2g Air Interface Baseband Radio Transmission

August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 1

Air Interface-Air Interface-Baseband Radio Baseband Radio

Transmission Transmission (AI-BRT)(AI-BRT)


AI-BRTAI-BRT

© Copyright 2001 Global Wireless Education Consortium

All rights reserved. This module, comprising presentation slides with notes, exercises, projects and Instructor Guide, may not be duplicated in any way without the express written permission of the Global Wireless Education Consortium. The information contained herein is for the personal use of the reader and may not be incorporated in any commercial training materials or for-profit education programs, books, databases, or any kind of software without the written permission of the Global Wireless Education Consortium. Making copies of this module, or any portion, for any purpose other than your own, is a violation of United States copyright laws.

Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners.


AI-BRTAI-BRT

Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039.

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Table of ContentsTable of ContentsOverview 5

Learning Objectives 6

Baseband Signaling 7

Analog to Digital Conversion 13

Digital Speech Coding 16

Channel Coding and Error Correction 21

Modulation and Demodulation 25

Baseband Filtering for Digital Signals 32

Multiplexing and Multiple Access 36

Digital Signal Processing 40

Summary 44

Contributors 47


OverviewOverview

This module covers the following topics: Baseband Signaling Analog to Digital Conversion Digital Speech Coding Channel Coding and Error Correction Modulation/Demodulation Multiplexing and Multiple Access Techniques Digital Signal Processing Summary of Baseband Signaling


Learning ObjectivesLearning Objectives

After completing this module participants will be able to: Describe the functions performed in baseband signal

processing for analog and digital transmission Describe the conversion of analog to digital signals Characterize the differences among speech coders Summarize the methods of channel coding and error

correction Summarize the basic techniques used in modulation

and demodulation of baseband signals Describe the techniques used for channel multiplexing

and multiple access for different wireless technologies


Baseband SignalingBaseband Signaling


Baseband SignalingBaseband Signaling

What is the baseband signal? The original band of frequencies produced by a

transducer, such as a microphone, telegraph key, or other signal-initiating device, prior to initial modulation. Note 1: In transmission systems, the baseband signal is usually

used to modulate a carrier. Note 2: Demodulation re-creates the baseband signal. Note 3: Baseband describes the signal state prior to

modulation, prior to multiplexing, following demultiplexing, and following demodulation.

Note 4: Baseband frequencies are usually characterized by being much lower in frequency than the frequencies that result when the baseband signal is used to modulate a carrier or subcarrier.


Steps in Baseband Signal Steps in Baseband Signal ProcessingProcessing

A/D MuxChannelDecoding D/AChannel

Coding DemuxMod Demod

Transmit/Receive

•Transmissi

on

•Multiple Access

•Demodulation

•Demultiplexing

•Channel Decoding

•Digital to Analog Conversion

•Analog to Digital Conversion

•Channel Coding

•Multiplexing

•Modulation

•Multiple Access

Multiple Access


Transmit versus ReceiveTransmit versus Receive Baseband processing takes place at both the mobile station and the base

station

The unmodified (baseband) signal can be: Analog voice: human speech received at the mobile station and delivered from

the mobile station on the receiving end Analog data: (i. e., modem data) transmitted from a mobile station to a base

station or from a base station to a mobile station Digital voice: the signal received by a base station from another base station or

from the PSTN to be transmitted to a mobile station may already be digitally encoded voice

Digital data: transmitted from a mobile station to a base station or from a base station to a mobile station

MobileStation

MobileStation

1

24

57

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0

3

6

9

#

1

24

57

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0

3

6

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#BaseStation

BaseStation


Analog versus Digital Analog versus Digital TechnologiesTechnologies

Baseband Processing Function

Analog Technology (e. g., AMPS, NAMPS)

Digital Technology (e. g., GSM, IS-136 TDMA, IS-95 CDMA)

Baseband Signal Type

Analog Voice

Analog Data

Digital Voice

Digital Data

Analog Voice

Analog Data

Digital Voice

Digital Data

Analog to Digital, Digital to Analog Conversion

No

No

N/A

N/A

Yes

N/A

No

No

Channel Coding, Channel Decoding

No No N/A N/A Yes N/A Yes Yes

Multiplexing, Demultiplexing

Yes Yes N/A N/A Yes N/A Yes Yes

Modulation, Demodulation

Yes Yes N/A N/A Yes N/A Yes Yes

Multiple Access Combining, Multiple Access Separation

Yes

Yes

N/A

N/A

Yes

N/A

Yes

Yes


Baseband Processing Baseband Processing (Analog Technologies)(Analog Technologies)

