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- 1. Air Interface-Baseband Radio Transmission (AI-BRT)
- 2. AI-BRT <ul><li>© Copyright 2001 Global Wireless Education Consortium </li></ul><ul><li>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. </li></ul><ul><li>Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners . </li></ul>
- 3. AI-BRT <ul><li>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. </li></ul><ul><li>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. </li></ul>
- 4. Table of Contents <ul><li>Overview 5 </li></ul><ul><li>Learning Objectives 6 </li></ul><ul><li>Baseband Signaling 7 </li></ul><ul><li>Analog to Digital Conversion 13 </li></ul><ul><li>Digital Speech Coding 16 </li></ul><ul><li>Channel Coding and Error Correction 21 </li></ul><ul><li>Modulation and Demodulation 25 </li></ul><ul><li>Baseband Filtering for Digital Signals 32 </li></ul><ul><li>Multiplexing and Multiple Access 36 </li></ul><ul><li>Digital Signal Processing 40 </li></ul><ul><li>Summary 44 </li></ul><ul><li>Contributors 47 </li></ul>
- 5. Overview <ul><li>This module covers the following topics: </li></ul><ul><li>Baseband Signaling </li></ul><ul><li>Analog to Digital Conversion </li></ul><ul><li>Digital Speech Coding </li></ul><ul><li>Channel Coding and Error Correction </li></ul><ul><li>Modulation/Demodulation </li></ul><ul><li>Multiplexing and Multiple Access Techniques </li></ul><ul><li>Digital Signal Processing </li></ul><ul><li>Summary of Baseband Signaling </li></ul>
- 6. Learning Objectives <ul><li>After completing this module participants will be able to: </li></ul><ul><li>Describe the functions performed in baseband signal processing for analog and digital transmission </li></ul><ul><li>Describe the conversion of analog to digital signals </li></ul><ul><li>Characterize the differences among speech coders </li></ul><ul><li>Summarize the methods of channel coding and error correction </li></ul><ul><li>Summarize the basic techniques used in modulation and demodulation of baseband signals </li></ul><ul><li>Describe the techniques used for channel multiplexing and multiple access for different wireless technologies </li></ul>
- 7. Baseband Signaling
- 8. Baseband Signaling <ul><li>What is the baseband signal? </li></ul><ul><li>The original band of frequencies produced by a transducer, such as a microphone, telegraph key, or other signal-initiating device, prior to initial modulation. </li></ul><ul><ul><li>Note 1: In transmission systems, the baseband signal is usually used to modulate a carrier. </li></ul></ul><ul><ul><li>Note 2: Demodulation re-creates the baseband signal. </li></ul></ul><ul><ul><li>Note 3: Baseband describes the signal state prior to modulation, prior to multiplexing, following demultiplexing, and following demodulation. </li></ul></ul><ul><ul><li>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. </li></ul></ul>
- 9. Steps in Baseband Signal Processing A/D Mux Channel Decoding D/A Channel Coding Demux Mod Demod Transmit/Receive <ul><li>Transmission </li></ul><ul><li>Multiple Access </li></ul><ul><li>Demodulation </li></ul><ul><li>Demultiplexing </li></ul><ul><li>Channel Decoding </li></ul><ul><li>Digital to Analog Conversion </li></ul><ul><li>Analog to Digital Conversion </li></ul><ul><li>Channel Coding </li></ul><ul><li>Multiplexing </li></ul><ul><li>Modulation </li></ul><ul><li>Multiple Access </li></ul>Multiple Access
- 10. Transmit versus Receive <ul><li>Baseband processing takes place at both the mobile station and the base station </li></ul><ul><li>The unmodified (baseband) signal can be: </li></ul><ul><ul><li>Analog voice: human speech received at the mobile station and delivered from the mobile station on the receiving end </li></ul></ul><ul><ul><li>Analog data: (i. e., modem data) transmitted from a mobile station to a base station or from a base station to a mobile station </li></ul></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>Digital data: transmitted from a mobile station to a base station or from a base station to a mobile station </li></ul></ul>Mobile Station Mobile Station 1 2 4 5 7 8 * 0 3 6 9 # 1 2 4 5 7 8 * 0 3 6 9 # Base Station Base Station
- 11. Analog versus Digital Technologies
