols", which are used to represent a zero and one.

20. Modulator Sequence Flagman Down
Similarly, the modulator creates a new "symbol" by generating a specific radio wave. We will now look at the ways in which the modulator can change the radio wave to represent different bits.

21. Sine Wave
Let's take a closer look at a radio wave, which is shown here. This is also called a sinusoidal signal, or sine wave. One way to change the sine wave is to change how fast the signal rises and falls. This is measured by its frequency.

22. Sine Wave
Namely, this picture shows one up-down cycle of the sine wave. The frequency of a radio wave is simply the number of cycles per second.

23. Sine Wave (The cycles repeat one-by-one up to 10 cycles)
In this example, if it takes one second to transmit 10 cycles,…

24. Sine Wave 10 Hz …then the frequency is 10 cycles per second, or 10 Hz.
(Fade-in “Ten Cycles…” label)

25. Bit
0 Bit FSK
Now, to transmit a zero bit,…

26. Bit
0 Bit FSK with Wave
…we might transmit the green radio wave shown here,…

27. 0 Bit FSK with 10 Hz Label Bit …which has 10 cycles in a second,…
(Fade-in “Ten Cycles…” label)

28. Bit
1 Bit FSK
...and to transmit a one bit,…

29. Bit
1 Bit FSK with Wave
…we might transmit this orange radio wave,…

30. 1 Bit FSK with 5 Hz Label Bit
…which has a lower frequency, namely, 5 cycles per second.
(Fade-in “Five Cycles…” label)

31. FSK Summary
By shifting frequencies, as shown here, we can transmit a combination of zeros and ones. In other words, the series of zeros and ones we want to transmit are converted to a series of frequencies, which can be read at the receiver, and converted back to the original bits. In this case, the frequencies we transmit are the symbols which represent the bits.

32. Bit
FSK 0 Flag Back
Using the flagman analogy,…

33. Bit
FSK 0 Wave+Flag Up
an "up" position might correspond to the higher frequency,…

34. Bit
FSK 0 10 Hz Label
(Fade-in “Ten Cycles…” label)

35. Bit
FSK 1 Flag Back
…and the "down" position might correspond to the lower frequency.

36. Bit
FSK 1 Wave+Flag Down

37. Bit
FSK 5 Hz Label
(Fade-in “Five Cycles…” label)

38. FSK Type Summary
Changing the radio wave in this manner is a type of digital modulation, which is called Frequency-Shift Keying, or FSK.
For reasons that I will explain in a moment, however, FSK modulation would not work to transmit voice in the air-to-ground system the FCC specified. There is another way to change the radio wave that would be a far more logical and apparent choice to someone experienced in digital communications, such as Hughes Network Systems.

39. Original Radio Wave
Let's return to the original radio wave. Another way to change the signal is to change the amplitude, or the height of the wave.

40. Bit
ASK 0 Bit
Namely, if this green radio wave again represents a zero bit,…

41. Bit
ASK 0 Wave
(Wipe right green wave)

42. Bit
ASK 1 Bit
…then to transmit a one bit, we may reduce the height, or amplitude, by a half.

43. Bit
ASK 1 Wave
(Wipe right orange wave)

44. Bit
ASK Amplitude Label
(Fade-in “Amplitude Reduced…” label)

45. ASK Summary
By shifting the amplitude back and forth, we can again transmit a combination of zeros of ones. In this case, the series of zeros and ones we want to transmit are converted to a series of amplitudes, which can be read at the receiver, and converted back to the original bits. The amplitudes we transmit are therefore the symbols used to represent the bits.

46. Bit
ASK 0 Flag Back
Using the flagman analogy again,…

47. Bit
ASK 0 Wave+Flag Up
…the larger amplitude corresponds to the "up" position…,
(Wipe right green wave)

48. Bit
ASK 1 Flag Back
…and the smaller amplitude corresponds to the "down" position.

49. Bit
ASK 1 Wave +Flag Down
(Wipe right orange wave)

50. Bit
ASK Amplitude Label
(Fade-in “Amplitude Reduced” label)

51. ASK Type Summary
This type of digital modulation is called Amplitude-Shift Keying, or ASK.
Amplitude modulation, however, would also not be the most logical or apparent choice for this air-to ground application, for reasons that I will explain in a moment.

