Overview of Digital Mobile...
Transcript of Overview of Digital Mobile...
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Overview of Digital Mobile Overview of Digital Mobile CommunicationsCommunications
Dong In Kim([email protected])
Wireless Communications Lab
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OutlineOutline
Digital CommunicationsDigital Communications
Multiple Access TechniquesMultiple Access Techniques
Power Control for CDMAPower Control for CDMA
IMTIMT--2000 System2000 System
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Why Digital? (1/3)Why Digital? (1/3)AdvantagesEasy to regenerate original function
Distance 1Distance 1OriginalOriginalpulse signalpulse signal
Distance 5Distance 5Amplification Amplification to regenerate pulseto regenerate pulse
Distance 2Distance 2Some signal Some signal distortiondistortion
Distance 3Distance 3DegradedDegradedsignalsignal
Distance 4Distance 4Signal is badlySignal is badlydegradeddegraded
1 2 3 4 5Propagation distance
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Why Digital? (2/3)Why Digital? (2/3)
Less subject to distortion & other interference
High signal fidelityHigh signal fidelity can be obtained
Digital circuits are more reliablereliable, lower costcost,
and more flexibleflexible to implementation in H/W
Protection against interference & jamming
Easy storageEasy storage & processingprocessing of data
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Why Digital? (3/3)Why Digital? (3/3)
Disadvantages
More technical complexitycomplexity
Require additional stepsadditional steps
Sampling
AD conversion
Require aa greater bandwidthgreater bandwidth
Require complicatedcomplicated synchronizationsynchronization issues
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Block Diagram of a Typical Digital Block Diagram of a Typical Digital Communication SystemCommunication System
Channel
InformationSource
InformationSink
ChannelBits
Synchron-ization
Digital Waveform
BitStream
FormatSourceencode Encrypt.
Channelencode
Multiplex ModulateMultipleAccess
FrequencySpread
XMT
FormatSourcedecode Decrypt. Channel
dncodeDemulti-
plexDemod-
lateMultipleAccess
FrequencyDepread
RCV
Digitalinput
Digitaloutput
SourceBits
ChannelBits
ChannelBits
SourceBits
ChannelBit
im∧
im
)t(s i
∧
)t(s i
EssentialOptional
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System ComponentsSystem ComponentsSystem components
Codec (Coder/Decoder)Modem (Modulator/Demodulator)RF (Transmitter/Receiver)
Data statusInformation source: digital, analog, textual informationBit stream
Digital waveform block diagramReciprocity between blocks in upper and lower branchesUpper block: transmitter (frequency up conversion stage, high power amp., antenna)Lower block: receiver (antenna, LNA, down-converter stage)
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Data StatusData StatusBit stream
Bandpass digital waveform
time
T T T
0 0 0 1 0 0 1 1 1 1 1 0 0 1 1 1 1 0 1 0 1
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Signal Flow in Digital CommunicationsSignal Flow in Digital Communications
FormattingFormatting: transforms the source information into digital symbolsSource codingSource coding: removes redundant or unneeded informationEncryptionEncryption: prevents unauthorized users fromunderstanding messages and from injecting false messages into the systemChannel codingChannel coding: reducing the probability of error, or SNR requirement, at the expense of bandwidth or decoder complexity
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CDMA Communication FlowCDMA Communication FlowConverting analog voice to digital
CODEC VOCODER 10011
Pulse CodedModulation
(PCM)
Vocodedvoice
Pulse Code Modulation (PCM) is a technique to convert analog voice to a digital representation of the voiceconvert 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.
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Example of FormattingExample of FormattingQuantization Level
CodeNumber X(t)(V)
Formatting Analog Information(Natural sample, quantized sample, pulse code modulation)
X(t)
t
001011100110111101134675
-2.5-0.50.52.53.51.5-2.4-0.80.523.31.3Natural sample value
PCM number
Code number
Quantized sample value-3.4-3.5
0
000
-2.5
3.5
0.5
-1.5
2.51.5
-0.5
-3.51
7
4
2
65
3
0-2
4
1
-1
32
0
-3-4
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Source coding, Encryption, Channel codingSource coding, Encryption, Channel coding
Three levels of codingThree levels of coding
TrashCan
Source Coding Source Coding (Data Compression)(Data Compression)
Encryption Encryption (Cryptography(Cryptography))
Channel Coding Channel Coding (Error Correction)(Error Correction)
ReducesReducesparcelparcelcostcost
PreventsPreventsthievesthieves
ProtectsProtectsagainst roughagainst roughhandinghanding
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Channel Encoding Example (1/2)Channel Encoding Example (1/2)
Actualmessage
Encodedtransmitted
message
Receivedmessage
Decodedmessage
D,D,DC,C,CB,B,BA,A,A
D,D,D?,?,CB,?,?A,A,A
DCBA
D??A
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Channel Encoding Example (2/2)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:
AA,, AA, , AA, , BB,, BB, , BB, , CC, , CC, , CC, , DD,, DD, , DD..
Note that errors as “?” in this example. The received message is:
AA, , AA, , AA, , BB, ?, ?, ?, ?,, ?, ?, ?, ?, CC,, DD,, DD, , DD..
