Basic cdma for 2 g and 3g

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  • January, 2002 1Course 136 v1.20 (c) 2001 Scott Baxter Understanding Basic CDMA For 2G and 3G Systems Understanding Basic CDMA For 2G and 3G Systems Course 136
  • January, 2002 2Course 136 v1.20 (c) 2001 Scott Baxter Purposes of This Course s This course would be a good choice for any of the following: a first introduction to CDMA technology a convenient review to solidify basic understanding of CDMA into a foundation for working in 2G or 3G systems an efficient way for engineers and techs in other technologies to understand the CDMA technology they will soon implement a glimpse into the similarities and differences between the two families of 3G systems around the world US/CDMA2000 International/WCDMA-UMTS
  • January, 2002 3Course 136 v1.20 (c) 2001 Scott Baxter Foreword: A Quick CDMA Family History s Code Division Multiple Access is a popular wireless communication technology based on spread-spectrum techniques s The first CDMA cellular systems began commercial operation in late 1994 and early 1995 (IS-95 standard, including AMPS/CDMA integration) s Many PCS systems chose CDMA and began operation in 1996 (J-Std008) s Performance/popularity of these IS-95/J-Std 008 systems has been good s Revised Standard IS-95B became available by early 1999 Offers improved access, handoffs, more flexible data capability Hasn't been widely deployed, most operators await CDMA2000 s In 1997-1998, the existing CDMA user community developed CDMA2000 for Third Generation systems, building on the IS-95 platform 1xRTT deployment underway now s In 1997-1998, the GSM community selected wideband CDMA technology (WCDMA, UMTS) to supersede GSM for Third Generation systems To avoid GPS dependence, uses other codes than IS-95/CDMA2000 s Wide successful deployment expected for both CDMA2000 and WCDMA
  • January, 2002 4Course 136 v1.20 (c) 2001 Scott Baxter CDMA Development & Deployment Leading up to Third Generation 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 TIA formally accepts CDMA As a standard in the USA Hong Kong and Los Angeles IS-95 commercial launch Korea IS-95 Commercial Launch Korea passes Million customers Ericsson, NTT Promote 3G ITU 3G team formed ETSI selects CDMA For GSM>3G migration! US CDMA2000 Specs finished ITU receives 10 IMTS2000 proposals Harmonization ratified 1xRTT Trials UMTS Trials 1xRTT Launch? UMTS Launch?
  • January, 2002 5Course 136 v1.20 (c) 2001 Scott Baxter Course Outline s Basic CDMA Principles Spreading Gain, Capacity, Link considerations s CDMA Coding Basics Forward and Reverse Code Schemes and Channels Differences between CDMA2000 and WCDMA/UMTS s IS-95 CDMA Operational Details Codes, Channels, Multiplexing, Power Control s IS-95 CDMA Handset Architecture s IS-95 CDMA Handoffs s IS-95 CDMA Network Architecture s IS-95 CDMA Messaging and Call Flow Procedures and Examples s IS-95 Basic Optimization - Drive-Test Tools, Post-Processing s Bibliography and Tools
  • January, 2002 6Course 136 v1.20 (c) 2001 Scott Baxter How Does CDMA Work? Introduction to Basic Principles How Does CDMA Work? Introduction to Basic Principles
  • January, 2002 7Course 136 v1.20 (c) 2001 Scott Baxter CDMA: Using A New Dimension s CDMA at first seems very strange when compared with 'normal' radio technologies s Every sector of every base station transmits simultaneously on the same frequency Not separated onto different frequencies, and not on different timeslots s The signal of a CDMA BTS sector carries all its individual channels simultaneously Not separated into timeslots, not separated onto different frequencies s All mobiles are transmitting simultaneously on their own frequency Not on different frequencies as in FDMA Not on different timeslots as in TDMA s How is it possible to distinguish the channels of individual users?!!? CDMA The Secret of CDMA Each CDMA user has a unique code pattern, different from the code pattern of any other user in the area. The user's actual "payload" information bits are combined with a much faster, yet unique, "noise" waveform. At the receiver, the received signal is combined with the same "noise" waveform locally generated. What is left is the user's information bits. Since all users' noise waveforms are random compared to each other, the presence of other users' signals does not confuse the detection process.