In an analog radio technology such as AMPS, baseband processing does very little conditioning to the raw audio (baseband) signal for voice before sending it to modulation

AMPS baseband processing functions include: Compression/Expansion Pre-emphasis/De-emphasis Limiting

Modulation and demodulation of analog signals takes place as described in the Modulation and Demodulation Section

Multiplexing and Multiple Access techniques are applied as appropriate (e. g., AMPS uses Frequency Division Multiple Access (FDMA) after modulation)


Analog to Digital Analog to Digital ConversionConversion



Conversion of an analog (continuous) signal to a digital (discrete) signal at the transmitting end requires the following: Initial analog signal (for example, analog voice) Sampling Quantization Encoding Transmission of the digital signal

At the receiving end, the original analog signal is reconstructed by decoding the digital signal



Quantizing

Time

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1.1 1.3

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3.2 2.53.1

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1111 100 001110 010 011011 011 011011001 100101

Analog Signal

Digital Signal

Encoding

Sampling


Digital Speech Digital Speech CodingCoding


Digital Speech CodingDigital Speech Coding Pulse Code Modulation (PCM) is a form of speech coding known

as “waveform coding”, and is commonly used to convert analog voice and data to digital transmission in the wireline network

In a wireless network, due to air interface (between the mobile station and the base station): PCM (which requires a transmission rate of 64 Kbps) is an inefficient

use of scarce bandwidth resources Higher error rates for wireless versus wireline transmission require the

adoption of error recovery techniques as part of digital transmission Classes of speech coders (coders/decoders, or “codecs”) that may

be used on the air interface in wireless networks include: Waveform coding algorithms Linear predictive coding algorithms (known as “vocoders”) Hybrid coders (combining waveform coding techniques and vocoder

techniques)


Digital Speech Coding Digital Speech Coding TechniquesTechniques

Waveform codec: Pulse Code Modulation (PCM) Adaptive Differential Pulse Code Modulation (ADPCM) Adaptive Predictive Coding (APC)

Linear Predictive codec (LPC): Models speech by encoding and transmitting a few key parameters,

which are used at the receiver to synthesize the original speech signal

Hybrid codec: Residual-Excited LPC (RELP) Code-Excited LPC (CELP) Algebraic Code-Excited LPC (ACELP) Vector-Sum Excited LPC (VSELP) Multi-pulse, multi-level quantization (MP-MLQ)


Standardization of Coding Standardization of Coding TechniquesTechniques

ITU-T G-series standards: G.711: Describes 64 Kbps PCM voice coding, including A-law and -

law encoding laws G.726: Describes ADPCM coding at 40, 32, 24, and 16 Kbps G.728: Describes 16 Kbps low-delay variation of CELP (LD-CELP) G.729: Describes 8 Kbps CELP (CS-ACELP) (G.729 and G.729

Annex A are similar standards that differ in computational complexity) G.723.1: Describes a compression technique for speech or audio

signal components at very low bit rates (5.3 Kbps (based on ACELP) or 6.3 Kbps (based on MP-MLQ))

ETSI standards for GSM: GSM EFR: Compresses 8 KHz sampled speed to 13 Kbps (based on

ACELP algorithm)


Comparison of CodecsComparison of CodecsCodec Advantages Disadvantages G.711 (PCM) – Highest speech quality

– Very low delay – High bit rate

G.726 (ADPCM) – Simple to implement – Very low delay

– Relatively high bit rate

G.728 (LD-CELP) – Relatively low delay – Lossy compression technique

G.729 (CS-ACELP) – Low complexity in compression algorithm

– Low bit rate

G.729a (CS-ACELP) – Low complexity in compression algorithm

– Low bit rate

G.723.1 (MP-MLQ) – Low bit rate

G.723.1 (ACELP) – Low complexity in compression algorithm

– Low bit rate

GSM EFR (ACELP) – Low complexity in compression algorithm

– Low bit rate

– Lossy compression technique


Channel Coding and Channel Coding and Error CorrectionError Correction


Channel CodingChannel Coding The purpose of channel coding in digital transmission of voice or

data is to introduce additional bits into the information bit stream that will allow errors to be detected and, in some cases, corrected at the receiving end

The radio air interface is more “hostile” than wireline: errors in transmission occur due to noise, co-channel interference (from users in adjacent cells), multipath fading (cancellation of the signal due to interference by multiple reflections of the signal)