- 12. Baseband Processing (Analog Technologies) <ul><li>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 </li></ul><ul><li>AMPS baseband processing functions include: </li></ul><ul><ul><li>Compression/Expansion </li></ul></ul><ul><ul><li>Pre-emphasis/De-emphasis </li></ul></ul><ul><ul><li>Limiting </li></ul></ul><ul><li>Modulation and demodulation of analog signals takes place as described in the Modulation and Demodulation Section </li></ul><ul><li>Multiplexing and Multiple Access techniques are applied as appropriate (e. g., AMPS uses Frequency Division Multiple Access (FDMA) after modulation) </li></ul>
- 13. Analog to Digital Conversion
- 14. Analog to Digital Conversion <ul><li>Conversion of an analog (continuous) signal to a digital (discrete) signal at the transmitting end requires the following: </li></ul><ul><ul><li>Initial analog signal (for example, analog voice) </li></ul></ul><ul><ul><li>Sampling </li></ul></ul><ul><ul><li>Quantization </li></ul></ul><ul><ul><li>Encoding </li></ul></ul><ul><ul><li>Transmission of the digital signal </li></ul></ul><ul><li>At the receiving end, the original analog signal is reconstructed by decoding the digital signal </li></ul>
- 15. Analog to Digital Conversion Quantizing Time Amplitude 7 5 3 1 7.0 1.1 1.3 4.8 4.2 3.1 2.4 2.9 3.2 2.5 3.1 6.2 3.8 T 0 1 111 100 001 110 010 011 011 011 011 011 001 100 101 Analog Signal Digital Signal Encoding Sampling
- 16. Digital Speech Coding
- 17. Digital Speech Coding <ul><li>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 </li></ul><ul><li>In a wireless network, due to air interface (between the mobile station and the base station): </li></ul><ul><ul><li>PCM (which requires a transmission rate of 64 Kbps) is an inefficient use of scarce bandwidth resources </li></ul></ul><ul><ul><li>Higher error rates for wireless versus wireline transmission require the adoption of error recovery techniques as part of digital transmission </li></ul></ul><ul><li>Classes of speech coders (coders/decoders, or “codecs”) that may be used on the air interface in wireless networks include: </li></ul><ul><ul><li>Waveform coding algorithms </li></ul></ul><ul><ul><li>Linear predictive coding algorithms (known as “vocoders”) </li></ul></ul><ul><ul><li>Hybrid coders (combining waveform coding techniques and vocoder techniques) </li></ul></ul>
- 18. Digital Speech Coding Techniques <ul><li>Waveform codec: </li></ul><ul><ul><li>Pulse Code Modulation (PCM) </li></ul></ul><ul><ul><li>Adaptive Differential Pulse Code Modulation (ADPCM) </li></ul></ul><ul><ul><li>Adaptive Predictive Coding (APC) </li></ul></ul><ul><li>Linear Predictive codec (LPC): </li></ul><ul><ul><li>Models speech by encoding and transmitting a few key parameters, which are used at the receiver to synthesize the original speech signal </li></ul></ul><ul><li>Hybrid codec: </li></ul><ul><ul><li>Residual-Excited LPC (RELP) </li></ul></ul><ul><ul><li>Code-Excited LPC (CELP) </li></ul></ul><ul><ul><li>Algebraic Code-Excited LPC (ACELP) </li></ul></ul><ul><ul><li>Vector-Sum Excited LPC (VSELP) </li></ul></ul><ul><ul><li>Multi-pulse, multi-level quantization (MP-MLQ) </li></ul></ul>
- 19. Standardization of Coding Techniques <ul><li>ITU-T G-series standards: </li></ul><ul><ul><li>G.711: Describes 64 Kbps PCM voice coding, including A-law and -law encoding laws </li></ul></ul><ul><ul><li>G.726: Describes ADPCM coding at 40, 32, 24, and 16 Kbps </li></ul></ul><ul><ul><li>G.728: Describes 16 Kbps low-delay variation of CELP (LD-CELP) </li></ul></ul><ul><ul><li>G.729: Describes 8 Kbps CELP (CS-ACELP) (G.729 and G.729 Annex A are similar standards that differ in computational complexity) </li></ul></ul><ul><ul><li>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)) </li></ul></ul><ul><li>ETSI standards for GSM: </li></ul><ul><ul><li>GSM EFR: Compresses 8 KHz sampled speed to 13 Kbps (based on ACELP algorithm) </li></ul></ul>
- 20. Comparison of Codecs
- 21. Channel Coding and Error Correction
- 22. Channel Coding <ul><li>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 </li></ul><ul><li>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) </li></ul><ul><li>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” </li></ul><ul><li>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 </li></ul>
