Data Rate, Bandwidth, and Modulation

Data Rate and Bandwidth

• Bandwidth refers to the frequency space used for transmitting data within modulation techniques.
• Bandwidth is related to the baud rate, as, for ASK and PSK, bandwidth equals 2 times the baud rate.
• Increased baud rates lead to increased bandwidth.
• Restrictions in available spectrum limit the ability to raise the baud rate for increasing data rate.
• Advanced modulation techniques are required for increased spectral efficiency, to maximize data within limited spectrum.

Advanced Digital Modulation

• Baud rate represents the rate of changing signal or the symbol rate.
• Representing more bits in a single symbol avoids increasing bandwidth.
• Advanced digital modulation types are QPSK and QAM.
• Goal of advanced modulation is to enhance spectral efficiency by encoding more bits into a single transmission symbol.

Amplitude, Frequency, Phase of a Wave

• Waves can be represented by the equations A(t) = Am sin(2πƒt ± Φ) or A(t) = Am cos(2πƒt ± Φ)
• Am = Amplitude
• f = Frequency
• Φ = Phase
• t = time

Binary Phase Shift Keying (BPSK)

• BPSK represents the same as PSK from previous lectures.
• In BPSK, Φ = π (180 degrees) represents 1, while Φ = 0 (0 degrees) represents 0.
• The constellation diagram visually represents symbols and their phase relationships.
• Waveforms of BPSK signals display a sudden phase change when data transitions between 1 and 0.

Analog Medium

• Signals transmitted over mediums such as radio frequencies are analog waveforms, even if the underlying data is digital.
• Applying digital modulation in analog mediums allows for the modulation of more bits into a single symbol.

Quadrature Phase Shift Keying (QPSK)

• QPSK can encode two bits into a single symbol.
• The possible two-bit numbers 00, 01, 10, and 11
• QPSK represents these numbers using 4 phases that are separated by 90°.
• Binary data 00 = 45°, 01 = 135°, 10 = 225°, 11 = 315°
• Constellation diagrams for QPSK show 4 points on a phasor, each representing two bits of data.

Quadrature Amplitude Modulation (QAM)

• QAM combines Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK) to have symbols represented by both amplitude and phase.
• QAM is a digital modulation method that encodes more bits into a signal symbol through amplitude and phase.
• QPSK is considered as a form of QAM named 4-QAM.
• QAM is widespread in wireless communications.
• QAM is named according to the number of symbols utilized.
• 16-QAM is capable of modulating 16 different waveforms/symbols.
• 16-QAM can modulate 4 bits in one symbol.
• In constellation diagrams each QAM symbol varies in amplitude and phase combination.
• Symbol distance from the origin indicates amplitude.
• The angle from the horizontal axis indicates phase.
• Modern WiFi technology uses 256-QAM and 1024-QAM (and the number correlates to the bits transmitted per symbol).

Impact of Noise and Interference on QAM

• Higher-order modulation rates enhance data transmission and spectral efficiency in radio communication systems.
• Higher-order modulation schemes are more susceptible to noise and interference.
• Increased order results show less difference between two symbols.
• Stringent requirements for the environment, like a high Signal-to-Noise Ratio, and the components of the receiver/transmitter.

Key Takeaways

• Symbols used in BPSK, QPSK, and QAM are representable on constellation diagrams.
• BPSK, QPSK, and QAM can enhance spectral efficiency by transmitting more data within the same bandwidth.
• QPSK is a subset of QAM also know as 4-QAM.
• QAM combines ASK and PSK.
• QAM uses both amplitude and phase to represent symbols.
• QAM names indicate of the number of symbols the technique uses.