Shannon’s Channel Capacity Theorem indicates that it is possible, in principle, to devise a coding technique such that the probability of error of information transmitted at a rate R less than the channel capacity C can be made “arbitrarily small”

In practice, there is a tradeoff: Reduction in error rate generally means a reduction in throughput as well, increasing the cost per subscriber in a capacity-limited network


Types of Channel CodingTypes of Channel Coding

The digital bit stream (voice, data, or call control) is typically segmented into blocks

Channel coding is used to add redundancy to each individual block Categories of channel coding:

Block Codes: Input block is mapped into output block containing parity bits Considered “memoryless”, i. e., dependent only on the individual code

Convolutional Codes: Incorporates “memory” - output is based on the previous m memory blocks

Interleaving: Corrects for “bursts” of errors May be used in conjunction with block codes or convolutional codes


Error Detection and Error Detection and CorrectionCorrection

Two main approaches exist for error correction and detection: Automatic Repeat Request (ARQ) Forward Error Correction (FEC)


Modulation and Modulation and DemodulationDemodulation


ModulationModulation Modulation allows the overlay of a signal containing “information”

(speech, data, or signaling) on a carrier wave in a different frequency band from the original signal

There are multiple reasons for modulating a signal prior to transmission on a radio network: Higher frequency transmission allows the use of a smaller antenna:

for example, radio signals in the range of audible speech (about 3 KHz) would require an antenna on the order of 50 kilometers in length

Licensing and/or statutory requirements constrain wireless service providers to transmit and receive in specific frequency bands. Transmission by the subscribers of one service provider is separated by frequency from all other radio transmission in the same geographic area


Modulation TechniquesModulation Techniques Modulation techniques vary with the technology supported:

Analog radio technology (e. g., AMPS) uses analog modulation techniques

Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) (considered a variation of Frequency Modulation)

Digital radio technology (digitized voice or digital data input) such as GSM, IS-95 CDMA, or IS-136 TDMA uses digital modulation techniques:

Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK)

Modulation techniques, including more complex forms of digital modulation, are discussed in detail in a separate module (AI-MOD)


Analog ModulationAnalog Modulation

Carrier Wave

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Time

Amplitude Modulation (AM)

Am

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Time

Baseband Voice Signal

Am

pli

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Time

Frequency Modulation (FM)

Am

pli

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Time


Digital ModulationDigital Modulation

0 1 1 10

Binary Digits

0

1

0

-1

Digital Signal1

0

-1

Am

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Time

Carrier Wave

Am

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Time

Frequency Shift Keying (FSK)1

0

-1

Am

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Time

Amplitude Shift Keying (ASK)1

0

-1

Am

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Time

Phase Shift Keying (PSK)

1

0

-1

Am

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Time


Intermediate FrequencyIntermediate Frequency An Intermediate Frequency is defined as a frequency to

which a carrier frequency is shifted as an intermediate step in transmission or reception Intermediate frequencies in a wireless system are generally in the

tens or low hundreds of MHz range The purpose of modulating a signal to an Intermediate Frequency

prior to modulation to the carrier frequency at the transmitter, and prior to demodulation to voiceband frequencies at the receiver, is that amplification of the signal can be accomplished more efficiently than if the same functions were performed at carrier frequencies

Intermediate Frequencies are used in modulation and demodulation in both analog and digital wireless technologies


DemodulationDemodulation

Demodulation of a carrier wave to recover the original signal involves several stages: Intermediate Frequency Amplification Demodulation Filtering


Baseband Filtering Baseband Filtering for Digital Signalsfor Digital Signals


Filtering in Digital Filtering in Digital Baseband ProcessingBaseband Processing

Filters can be applied at both the transmit and receive end, for analog and digital radio technologies

Depending on whether the signal at the receiver has been sampled and converted to digital, filtering can be done using Digital Signal Processing as well as with an analog filter

Equalization involves application of a filter to the received signals in order to reverse the time dispersion caused by multi-path effects

Time dispersion causes inter-symbol interference (ISI), or, more generally, distortion of the received signal


Effects of FilteringEffects of Filtering

Allows the transmitted bandwidth to be significantly reduced without losing the content of the digital data

Eliminates much of the distortion of the received signal, Intersymbol Interference (ISI)

Removes high frequency replicas of the signal that arise due to modulation

Removes as much noise as possible, while affecting the information signal as little as possible