52. Original Radio Wave
Returning again to the original radio wave, a third way to represent the input bits is to change the phase of the wave.

53. Bit
PSK 0 Bit
Namely, if this again represents a zero bit,…

54. Bit
PSK 0 Wave
(Fade-in circle, wipe up arrow. Wipe right green wave)

55. Bit
Bit
PSK Lop off 0 Wave
See PowerPoint: the first 1/2 cycle of the green wave is deleted, then the wave shifts to the left as it turns orange)
…a one bit might be transmitted by shifting the wave, so that it starts by going down, instead of up.

56. Bit
PSK 1 Wave
In other words, we start the sine wave at a different point in the cycle, which we also refer to as the "phase".
(Fade-in circle, wipe down arrow)

57. Bit
PSK Shift Label
(Fade-in “Phase Shifted…” label)

58. BPSK 0˚
360˚
This picture shows a single cycle of the sine wave with the two starting points for zero and one. The corresponding radio wave for a zero is again shown in green, and the radio wave for a one is again shown in orange. Going through one cycle of the sine wave corresponds to a phase shift of 360 degrees. The starting point on the left corresponds to a phase shift of zero degrees,…

59. BPSK 0˚
180˚
360˚
…and the starting point on the right corresponds to a phase shift of one-half of a cycle, or 180 degrees.
(Wipe right shaded 1/2 cycle area)

60. Modulator Sequence 0 Bitstream in/Wave out
The modulator then determines the phase, or starting point, of the radio wave every time it gets a new bit to transmit.

61. Modulator Sequence 1 Bitstream in/Wave out
This picture shows the sine waves that are generated by the modulator corresponding to the sequence of zeros and ones shown. This time the zeros and ones are converted to phases, namely, up or down, which again can be read at the receiver.

62. Modulator Flagman Up Sequence w/Clock 1 Bitstream in/Wave out
The phases are the symbols used to represent the bits, again analogous to the two different flag positions.

63. Modulator Flagman Down Sequence w/Clock 1 Bitstream in/Wave out
(Flagman, down position)

64. Modulator Sequence w/Clock 0 Bitstream in/Wave out
The rate, or speed with which the modulator changes the phase is determined by a clock, or timer, inside the transmitter.
(Clock ticks every second)

65. Modulator Sequence w/Clock 1 Bitstream in/Wave out

66. Modulator Flagman Up Sequence w/Clock 1 Bitstream in/Wave out
This is analogous to the rate, or speed at which the flagman moves the flag up and down. Here we have increased the rate at which the modulator is transmitting the symbols.
(Bits and waves enter and exit at a faster rate. Flagman in “up” position)

67. Modulator Flagman Down Sequence w/Clock 1 Bitstream in/Wave out
(Flagman “down” position)

68. PSK Summary
At the receiver, the phase of the radio wave must be measured for each new symbol, namely, at the times shown in the picture.
(See next slide)

69. PSK Summary measure phase
(Measure phase arrows wipe up successively)
Each phase is then converted back to the corresponding bit.

70. PSK Type Summary
This type of digital modulation is called Phase-Shift Keying, or PSK.

71. ALL Type Summary
So, which modulation scheme is the most logical choice, Frequency-, Amplitude-, or Phase-Shift Keying? This generally depends on the application, but for the air-to-ground voice communications service we are considering,…

72. PSK HL Type Summary
…Phase-Shift Keying is generally the most efficient in terms of power and bandwidth, both of which are limited by the Federal Communications Commission, or FCC.

73. Frequency range
Bandwidth refers to the limitation on frequency range of the radio signal. This picture shows how different types of services, such as broadcast TV, mobile cellular telephony, and air-to-ground communications, are assigned to different ranges of frequencies.

74. 6000 Hz Available Bandwidth
Frequency range
6000 Hz Available Bandwidth
For our air-to-ground application, the FCC allows 6000 Hz, which is shown as the narrow yellow slice in the air-to-ground frequency range.