  • January, 2002 8Course 136 v1.20 (c) 2001 Scott Baxter Claude Shannon: The Einstein of Information Theory s The core idea that makes CDMA possible was first explained by Claude Shannon, a Bell Labs research mathematician s Shannon's work relates amount of information carried, channel bandwidth, signal-to-noise-ratio, and detection error probability It shows the theoretical upper limit attainable In 1948 Claude Shannon published his landmark paper on information theory, A Mathematical Theory of Communication. He observed that "the fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point." His paper so clearly established the foundations of information theory that his framework and terminology are standard today. Shannon died Feb. 24, 2001, at age 84. SHANNONS CAPACITY EQUATION C = B log2 [ 1 + ]S N B = bandwidth in Hertz C = channel capacity in bits/second S = signal power N = noise power
  • January, 2002 9Course 136 v1.20 (c) 2001 Scott Baxter Two Types of CDMA CDMA can be implemented two ways: s Frequency-Hopping Each users narrowband signal hops among discrete frequencies, and the receiver follows in sequence Frequency-Hopping Spread Spectrum (FHSS) CDMA is NOT currently used in wireless systems, although used in military applications s Direct Sequence narrowband input from a user is coded (spread) by a user-unique broadband code, then transmitted broadband signal is received; receiver knows, applies users code, recovers users data Direct Sequence Spread Spectrum (DSSS) CDMA IS the method used in IS-95 commercial systems User 1 Code 1 Composite Time Frequency + = Direct Sequence CDMA User 1 User 2 User 3 User 4 Frequency Hopping CDMA User 3 User 4 User 1 unused User 2 User 1 User 4 User 3 User 2 unused Frequency unused User 1 User 2 User 4 User 3
  • January, 2002 10Course 136 v1.20 (c) 2001 Scott Baxter The Mechanics of DSSS Spreading: Viewed as Waveforms in Time At Originating Site: s Input A: Users Data @ 19,200 bits/second s Input B: Walsh Code #23 @ 1.2288 Mcps s Output: Spread spectrum signal At Destination Site: s Input A: Received spread spectrum signal s Input B: Walsh Code #23 @ 1.2288 Mcps s Output: Users Data @ 19,200 bits/second just as originally sent Drawn to actual scale and time alignment via air interface XOR Exclusive-OR Gate 1 1 Input A: Received Signal Input B: Spreading Code Output: Users Original Data Input A: Users Data Input B: Spreading Code Spread Spectrum Signal XOR Exclusive-OR Gate Originating Site Destination Site
  • January, 2002 11Course 136 v1.20 (c) 2001 Scott Baxter Direct-Sequence Spreading Viewed as Frequency Spectra s Traditional technologies try to squeeze a user's signal into a minimum required bandwidth s CDMA uses larger bandwidth but exploits the resulting processing gain to get capacity Spread Spectrum Payoff: Processing Gain Spread Spectrum TRADITIONAL COMMUNICATIONS SYSTEM Slow Information Sent TX Slow Information Recovered RX Narrowband Signal SPREAD-SPECTRUM SYSTEM Fast Spreading Sequence Slow Information Sent TX Slow Information Recovered RX Fast Spreading Sequence Wideband Signal
  • January, 2002 12Course 136 v1.20 (c) 2001 Scott Baxter The CDMA Spread Spectrum Payoff: Would you like a lump-sum, or monthly payments? s Shannon's work suggests that a certain bit rate of information deserves a certain bandwidth s If one CDMA user is carried alone by a CDMA signal, the processing gain is large - roughly 21 db for an 8k vocoder. Each doubling of the number of users consumes 3 db of the processing gain Somewhere above 32 users, the signal-to-noise ratio becomes undesirable and the ultimate capacity of the sector is reached s Practical CDMA systems restrict the number of users per sector to ensure processing gain remains at usable levels # Users Processing Gain 1 21 db 2 18 db 4 15 db 8 12 db 16 9 db 32 6 db 64..Uh, Regis, can I just take the money I've already won, and go home now? CDMA Spreading Gain Consider a user with a 9600 bps vocoder talking on a CDMA signal 1,228,800 hz wide. The processing gain is 1228800/9600 = 128, which is 21 db. What happens if additional users are added?
  • January, 2002 13Course 136 v1.20 (c) 2001 Scott Baxter Real-World CDMA Channel Coding Real-World CDMA Channel Coding
  • January, 2002 14Course 136 v1.20 (c) 2001 Scott Baxter Real-World CDMA Codes - Forward Link s In actual commercial systems, a user's unique code is actually constructed from a combination several simpler codes s A base station uses a different unique channelization code to mix with and encode the signal of each individual user before combining their signals s The combined signal is then coded with a unique pattern to distinguish this sector's si