- 23. Types of Channel Coding <ul><li>The digital bit stream (voice, data, or call control) is typically segmented into blocks </li></ul><ul><li>Channel coding is used to add redundancy to each individual block </li></ul><ul><li>Categories of channel coding: </li></ul><ul><ul><li>Block Codes: </li></ul></ul><ul><ul><ul><li>Input block is mapped into output block containing parity bits </li></ul></ul></ul><ul><ul><ul><li>Considered “memoryless”, i. e., dependent only on the individual code </li></ul></ul></ul><ul><ul><li>Convolutional Codes: </li></ul></ul><ul><ul><ul><li>Incorporates “memory” - output is based on the previous m memory blocks </li></ul></ul></ul><ul><ul><li>Interleaving: </li></ul></ul><ul><ul><ul><li>Corrects for “bursts” of errors </li></ul></ul></ul><ul><ul><ul><li>May be used in conjunction with block codes or convolutional codes </li></ul></ul></ul>
- 24. Error Detection and Correction <ul><li>Two main approaches exist for error correction and detection: </li></ul><ul><ul><li>Automatic Repeat Request (ARQ) </li></ul></ul><ul><ul><li>Forward Error Correction (FEC) </li></ul></ul>
- 25. Modulation and Demodulation
- 26. Modulation <ul><li>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 </li></ul><ul><li>There are multiple reasons for modulating a signal prior to transmission on a radio network: </li></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>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 </li></ul></ul>
- 27. Modulation Techniques <ul><li>Modulation techniques vary with the technology supported: </li></ul><ul><ul><li>Analog radio technology (e. g., AMPS) uses analog modulation techniques </li></ul></ul><ul><ul><ul><li>Amplitude Modulation (AM) </li></ul></ul></ul><ul><ul><ul><li>Frequency Modulation (FM) </li></ul></ul></ul><ul><ul><ul><li>Phase Modulation (PM) (considered a variation of Frequency Modulation) </li></ul></ul></ul><ul><ul><li>Digital radio technology (digitized voice or digital data input) such as GSM, IS-95 CDMA, or IS-136 TDMA uses digital modulation techniques: </li></ul></ul><ul><ul><ul><li>Amplitude Shift Keying (ASK) </li></ul></ul></ul><ul><ul><ul><li>Frequency Shift Keying (FSK) </li></ul></ul></ul><ul><ul><ul><li>Phase Shift Keying (PSK) </li></ul></ul></ul><ul><li>Modulation techniques, including more complex forms of digital modulation, are discussed in detail in a separate module (AI-MOD) </li></ul>
- 28. Analog Modulation Carrier Wave Amplitude Time Amplitude Modulation (AM) Amplitude Time Baseband Voice Signal Amplitude Time Frequency Modulation (FM) Amplitude Time
- 29. Digital Modulation 0 0 1 1 1 0 Binary Digits 1 0 -1 Digital Signal 1 0 -1 Amplitude Time Carrier Wave Amplitude Time Frequency Shift Keying (FSK) 1 0 -1 Amplitude Time Amplitude Shift Keying (ASK) 1 0 -1 Amplitude Time Phase Shift Keying (PSK) 1 0 -1 Amplitude Time
- 30. Intermediate Frequency <ul><li>An Intermediate Frequency is defined as a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception </li></ul><ul><ul><li>Intermediate frequencies in a wireless system are generally in the tens or low hundreds of MHz range </li></ul></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>Intermediate Frequencies are used in modulation and demodulation in both analog and digital wireless technologies </li></ul></ul>
- 31. Demodulation <ul><li>Demodulation of a carrier wave to recover the original signal involves several stages: </li></ul><ul><ul><li>Intermediate Frequency </li></ul></ul><ul><ul><li>Amplification </li></ul></ul><ul><ul><li>Demodulation </li></ul></ul><ul><ul><li>Filtering </li></ul></ul>
- 32. Baseband Filtering for Digital Signals
- 33. Filtering in Digital Baseband Processing <ul><li>Filters can be applied at both the transmit and receive end, for analog and digital radio technologies </li></ul><ul><li>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 </li></ul><ul><li>Equalization involves application of a filter to the received signals in order to reverse the time dispersion caused by multi-path effects </li></ul><ul><li>Time dispersion causes inter-symbol interference (ISI), or, more generally, distortion of the received signal </li></ul>
- 34. Effects of Filtering <ul><li>Allows the transmitted bandwidth to be significantly reduced without losing the content of the digital data </li></ul><ul><li>Eliminates much of the distortion of the received signal, Intersymbol Interference (ISI) </li></ul><ul><li>Removes high frequency replicas of the signal that arise due to modulation </li></ul><ul><li>Removes as much noise as possible, while affecting the information signal as little as possible </li></ul>