75. 6000 Hz Available Bandwidth
FSK vs Bandwidth
6000 Hz Available Bandwidth
Frequency-Shift Keying would require a larger bandwidth than this to transmit our voice signal, so it would not be suitable.
(Broader shade around 6000 Hz fades-in, then arrows)

76. 6000 Hz Available Bandwidth
FSK vs Bandwidth
6000 Hz Available Bandwidth
(Fade-in “No” symbol)

77. ASK vs PSK
Also, for our application, Amplitude-Shift Keying would require more transmitter power than Phase-Shift Keying. More power may require more expensive equipment and generates more interference to other users. We would therefore like to conserve power, which means that Phase-Shift Keying is a more logical and readily apparent choice than Amplitude-Shift Keying.
(Note: Left half of slide fades out)

78. PSK HL Type Summary
So for our application, which is limited in both power and bandwidth, Phase-Shift Keying offers the best performance and would be readily apparent as the leading, logical choice with little analysis by someone skilled in designing digital communications systems, such as Hughes Network Systems.

79. 2 Flag Positions
So far, we have talked about modulation schemes in which each symbol represents one bit, either a zero or one. Using the flagman analogy, this corresponds to holding the flag either up or down.

80. 4 Flag Positions
But notice that we could communicate more information by allowing more than just two flag positions. For example, we could allow the four different flag positions shown here. In that case, each flag position can be used to represent two bits instead of one, as shown by the labels in the figure.

81. BPSK
180˚ 0˚
360˚
Bit
Bit
So now let's take a look at other types of Phase-Shift Keying, or PSK schemes, which send more than 1 bit for each symbol. First, let's go back to the previous example with two phases. In that case, the modulator converts a zero bit to the green sine wave on the left, and converts a one bit to the orange sine wave on the right. These two phases can be labeled with a single bit, zero or one.

82. TYPES BPSK
This type of modulation is called Binary Phase-Shift Keying, or BPSK.

83. QPSK
Instead of choosing from one of just two phases of the sine wave, we could add two more phases,…
(Fade-in circle at 0 degrees, wipe down line and circle to wave. Repeat for orange point)

84. QPSK
QPSK
…corresponding to the purple and red points. So now we can choose from among four different phases.
(repeat fade-in for purple and red points)

85. QPSK QPSK 1/4 Cycle Shade 90˚ 0˚
The top picture shows a single cycle of a sine wave where the four colored points correspond to four different phases, or starting points of the radio wave. The neighboring phases in this case are separated by 1/4 of a cycle, or 90 degrees.
(Fade-in 0 and 90 degrees labels, wipe right 90 degree shading between first two points)
Namely, the green point is at zero phase, and corresponds to the green transmitted wave. The purple point at 1/4 cycle, or 90 degrees corresponds to the purple transmitted wave. The starting point for the purple wave is therefore shifted from the starting point for the green wave by 1/4 of a cycle, or 90 degrees.

86. QPSK 1/2 Cycle Shade
90˚ 0˚
180˚
The orange point shows a phase shift of 1/2 of a cycle, or 180 degrees, from the green point, and corresponds to the orange transmitted wave.

87. QPSK 3/4 Cycle Shade
90˚ 0˚
180˚
270˚
Finally, the red point is at a phase shift of 3/4 of a cycle from the green point, or 270 degrees, and corresponds to the red transmitted wave.

88. QPSK w/Flagmen
90˚ 0˚
180˚
270˚
These four phase shifts correspond to the four flag positions shown next to each wave.

89. QPSK Green
90˚ 0˚
180˚
270˚
Namely, the green wave at 0 degrees corresponds to the "up" position,…

90. QPSK Purple
90˚ 0˚
180˚
270˚
…the purple wave at 90 degrees corresponds to the "right" position,…

91. QPSK Orange
90˚ 0˚
180˚
270˚
…the orange wave at 180 degrees corresponds to "down",…

92. QPSK Red
90˚ 0˚
180˚
270˚
…and the red wave at 270 degrees corresponds to "left". Each flag position can therefore be identified by the angle from the up position. For example, the “right” position is 90 degrees from the up position, and “down” is 180 degrees from the up position. The different flag positions therefore correspond directly to the four different phases of the sine wave.

