Simulink 4.1. CDMA Reference Blockset 1.1. Organization of Contents. Contents. Introduction to CDMA. Description of Some Blocks

Transcription

1 Simulink 4.1 CDMA Reference Blockset 1.1 Organization of Contents Contents Introduction to CDMA Description of Some Blocks Description of a Complete Demo

2 Introduction to CDMA (1) History of CDMA General Concepts of CDMA CDMA Transmission Different CDMA methods CDMA Capacity & Processing Gain Suitable PN Codes Intrinsic CDMA Advantages CDMA Disadvantages

3 Introduction to CDMA (2) Power Control Methods Cellular Systems Introduction Cluster Frequency Reuse Handoff CDMA Advantages in Cellular Systems Generations of Cellular Systems CDMA in Different Standards History of CDMA (1) A method for multiple access {single channel ; multiple users} The conventional method was FDMA Then, after developing synchronization methods, TDMA was introduced {T1,E1}

4 History of CDMA (2) In early 1980s CDMA was introduced First for military usage. In early 1990s used in Cellular Communication Systems {IS-95} First commercial large scale usage in Seul. General Concepts of CDMA Transmitted signal occupies a bandwidth much larger than the original signal! The bandwidth is spread by means of a PN code independent of the data The receiver synchronizes to the code to recover the data Many signals share the same times and frequencies but independent codes

5 Spreading a Bit by means of many Chips Relative Bandwidth of coded and original signal

6 Steps of signal Transmission A pseudo-random code is generated, different for each channel The Information data modulates the pseudo-random code ( spreading step). The resulting signal modulates a carrier. The modulated carrier is amplified and broadcast. Block Diagram of CDMA Transmitter

7 THjj Different CDMA methods CDMA is in fact a Spread Spectrum method Direct Sequence Frequency Hopping Time Hopping Hybrid Modulation Classification of CDMA methods CDMA PURE Hybrid DS WB/DS FHh FAST DS/FH DS/TH FH/TH DS/FH/TH MC-CDMA MT-CDMA NB/DS SLOW TDMA/CDMA

8 Gp:Processing Gain Processing Gain (Gp): Gp = fc/fi Gp reflects the relative advantage that frequency spreading provides Usually Gp is 128 or even more Importance of High Gp There are two major benefits from high processing gain: Interference rejection: the ability of the system to reject interference is directly proportional to Gp. System capacity: the capacity of the system is directly proportional to Gp. the higher the PN code bit rate wider the CDMA bandwidth the better the system performance

9 Suitable PN Codes Codes should be almost uncorelated. Each code should have small autocorelation System Capacity The capacity of a system is approximated by: Cmax=Gp/(Eb/No)* (1/(1+B)) Cmax is the maximum number of simultaneous calls Gp is the processing gain Eb/No is the total signal to noise ratio per bit B is the inter-cell interference factor

10 Intrinsic CDMA Advantages Multipath access capability Multipath interference rejection Interference rejection Anti-jamming {esp. NB jamming} capability Low prabablity of interception {LPI} {in fact extreme Privacy} CDMA Disadvantages Highly Accurate Synchronization {esp. for W-CDMA}: 0.2*Tc Synchronization is needed Accurate Power Control is needed {esp. in Reverse Channel} Near-End/Far-End Problem

11 Power Control Methods Power Control compensates Shadowing & FastFading effects also Open-Loop Power Control {by means of a Pilot signal only} Closed-Loop Power Control Voice Activity Consideration Generally Power Control increases the system capacity Cellular Systems Introduction Low Power Limited Frequency Band Point to Point ; Not Broadcast The Only solution is: Frequency Reuse!

12 Covering an area by many Cells A Cluster

13 Frequency Reuse Handoff

14 CDMA Advantages in Cellular Systems No need to Frequency Managements {Number of Cells in a Cluster is (N)=1} Soft Capacity Limit Prabablity of Force Termination is negligible Soft Handoff {No Ping-Pong effect!} Generations of Cellular Systems G1 : in 1980s, Analog, Vehicle Sets, Voice only, Uncompatible standards G2 : in 1990s, Digital, Hand Sets, Voice+Low Data Rates, Few Global standards {Current Sys} G3 : in 2000s, High Data Rates, 3 Global standards, {Designed but not Commercial Implemented} G4: is being Designed!

