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Introduction To Modulation: Baseband and Passband Signals

This document provides an overview of modulation techniques used in communication systems. It discusses analog modulation methods like AM, PM, and FM, as well as digital modulation techniques including BPSK, QPSK, GMSK, and QAM. It also covers multiple access methods such as FDMA, TDMA, and CDMA that allow multiple users to access the same frequency band. Finally, it summarizes several wireless communication standards including GSM, CDMA, Bluetooth, and 802.11a that specify protocols for cell phones and wireless networks.

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praveen alwis
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241 views7 pages

Introduction To Modulation: Baseband and Passband Signals

This document provides an overview of modulation techniques used in communication systems. It discusses analog modulation methods like AM, PM, and FM, as well as digital modulation techniques including BPSK, QPSK, GMSK, and QAM. It also covers multiple access methods such as FDMA, TDMA, and CDMA that allow multiple users to access the same frequency band. Finally, it summarizes several wireless communication standards including GSM, CDMA, Bluetooth, and 802.11a that specify protocols for cell phones and wireless networks.

Uploaded by

praveen alwis
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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EE215C B.

Razavi
Win. 10 HO #2

Introduction to Modulation

Baseband and Passband Signals

• A passband signal can be expressed as:

• Modulation converts a baseband signal to a passband signal (in most


cases):

• Modulation Characteristics:
- Signal quality in the presence of noise
- Bandwidth efficiency
- Power efficiency
Analog Modulation

Quality is quantified by SNR.

• AM

- Sensitive to noise
and nonlinearity
- Requires linear PA.

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EE215C B. Razavi
Win. 10 HO #2

• PM and FM

- Insensitive to nonlinearity
- Can operate with nonlinear PA.
- But occupies more BW.

- Insensitive to nonlinearity
- Can operate with nonlinear PA.
- But occupies more BW.

How to build a frequency modulator?

• Narrowband FM Approximation
If the phase component is much less than 1 rad, then:

For example, if

Digital Modulation
-3
Quality is quantified by bit error rate (BER). For voice, BER = 10 .

• Binary Shift Keying

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EE215C B. Razavi
Win. 10 HO #2

Simple BPSK Modulator:

• Quadrature Modulation

The occupied bandwidth can be reduced by converting the data to two


slower streams and impressing each stream on the sine and cosine of
carrier. For example, “quadrature phase shift keying” (QPSK) is
performed as follows:

There are other variants of QPSK, e.g., DQPSK, π/4-QPSK, OQPSK.

• Gaussian Minimum Shift Keying


FSK occupies too much BW:

Try “shaping” the baseband pulses:

How to build a
GMSK modulator?

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EE215C B. Razavi
Win. 10 HO #2

Important Difference Between QPSK and GMSK

QPSK also usually incorporates baseband pulse shaping:

Shaped

Thus, shaped QPSK has a variable amplitude (envelope) and hence


requires a linear PA. GSMK, on the other hand, has a constant
envelope and can operate with nonlinear PAs.

QPSK is used in CDMA cellphones and in 802.11b. GMSK is used in


GSM cellphones and in Bluetooth (in which case it is called GFSK).

Many other types of modulation are used: QAM, OOK, PAM,

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EE215C B. Razavi
Win. 10 HO #2

Multiple Access and Wireless Standards


Multiple Access Techniques

• Time and Frequency Division Duplexing

Cellphones use FDD; most other systems use TDD.

• Multiple Access

- Frequency Division Multiple Access


(FDMA) --- used in cellphones

- Time Division Multiple Access


(TDMA) --- used in cellphones and
most other systems

- Code Division Multiple Access


(CDMA) --- used in some cellphones

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EE215C B. Razavi
Win. 10 HO #2

• Near-Far Problem in CDMA

The basestation therefore needs to constantly monitor and control


each mobile’s output power.

Wireless Standards

A “standard” specifies all of the details of how a communication


system must operate, e.g., modulation, bit rate, duplexing, multiple
access, frequency band, channel bandwidth.

The standard also specifies exact performance tests.

• GSM

Now we have GPRS and EDGE. EDGE uses variable-envelope


modulation and requires a linear PA.

• CDMA

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EE215C B. Razavi
Win. 10 HO #2

• Bluetooth

- Frequency band = 2.400-2.480 GHz


- Channel BW = 1 MHz
- Bit rate = 1 Mb/s
- TDD

• 802.11a

- Frequency band = 5.18-5.24 GHz, 5.26-5.32 GHz, 5.745-5.805 GHz


- Channel BW = 20 MHz
- Bit rate = 54 Mb/s
- TDD

And there are others …

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