Overview of Digital Mobile Communications Dong In Kim (dikim@ece.skku.ac.kr) Wireless Communications Lab 1
Outline Digital Communications Multiple Access Techniques Power Control for CDMA IMT-2000 System 2
Why Digital? (1/3) Advantages Easy to regenerate original function Distance 1 Distance 2 Distance 3 Distance 4 Distance 5 Original Some signal Degraded Signal is badly Amplification pulse signal distortion signal degraded to regenerate pulse 1 2 3 4 5 Propagation distance 3
Why Digital? (2/3) Less subject to distortion & other interference High signal fidelity can be obtained Digital circuits are more reliable, lower cost, and more flexible to implementation in H/W Protection against interference & jamming Easy storage & processing of data 4
Why Digital? (3/3) Disadvantages More technical complexity Require additional steps Sampling AD conversion Require a greater bandwidth Require complicated synchronization issues 5
Block Diagram of a Typical Digital Communication System Channel Bit Format Source encode Source Bits Encrypt. Channel Bits Channel Bits Channel Multiplex encode Modulate Frequency Spread Multiple Access XMT Information Source Digital input m i Bit Stream Synchron -ization s i (t) Digital Waveform Channel Information Sink Digital output m i s i (t) Format Source decode Decrypt. Channel dncode Demultiplex Demodlate Frequency Depread Multiple Access RCV Essential Optional Source Bits Channel Bits 6
System Components System components Codec (Coder/Decoder) Modem (Modulator/Demodulator) RF (Transmitter/Receiver) Data status Information source: digital, analog, textual information Bit stream Digital waveform block diagram Reciprocity between blocks in upper and lower branches Upper block: transmitter (frequency up conversion stage, high power amp., antenna) Lower block: receiver (antenna, LNA, down-converter stage) 7
Data Status Bit stream 0001 0 0 111110011110 1 0 1 Bandpass digital waveform time T T T 8
Signal Flow in Digital Communications Formatting: transforms the source information into digital symbols Source coding: removes redundant or unneeded information Encryption: prevents unauthorized users from understanding messages and from injecting false messages into the system Channel coding: reducing the probability of error, or SNR requirement, at the expense of bandwidth or decoder complexity 9
CDMA Communication Flow Converting analog voice to digital Pulse Coded Modulation (PCM) Vocoded voice CODEC VOCODER 10011 Pulse Code Modulation (PCM) is a technique to convert analog voice to a digital representation of the voice. The Vocoder (Voice Coder) is used to compress the digital representations that came from the Codec (Code/Decoder). In other words, in compression, redundant digits are eliminated. Voice quality is not adversely affected. 10
Example of Formatting Code Number 7 6 5 4 3 2 1 0 Quantization Level X(t)(V) 3.5 2.5 1.5 0.5-0.5-1.5-2.5-3.5 Natural sample value Quantized sample value Code number PCM number 4 3 2 1 0-1 -2-3 -4 1.3 1.5 5 101 Formatting Analog Information (Natural sample, quantized sample, pulse code modulation) 3.3 3.5 7 111 2 2.5 6 110 0.5 0.5 4 100 X(t) -0.8-0.5 3 011-2.4-2.5 1 001-3.4-3.5 0 000 11 t
Source coding, Encryption, Channel coding Three levels of coding Source Coding (Data Compression) Encryption (Cryptography) Channel Coding (Error Correction) Trash Can Reduces parcel cost Prevents thieves Protects against rough handing 12
Channel Encoding Example (1/2) Actual message A B C D Encoded transmitted message A,A,A B,B,B C,C,C D,D,D Received message A,A,A B,?,??,?,C D,D,D Decoded message A?? D 13
Channel Encoding Example (2/2) A simple coding scheme is shown in this example. The actual digit message consists of four letters (A,B,C,D). Each of the bits are then encoded into 3 symbols to represent one bit. The encoded transmitted message would be as follows: A, A, A, B, B, B, C, C, C, D, D, D. Note that errors as? in this example. The received message is: A, A, A, B,,?,?,?,?, C, D, D, D. The decoder uses majority logic rule (two out of three wins). 14
Interleaving Example (1/2) A B C D Bit pattern A A A B B B C C C D D D Encoded symbols A B C D A B C D A B C D Interleaved symbols A B C???? D A B C D Burst errors A? A B? B C? C D? D Deinterleaved symbols A B C D Decoded bits 15
Interleaving Example (2/2) Interleaving is a simple, but powerful, method of reducing errors and recovering bits when errors occur. In this example, the first 3 symbols for the 1 st code (1,1,1) in the actual message are transmitted at locations 1, 5, and 9. The 3 symbols for the 3 rd code (1,1,1) are transmitted at locations 3, 7, and 11. Using the same error pattern as used in the previous example the received message is: 1?1,0?0,1?1,0?0. What is the decoded message? Note that with the same error pattern as before, the original message is completely recovered using majority logic decoding. 16
Modulation (1/3) Modulation: process by which the symbols are converted to waveforms that are compatible with the transmission channel PSK (Phase Shift Keying): the carrier phase varies with corresponding digital symbol. FSK (Frequency Shift Keying): the carrier frequency varies with corresponding digital symbol. ASK (Amplitude Shift Keying): the carrier amplitude varies with corresponding digital symbol. 17
Modulation (2/3) Note Modulation: translate a baseband message signal to a bandpass signal at high frequencies Baseband signal: information-bearing signal Carrier: high frequency signal Modulated wave: bandpass signal Cellular system: 800MHz, 1.7 ~1.8GHz (PCS) 18
Modulation (3/3) PSK time T T T FSK time T T T ASK time T T T 19