15 CDMA in Different Standards No CDMA in G1! {Only FDMA} G2: Only IS-95 System CDMA Others {GSM,IS-54,PDC}: FDMA/TDMA G3: WCDMA & CDMA2000 are CDMA Only UWC-136 is FDMA/TDMA G4: Proposals for various Spread Spectrum methods, other than DS-CDMA!

16 Description of Some Blocks of CDMA Blockset (1) IS-95 PN Codes Fwd Link Logical Chs. Rev Link Logical Chs. Forward Channel Coding Coherent Rake Receiver Non-Coherent rake Receiver Rates and Rate Sets Description of Some Blocks of CDMA Blockset (2) CDMA Reference Blockset Library IS-95 Base Station Transmitter IS-95 Mobile Station Receiver IS-95 Mobile Station Transmitter IS-95 Base Station Receiver IS-95 Common IS-95 Forward Ch. IS-95 Reverse Ch.

17 IS-95 PN Codes (1) Chip rate: Mchips/s So,Spread Signal Bandwidth: 1.25MHz Walsh Codes: 64 chips, mutually Orthogonal Ensure Orthogonality between signals from a Base for different Mobiles. Also used for modulation for the Rev. Ch. IS-95 PN Codes (2) Short PN Codes: A pair of periodic Binary sequences with period 2^15 chips. used for spreading and despreading signals into in-phase and quadrature components.

18 IS-95 PN Codes (3) Long PN Codes: Period: 2^42-1. Used for Data Scrambling. Used for Spreading on the Reverse link. Used for Power Control Burst randomization Fwd Link Logical Chs. Pilot Ch: modulates a constant sumbol! used for ch. Estimation, Coherent demodulation of other channels. Sync Ch. Paging Ch: for Controling, paging the requested Mobile. Traffic Ch: Speech or Data.

19 Rev Link Logical Chs. Access Ch: for Controling, sending Requests, responding to pagings, etc. Traffic Ch: Speech or Data. Forward Channel Coding Uses 20 ms frames for all channels Only Sync channel uses ms frames Error protection : 1/2-rate convolutional code Viterbi algorithm for optimal decoding For protection against bursts of errors, Interleaving before modulation.

20 Coherent Rake Receiver In Fwd Ch. Several paths with different propagation delays. Detects the transmitted data from each delayed-path component and combines the results. In fact we have a diversity gain! Non-Coherent rake Receiver In Rev Ch. No Pilot available! Each of 64 possible walch Codes are multiplied bye both In-phase and Qaudrature components and then are added. The biggest result is selected!

21 Rates and Rate Sets CDMA Reference Blockset Library

22 IS-95 Base Station Transmitter IS-95 Mobile Station Receiver

23 IS-95 Mobile Station Transmitter IS-95 Base Station Receiver

24 IS-95 Common IS-95 Forward Ch.

25 IS-95 Reverse Ch.

26 IS-95A Forward Traffic Channel Detection Library Blocks in the Demo IS-95A Fwd Ch Base Station Transmitter Interface (inside Spreading and Modulation subsystem) IS-95A Fwd Ch Detector IS-95A Long Code Generator (inside Data Source subsystem) IS-95A Short Code Generator (inside Data Source subsystem) IS-95A Walsh Code Generator (inside Data Source subsystem) IS-95A Fwd Ch Scrambler

27 Data Source Subsystem Components of Data Source Random number generator: generates the bipolar random data symbols for the Traffic, Sync and Paging channels. A signal that represents the data rate. The IS-95A Long Code Generator library block: used to scramble the data. The IS-95A Walsh Code Generator and IS-95A Short Code Generator library blocks: used for orthogonal spreading.

28 Inside Spreading and Modulation Subsystem Rayleigh Multipath and AWGN Channel subsystem

29 Variable Parameters Simulation Duration Data Rate Rate Set Doppler Frequency in Channel S/N Ratio Random Seed Ch Paths & Rake Fingers