Types of Filters in Digital Types of Filters in Digital TransmissionTransmission

Raised Cosine filter Square Root Raised Cosine filter (IS-136 TDMA) Gaussian filter (GSM) Chebyshev lowpass Finite Impulse Response (FIR) filter

(IS-95 CDMA)


Multiplexing and Multiplexing and Multiple AccessMultiple Access


Multiplexing and Multiple Multiplexing and Multiple AccessAccess

Channel multiplexing is used to merge speech with call control signaling, synchronization, etc. into a single digital bit stream prior to modulation

Multiple Access techniques multiplex active calls from multiple users onto a single frequency channel by using: Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA)


Multiple AccessMultiple Access

Multiple Access techniques may be applied before or after modulation, depending on the technique used: FDMA: applied after the signals have been modulated up to the

carrier frequencies to be used for transmission TDMA: applied before modulation, so that the combined bit

stream for all active calls on a given frequency channel can be modulated onto the same carrier frequency

CDMA: applied before modulation, so that all active calls on a given frequency channel can be modulated onto the same carrier frequency


Multiple Access Multiple Access MultiplexingMultiplexing

FDMA:

Combiner

Call 1

Call 2Call n

f1

f2

fn

Modulator f1

Modulator f2

Modulator fn

f1

f2

fn

f1

f2

fn

TDMA:

Modulator f1

Modulator f2

Modulator fn

Call 1

Call 2Call i

Call 1

Call 2Call i

Time DivisionMultiplexer 1

Time DivisionMultiplexer 2

f1

f2

fn

f1

f2

fn

Modulator f1

Modulator f2

Modulator fn

CDMA:Call 1

Call 2Call j

Call 1

Call 2Call j

Code DivisionMultiplexer 1

Code DivisionMultiplexer 2

f1

f2

fn


Digital Signal Digital Signal ProcessingProcessing


Digital Signal ProcessingDigital Signal Processing Digital Signal Processing operates on signals of interest

(speech or data) as sequences of binary numbers, using numeric techniques

“Digital” radio technologies are still partly analog The result is selected applicability of Digital Signal

Processing


Hardware Options for Hardware Options for Digital Signal ProcessingDigital Signal Processing

Application Specific Integrated Circuit (ASIC) Digital Signal Processor (DSP) Field Programmable Gate Array (FPGA)


Software Defined Radio Software Defined Radio (SDR)(SDR)

Software defined radio (SDR) or “soft radio”: Will use Digital Signal Processing to allow service providers to

reprogram base stations as standards change or to develop mobile stations that will be able to communicate with any wireless technology base station in any frequency band

SDRs currently use a combination of DSP, ASIC, and FPGA technology with hardware support Ultimate goal is to have all processing done by software Battery power, size, weight, and cost requirements are all

issues, especially in handheld mobile stations The closer to the antenna that an incoming signal can be

sampled and converted back to a digital data stream, the more baseband processing functions can be programmed into software


SummarySummary


Summary of Baseband Summary of Baseband Processing by TechnologyProcessing by Technology

AMPS N-AMPS

IS-136 TDMA

IS-95 CDMA

GSM

Voice Transmission

Analog Digital Digital Digital

Codec None VSELP QCELP (Qualcomm CELP)

GSM EFR (ACELP)

Coding Rate Analog 8 Kbps 9.6 Kbps, 14.4 Kbps

13 Kbps

Modulation Frequency Modulation (FM)

Phase Shift Keying (specifically, DQPSK)

Direct Sequence Code Modulation and QPSK

Phase Shift Keying (specifically, GMSK)

Multiple Access

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)

Time Division Multiple Access (TDMA)

Digital Filter None

Square Root Raised Cosine

Chebyshev FIR Gaussian


Summary of Baseband Summary of Baseband Radio TransmissionRadio Transmission

Analog to Digital Conversion Speech Coding Channel Coding and Error Detection Modulation, Demodulation, and Filtering Multiplexing and Multiple Access Digital Signal Processing/Software Defined Radio


Industry ContributorsIndustry Contributors

Telcordia Technologies, Inc (http://www.telcordia.com)

The following companies provided materials and resource support for this module:


Individual ContributorsIndividual ContributorsThe following individuals and their organization or institution provided materials, resources, and development input for this module: Dr. Cheng Sun

Cal Poly http://www.calpoly.edu

Dr. David Voltmer Rose-Hulman Institute of Technology http://www.rose-hulman.edu


Documents

2g Air Interface Baseband Radio Transmission