The decoder uses majority logic rule majority logic rule (two out of three wins).
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Interleaving Example (1/2)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 symbolsInterleaved symbols
A B C ? ? ? ? D A B C D Burst errors
A ? A B ? B C ? C D ? D DeinterleavedDeinterleaved symbolssymbols
A B C D Decoded bits
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Interleaving Example (2/2)Interleaving Example (2/2)Interleaving is a simple, but powerful, method of reducing reducing errorserrors and recovering bitsrecovering bits when errors occur. In this example, the first 3 symbols for the 1st code (1,1,1) in the actual message are transmitted at locations 1, 5, and 9. The 3 symbols for the 3rd 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 decodingmajority logic decoding.
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Modulation (1/3)Modulation (1/3)ModulationModulation: process by which the symbols areconverted to waveforms that are compatible with the transmission channel
PSKPSK (Phase Shift Keying): the carrier phasephase varies
with corresponding digital symbol.
FSKFSK (Frequency Shift Keying): the carrier frequencyfrequency
varies with corresponding digital symbol.
ASKASK (Amplitude Shift Keying): the carrier amplitudeamplitude
varies with corresponding digital symbol.
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Modulation (2/3)Modulation (2/3)
Note
ModulationModulation: translate a baseband message signal
to a bandpass signal at high frequencies
BasebandBaseband signalsignal: information-bearing signal
CarrierCarrier: high frequency signal
Modulated waveModulated wave: bandpass signal
Cellular systemCellular system: 800MHz, 1.7 ~1.8GHz (PCS)
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Modulation (3/3)Modulation (3/3)PSKPSK
FSKFSK
ASKASK
T TT
time
T TT
time
T TT
time
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Methods to Improve Methods to Improve Frequency Efficiency (1/2)Frequency Efficiency (1/2)
Frequency reuse
Multiple access method
Multiple access technique permits the communication resources of the channel to be shared by a largeshared by a largenumber of usersnumber of users seeking to communicate with each other.
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Methods to Improve Methods to Improve Frequency Efficiency (2/2)Frequency Efficiency (2/2)
FDMAFDMA: disjoint subbands of frequencies are allocated to the different users on a continuous-time basis.
TDMATDMA: each user is allocated the full spectral occupancy of the channel, but only for a short duration of time called a time slot.
CDMACDMA: an application of spread-spectrum techniques (SS). using orthogonal codes permits multiple users to access the same frequency band at the same time.
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FDMAFDMAFFrequency DDivision MMultiple AAccess
ch2 ☆☆☆☆☆☆☆☆ ☆☆☆☆☆☆☆
Multiple Access
B
C ch3
ch1 ☆☆☆☆A
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FDMA (cont.)FDMA (cont.)
GOGO
HIHI
II HH
Frequency channel 1
Frequency channel 2
OO GG
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TDMATDMA
TTime DDivision MMultiple AAccess
Multiple Access
Time slot Time Gate
☆☆
B
C
☆☆A
☆
☆
☆
☆
ch3 ch2 ch1
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TDMA (cont.)TDMA (cont.)
Time
slot4
Time
slot3
Time
slot2
Time
slot1
GOGO
HIHI
II OO HH GG
HIHI
GOGO
Common Frequency ChannelCommon Frequency Channel
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CDMACDMACCode DDivision MMultiple AAccess
A ☆☆☆ 1010 1010 ☆☆☆ AB 1100 1100 B
X 1001 1001 X
11001100
11001100
11001100
10011001
10011001
10101010☆
10101010☆
Multiple Access / Spread Spectrum
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CDMA (cont.)CDMA (cont.)
Common ChannelCommon Channel
Red codeRed code
Green codeGreen code
HIHI
GOGO
HIHI
GOGO
OOGG
HHII
Green codeGreen code
Red codeRed code
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Codes for CDMACodes for CDMAOrthogonal Code (Walsh Code)Orthogonal Code (Walsh Code)
Generated by Hadamard MatrixHadamard MatrixIts important property is “orthogonal” among codes
PN CodePN CodeGenerated by hardware such as shift registershift registerIt is not perfectly orthogonalPseudo random noise codeShort code: length Long code: length
768,32215 ==
Billion44002 42 ==
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Orthogonal Code (Walsh Code)Orthogonal Code (Walsh Code)
Orthogonal functions have zero correlation !Orthogonal functions have zero correlation !
Two binary sequences are orthogonal if the process
of “XORingXORing” them results in an equal number of
1’s and 0’s.
