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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)
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 2
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.
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 3
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.
GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 4
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 5
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 6
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
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Baseband SignalingBaseband Signaling
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 8
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.
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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
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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
8*
0
3
6
9
#
1
24
57
8*
0
3
6
9
#BaseStation
BaseStation
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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
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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)
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Analog to Digital Analog to Digital ConversionConversion
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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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 15
Analog to Digital Analog to Digital ConversionConversion
Quantizing
Time
Am
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e 7
5
3
1
7.0
1.1 1.3
4.84.2
3.1 2.42.9
3.2 2.53.1
6.2
3.8
T
0
1111 100 001110 010 011011 011 011011001 100101
Analog Signal
Digital Signal
Encoding
Sampling
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Digital Speech Digital Speech CodingCoding
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 17
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)
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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)
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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)
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 20
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 21
Channel Coding and Channel Coding and Error CorrectionError Correction
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 22
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 23
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 24
Error Detection and Error Detection and CorrectionCorrection
Two main approaches exist for error correction and detection: Automatic Repeat Request (ARQ) Forward Error Correction (FEC)
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Modulation and Modulation and DemodulationDemodulation
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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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 27
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)
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 28
Analog ModulationAnalog Modulation
Carrier Wave
Am
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e
Time
Amplitude Modulation (AM)
Am
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Time
Baseband Voice Signal
Am
pli
tud
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Time
Frequency Modulation (FM)
Am
pli
tud
e
Time
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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
pli
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e
Time
Phase Shift Keying (PSK)
1
0
-1
Am
pli
tud
e
Time
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 30
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 31
DemodulationDemodulation
Demodulation of a carrier wave to recover the original signal involves several stages: Intermediate Frequency Amplification Demodulation Filtering
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 32
Baseband Filtering Baseband Filtering for Digital Signalsfor Digital Signals
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 33
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 34
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 35
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)
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 36
Multiplexing and Multiplexing and Multiple AccessMultiple Access
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 37
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)
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 38
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 39
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 40
Digital Signal Digital Signal ProcessingProcessing
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 41
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 42
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)
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 43
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 44
SummarySummary
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 45
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 46
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 47
Industry ContributorsIndustry Contributors
Telcordia Technologies, Inc (http://www.telcordia.com)
The following companies provided materials and resource support for this module:
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 48
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
August 2001 Copyright 2001 Global Wireless Education Consortium AI-BRT 49