93. QPSK w/Bit Labels
90˚ 0˚
180˚
270˚
Because we can choose from one of four phases, each phase can be used to represent two bits, instead of one as before. This is shown in the figure by labeling each phase with two bits. Namely, zero phase corresponds to transmitting 00, shown in green, 90 degrees corresponds to transmitting 11, shown in purple, 180 degrees corresponds to 10, shown in orange, and 270 degrees corresponds to 01, shown in red.
(Bit labels fade-in one at a time)

94. TYPES QPSK
This scheme is called 4-PSK, and is also referred to as “Quadrature Phase-Shift Keying” or QPSK.

95. 00 Bit
This picture shows how the 4-PSK radio wave changes with each pair of bits that enters the modulator.

96. 10 Bit
(4-PSK bitstream animation continues)

97. 01 Bit
In this case, each pair of bits is converted to a phase with matching color, which is the transmitted symbol.

98. 11 Bit
That is, each transmitted phase corresponds to one of the four flag positions.

99. 00 Bit w clock
Again, the speed with which the modulator changes the phase is determined by a clock inside the transmitter, and corresponds to how fast the flagman can move the flag from one position to another.
(Clock ticks once per second)

100. 10 Bit w clock
(Clock ticks once per second, bits enter, corresponding colored waves exit)

101. 01 Bit w clock
(Clock ticks once per second, bits enter, corresponding colored waves exit)

102. 11 Bit w clock
(Clock ticks once per second, bits enter, corresponding colored waves exit)

103. 4 PSK Summary Measure Phase
This figure shows how the sequence of phases and the sine wave change with time for a particular sequence of zeros and ones to transmit. At the receiver, the phase of the received wave is detected for each new symbol at the times shown by the arrows.
(See next slide)

104. 4 PSK Summary Measure Phase
(Measure phase arrows wipe up successively)
Because each 4-PSK symbol represents 2 bits, 4-PSK conveys twice as much information as Binary, or two-level PSK, in which one of two different phases are transmitted.
Measure
Phase
Measure
Phase
Measure
Phase

105. 8PSK
Finally, if we want to transmit more than 2 bits for each symbol, we could choose from one of eight phases, as shown in this picture.

106. 8PSK
Here we choose from among 8 different starting points of the sine wave, which are separated by 1/8 of a cycle, which is 45 degrees. Using the flagman analogy, we select from one of the 8 different flag positions, which are shown over each wave.

107. 8PSK 0˚
We can again identify the flag positions in terms of an angle, or phase, where "up" is 0 degrees.

108. 8PSK 45˚ 1/8 Cycle (Wipe right 45 degree shade)
This flag position is 45 degrees, corresponding to a shift of 1/8 of a cycle

109. 8PSK
90˚
45˚ 0˚
1/4
Cycle
This flag position is 90 degrees, corresponding to a shift of 1/4 of a cycle.

110. 8PSK
90˚
45˚
135˚ 0˚
3/8
Cycle
This flag position is 135 degrees, corresponding to a shift of 3/8 of a cycle.

111. 8PSK
90˚
45˚
135˚ 0˚
180˚
1/2
Cycle
This flag position is 180 degrees, corresponding to a shift of 1/2 of a cycle.

112. 8PSK
90˚
45˚
135˚ 0˚
180˚
5/8
Cycle
225˚
This flag position is 225 degrees, corresponding to a shift of 5/8 of a cycle.

113. 8PSK
90˚
45˚
135˚ 0˚
180˚
3/4
Cycle
225˚
270˚
This flag position is 270 degrees, corresponding to a shift of 3/4 of a cycle.

114. 8PSK
90˚
45˚
135˚ 0˚
180˚
7/8
Cycle
225˚
270˚
315˚
This flag position is 315 degrees, corresponding to a shift of 7/8 of a cycle.

115. 8PSK
90˚
45˚
135˚ 0˚
180˚
225˚
270˚
315˚
Notice that the angle of each flag position again corresponds directly to the starting phase of the radio wave.
(Each pair of colored dots connected by a line wipe down successively)

116. 8PSK
45˚
1/8
Cycle
For example, this flag position at 45 degrees corresponds to the starting point at 1/8 of a cycle, which is again 45 degrees.