Example:0 0 0 00 1 0 10 1 0 1
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Generation of Orthogonal CodeGeneration of Orthogonal CodeRepeat
Right ComponentBelow Component
InvertDiagonal Component
Procedure:0 0 0 00 1 0 10 0 1 10 1 1 0
0 00 1
0Seed
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Multiple access using SS technique (1/3)Multiple access using SS technique (1/3)( )tm1 ( )tc11 -1 1
bT b2T b3T
( ) ( )tctm 11
( )tm2
bT b2T b3T
-1
c2T c4T c12Tc8T( )tc21 1
( ) ( )tctm 22
c2T c4T c8T c12T
mi(t): Baseband messages
cj(t): Orthogonal codes
mi(t)cj(t): Spread messages
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( ) ( ) ( ) ( )tctmtctmA 2211 +=
( ) ( )tcAtB 11 ×=
( ) ( )tcAtB 22 ×=
bT b2T b3T
bT b2T b3T
bT b2T b3T
The waveforms for the signals at different points of the receiveThe waveforms for the signals at different points of the receiverr
--2 0 2 0 0 2 0 2 0 2 0 0 2 0 ––2 0 2 0 ––2 0 22 0 2
2 0 2 0 0 2 0 2 0 0 --2 0 2 0 ––2 0 2 0 22 0 2 0 2
2 0 2 0 0 2 0 2 0 2 0 2 0 0 2 0 2 0 --2 0 2 0 --22
Multiple access using SS technique (2/3)Multiple access using SS technique (2/3)
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( )tm 1~
2c
( )tm 2~
cT bT b2T b3T
cT bT b2T b3T
cT bT b2T b3T
cT bT b2T b3T
1cMultiple access using SS technique (3/3)Multiple access using SS technique (3/3)
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Basic Principle of CDMABasic Principle of CDMA0110100110010110011001101001100101100110
Base Mobile
User input User output
10011 10011
1001100110011001100110011001100110011001 1001100110011001100110011001100110011001Code Code
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CDMA techniqueCDMA technique(Direct(Direct--Sequence Spreading)Sequence Spreading)
1 0 0 1 1User Input
Spreading Sequence
1001 1001 10011001 1001 1001 10011001 10011001
0110 1001 10010110 1001 1001 01100110 01100110Tx Data
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CDMA techniqueCDMA technique(Decoding using Correct User Code)(Decoding using Correct User Code)
0110 1001 1001 01100110 1001 1001 0110 01100110Rx Data
Correct Code
1001 1001 10011001 1001 1001 10011001 10011001
1111 0000 0000 1111 1111
1 0 0 1 1User Output
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CDMA techniqueCDMA technique(Decoding with Incorrect User Code)(Decoding with Incorrect User Code)
0110 1001 1001 01100110 1001 1001 0110 01100110Rx Data
Incorrect Code
01010101 0101 0101 0101 0101 0101 0101 0101 0101
0011 1100 1100 0011 0011
? ? ? ? ?User Output
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CDMA Channel GenerationCDMA Channel Generation
Background
Noise
External
Interference
Other Cell
Interference (Ioc)
Other User
Noise(isc)
10KHz1.25MHz
f
1.25MHz
0
10KHz
f f
= -1.69dB/Hz
1.25MHz
Wideband
Spectrum
Data
DataData
(9.6Kbps)
Encoding &
Interleaving
PN
Source
1.25MHz 1.25MHz
BPF
1.25MHz
Digital
Filter
PN
Source
Deinterleave
& Decode
1.2288
McpsCarrier
Carrier
1.2288
Mcps
Correlator
f0
f f
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Receive Sensitivity in Free SpaceReceive Sensitivity in Free Space
Base station Mobile station
R (distance)
Receive sensitivity is in inverse proportion to Receive sensitivity is in inverse proportion to the square of distance in free space.the square of distance in free space.
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Receive Sensitivity in Cellular RadioReceive Sensitivity in Cellular Radio
Base station Mobile station
Multipath
Reflector
R (distance)
Receive sensitivity is in inverse proportion to Receive sensitivity is in inverse proportion to the third or fifth power of distance.the third or fifth power of distance.
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Receive SensitivityReceive SensitivityAverage path loss as a function of distanceAverage path loss as a function of distance
: reference distance
Free space20dB/decade pass loss
Cellular radio30~50dB/decade pass loss
( )n
ddd Pathloss ⎟⎟⎠
⎞⎜⎜⎝
⎛∝
0
0d
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NearNear––Far ProblemFar Problem
A B Signal B after despreading
Impossible for the receiver to detect signal B
A
B
A
~~ ~~
~~~~
BNearNear--far problemfar problem
Received power level at base
When the difference in distance from base station to mobile A and B is ten times, their received powers differ by the third or fifth power of 10 (30-50dB). Then, it is hard to detect the signal from mobile B.
100M 1000M0
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Need for Power ControlNeed for Power ControlTo avoid “nearnear--farfar” problemObservationsObservations:
Need to control the transmit power level to meetthe minimum required SIRminimum required SIR.Save the powerSave the power at the mobile station.
If Tx/Rx power level is too low, it will increase BER.If the power level is too high, undesired interferencewill degrade the BER of other mobiles.
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Forward Power ControlForward Power Control
OK!!OK!!
BaseStation
What? I can’t hear you
What? I can’t hear you
with forward with forward power controlpower control
Base station transmission power
AB C
DC
B
B
DA
C
D
A
without forward without forward power controlpower controlhigh BER in Chigh BER in C
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Reverse Power ControlReverse Power Control
Please raise your powerPlease raise your power
without reverse without reverse power controlpower control
with reverse with reverse power controlpower control
Base station received power
A B C
D AA
D
D
BC
B
C
BaseStation