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Digital Modulation

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- Better performance and more cost effective than analog modulation methods (AM, FM, etc.)
- Used in 2nd generation (2G) cellular systems in U.S. from 1998 - present
- AT&T Wireless, Verizon Wireless, Sprint, T-mobile, Cingular (now AT&T), Nextel (now Sprint), etc.

- Advancements in VLSI, DSP, ASICs, etc. have made digital solutions practical and affordable

ECE 4730: Lecture #12

- Performance advantages:
1) Resistant to noise, fading, & interference

2) Combine multiple information types (voice, data, & video)

in single transmission channel

3) Improved security (e.g. encryption) deters phone

cloning + eavesdropping

4) Error coding to detect/correct transmission errors

5) Signal conditioning to combat hostile MRC environment

6) Implement mod/dem functions using DSP software

ECE 4730: Lecture #12

- Many types of digital modulation methods subtle differences
- How to choose appropriate method??
- Performance factors to consider
1) Low Bit Error Rate (BER) at low SNR power efficiency

2) Resistance to interference (ACI & CCI) & fading

3) Occupies minimum amount of BW spectral efficiency

4) Easy and cheap to implement mobile unit

5) Efficient use of battery power mobile unit

ECE 4730: Lecture #12

Digital Modulation Performance

- Power Efficiency p
- Ability of modulation technique to preserve quality of digital message at low power levels (low SNR)

- Specified as Eb/No @ some BER (e.g. 10-5) where
- Eb : energy/bit and No : noise energy/bit

- Tradeoff between signal power vs. signal quality BER as Eb / No
- **Note that this is NOT related to DC/RF efficiency of Tx power amplifier**

ECE 4730: Lecture #12

- Bandwidth Efficiency B
- Ability of modulation technique to accommodate data in a limited BW

- Tradeoff between data rate (R) and occupied BW
- BW as R

- Symbol Period = Ts
- Signal BW = Bs1 / Ts R

ECE 4730: Lecture #12

PSD

. . .

f

1 / Ts = FNBW

0

0

0

1

0

0

1

1

Symbol Period = Ts

Signal BW = Bs1 / Ts

- For a digital signal :
- Each pulse or “symbol” having mfinite states represents n = log2m bits/symbol e.g. m = 0 or 1 (2 states) n = 1 bit/symbol

ECE 4730: Lecture #12

- Maximum BW efficiency Shannon’s Theorem
C: channel capacity (bps)

B : RF BW

- Note that CB (expected) but also C S/N unexpected??
- Increase in signal power translates to increase in channel capacity!!
- Large S/N easier to differentiate between multiple signal states (m) in one symbol n

- is fundamental limit that cannot be achieved in practice typically only 4060% is realizable

where

ECE 4730: Lecture #12

- Fundamental tradeoff between pand B (in general)
- If p then B (or vice versa)

- Example: add error control bits to data stream and keep same data rate
- Ts so Bs so B … but error bits will allow lower Eb / No for same BER so p

- Is p vs. Btradeoff worth it??
- Use other factors to evaluate complexity, resistance to MRC impairments, etc.

ECE 4730: Lecture #12

1

B’

0.5

f

fc

B”

B”’

- Many definitions depending on application
- All use Power Spectral Density (PSD) of modulated bandpass signal
- FCC definition of occupied BW BW contains 99% of signal power

B’ : half-power (3 dB) BW

B” : null-to-null BW

B’” : absolute BW

range where PSD > 0

ECE 4730: Lecture #12

Tb

0

0

0

0

1

1

1

Tb

0

1

0

1

0

1

0

- Different types of coding used for baseband digital signals
- Choice depends on desired spectral properties

- Unipolar 0 or V
- Bipolar V or V
- Non Return to Zero (NRZ)
- Return to Zero (RZ)

ECE 4730: Lecture #12

PSD

Tb

V

0

1

1

0

f

. . .

Rb = 1 / Tb

- Unipolar NRZ
- Advantage : narrow spectral width
- Disadvantage : large DC component data can’t be passed thru circuits that block DC (e.g. phone line!)
- Disadvantage : no zero return hard to synchronize (decoding errors)

ECE 4730: Lecture #12

PSD

Tb

V

0

. . .

1

0

1

f

2Rb = 2 / Tb

- Unipolar RZ
- Disadvantage : wide spectral width
- Advantage : smaller DC component
- Advantage : better synchronization easier to decode

ECE 4730: Lecture #12

PSD

Tb

+V

0 V

V

f

1

0

1

0

Rb

0.7Rb

- Manchester NRZ
- Advantage : zero DC component
- Advantage : zero crossing excellent synchronization
- Moderate spectral width

ECE 4730: Lecture #12

MRC

- Rectangular pulses passed thru bandlimited channel (MRC)
- Symbols smear into adjacent time slots
- Inter-Symbol Interference (ISI)
- Increases probability that symbol error will occur

0 1 0 1

0 1 0 1

ECE 4730: Lecture #12

- Spectral shaping of digital pulses done at baseband
- Reduce ISI due to pulse smearing
- Reduce spectral width of signal
- Improve BW efficiency
- Achieve better control of ACI!

- Nyquist Criterion
- Design overall response of system (Tx + MRC + Rx) so at every sampling instant in Rx (0/1 decision point) the response of all other symbols is zero
- Leads to ideal “brick wall” filter in frequency domain

ECE 4730: Lecture #12

Heff (f)

f

- Ideal Brick Wall Filter
- In Time Domain

-B +B

ECE 4730: Lecture #12

- Ideal brick wall filter in frequency domain and unlimited time domain pulse response cannot be achieved in practice
- Other filters can satisfy the Nyquist criterion
- Raised Cosine (RC) Filter

- Other pulse shaping filters can also be used that do NOT satisfy Nyquist criterion
- Gaussian Filter

ECE 4730: Lecture #12