117. 8 Flag Positions
Notice that we now need three bits to identify each phase, or symbol, in the figure. In other words, each phase conveys 3 bits of information.

118. Stream Mod Sequence 1
For example, this figure shows the three bits 000 going into the modulator, which converts these to a 0 degree phase shift, corresponding to the green sine wave.

119. Stream Mod Sequence 2
The second set of three bits are 001, and the modulator converts these to a 45 degree phase shift, and generates the corresponding blue sine wave. The starting point of the 45 degree wave differs from the starting point of the 0 degree wave by 1/8 of a cycle.

120. Stream Mod Sequence 3
(3 bit animation continues)

121. Stream Mod Sequence 4
As each group of 3 bits enters the modulator, the modulator then selects the phase with the same color, and generates the corresponding radio wave, as shown here.

122. Stream Mod Sequence 5
(3 bit animation continues)

123. Stream Mod Sequence 6
(3 bit animation continues)

124. Stream Mod Sequence 7
(3 bit animation continues)

125. Stream Mod Sequence 8
(3 bit animation continues)

126. Clock Stream Mod Sequence 0
(Add clock)
The speed with which the modulator changes the phases, or the colors shown in the animation, is again determined by a clock inside the transmitter.

127. Clock Stream Mod Sequence 1
(3 bit animation with clock and flagman continues. Clock ticks once per second, colored bits and corresponding waves enter and exit at random)

128. Clock Stream Mod Sequence 2
(3 bit animation with clock and flagman continues)

129. Clock Stream Mod Sequence 3
(3 bit animation with clock and flagman continues)

130. Clock Stream Mod Sequence 4
(3 bit animation with clock and flagman continues)

131. Clock Stream Mod Sequence 5
(3 bit animation with clock and flagman continues)

132. Clock Stream Mod Sequence 6
(3 bit animation with clock and flagman continues)

133. Clock Stream Mod Sequence 7
(3 bit animation with clock and flagman continues)

134. Clock Stream Mod Sequence 8
(3 bit animation with clock and flagman continues)

135. 8 PSK Summary
This figure shows how the sequence of phases and the sine wave change with time for a particular sequence of transmitted bits. Each group of three bits with the corresponding flag position is shown above the corresponding radio wave segment.

136. 8 PSK Summary Measure Phase
The receiver again detects the phase for each symbol at the times shown by the arrows, and converts this back into the corresponding three transmitted bits. With 8-PSK, we are transmitting 3 bits every symbol, so that 8-PSK conveys three times as much information as Binary PSK, which only transmits 1 bit every symbol.
Measure
Phase
Measure
Phase
Measure
Phase
Measure
Phase
Measure
Phase

137. TYPES ALL PSK
As we increase the number of phases, or symbols, to choose from, we increase the data rate, or number of bits transmitted every second. That's the good news, because a higher data rate means we can get a better quality voice signal, or support additional services or features. The bad news, however, is that the symbols, or phases, become harder to distinguish at the receiver, increasing the likelihood of making errors.

138. 2 Flag Positions
Using the flagman analogy, it is easier to distinguish between only two flag positions, up and down,…

139. 8 Flag Positions
…than it is to distinguish from among 8 different flag positions.

140. MOTION BLUR ?
Hence the person reading the flagman's signals is more likely to confuse one of 8 flag positions than he or she is to confuse one of 2 flag positions. To reduce the likelihood of making errors in this way, the transmitter must use more power.

141. Bullet Points • More phases Higher data rate
More transmitter power to maintain voice quality
• Want minimum number of phases (bits per symbol) which provides a sufficient data rate
Consequently, if we increase the number of phases to choose from, we increase the data rate, but at the expense of requiring more transmitter power to maintain the same voice quality. The communications engineer can therefore be expected to choose the minimum number of phases, or bits per symbol, which provide a sufficient data rate for the desired service (for example, voice communications).

142. Blank
(Cross-fade to end slide)