- 35. Types of Filters in Digital Transmission <ul><li>Raised Cosine filter </li></ul><ul><li>Square Root Raised Cosine filter (IS-136 TDMA) </li></ul><ul><li>Gaussian filter (GSM) </li></ul><ul><li>Chebyshev lowpass Finite Impulse Response (FIR) filter (IS-95 CDMA) </li></ul>
- 36. Multiplexing and Multiple Access
- 37. Multiplexing and Multiple Access <ul><li>Channel multiplexing is used to merge speech with call control signaling, synchronization, etc. into a single digital bit stream prior to modulation </li></ul><ul><li>Multiple Access techniques multiplex active calls from multiple users onto a single frequency channel by using: </li></ul><ul><ul><li>Frequency Division Multiple Access (FDMA) </li></ul></ul><ul><ul><li>Time Division Multiple Access (TDMA) </li></ul></ul><ul><ul><li>Code Division Multiple Access (CDMA) </li></ul></ul>
- 38. Multiple Access <ul><li>Multiple Access techniques may be applied before or after modulation, depending on the technique used: </li></ul><ul><ul><li>FDMA: applied after the signals have been modulated up to the carrier frequencies to be used for transmission </li></ul></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>CDMA: applied before modulation, so that all active calls on a given frequency channel can be modulated onto the same carrier frequency </li></ul></ul>
- 39. Multiple Access Multiplexing FDMA: Combiner Call 1 Call 2 Call n f 1 f 2 f n Modulator f 1 Modulator f 2 Modulator f n f 1 f 2 f n f 1 f 2 f n TDMA: Modulator f 1 Modulator f 2 Modulator f n Call 1 Call 2 Call i Call 1 Call 2 Call i Time Division Multiplexer 1 Time Division Multiplexer 2 f 1 f 2 f n f 1 f 2 f n Modulator f 1 Modulator f 2 Modulator f n CDMA: Call 1 Call 2 Call j Call 1 Call 2 Call j Code Division Multiplexer 1 Code Division Multiplexer 2 f 1 f 2 f n
- 40. Digital Signal Processing
- 41. Digital Signal Processing <ul><li>Digital Signal Processing operates on signals of interest (speech or data) as sequences of binary numbers, using numeric techniques </li></ul><ul><li>“Digital” radio technologies are still partly analog </li></ul><ul><li>The result is selected applicability of Digital Signal Processing </li></ul>
- 42. Hardware Options for Digital Signal Processing <ul><li>Application Specific Integrated Circuit (ASIC) </li></ul><ul><li>Digital Signal Processor (DSP) </li></ul><ul><li>Field Programmable Gate Array (FPGA) </li></ul>
- 43. Software Defined Radio (SDR) <ul><li>Software defined radio (SDR) or “soft radio”: </li></ul><ul><ul><li>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 </li></ul></ul><ul><li>SDRs currently use a combination of DSP, ASIC, and FPGA technology with hardware support </li></ul><ul><ul><li>Ultimate goal is to have all processing done by software </li></ul></ul><ul><ul><li>Battery power, size, weight, and cost requirements are all issues, especially in handheld mobile stations </li></ul></ul><ul><ul><li>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 </li></ul></ul>
- 44. Summary
- 45. Summary of Baseband Processing by Technology
- 46. Summary of Baseband Radio Transmission <ul><li>Analog to Digital Conversion </li></ul><ul><li>Speech Coding </li></ul><ul><li>Channel Coding and Error Detection </li></ul><ul><li>Modulation, Demodulation, and Filtering </li></ul><ul><li>Multiplexing and Multiple Access </li></ul><ul><li>Digital Signal Processing/Software Defined Radio </li></ul>
- 47. Industry Contributors <ul><li>Telcordia Technologies, Inc ( http://www.telcordia.com ) </li></ul>The following companies provided materials and resource support for this module:
- 48. Individual Contributors <ul><li>The following individuals and their organization or institution provided materials, resources, and development input for this module: </li></ul><ul><li>Dr. Cheng Sun </li></ul><ul><ul><li>Cal Poly </li></ul></ul><ul><ul><li>http://www.calpoly.edu </li></ul></ul><ul><li>Dr. David Voltmer </li></ul><ul><ul><li>Rose-Hulman Institute of Technology </li></ul></ul><ul><ul><li>http://www.rose-hulman.edu </li></ul></ul>

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