Revision Of Spread Spectrum Technology ?

Revision of Spread Spectrum Technology ?

1

Revision What is Spread Spectrum Technology the bandwidth − In Spread Spectrum communication, ? occupancy of a single transmitted signal is much higher than in systems using conventional modulation methods. − This band-spreading is achieved by selecting appropriate transmission waveforms with a wide bandwidth. − A very popular method is to multiply the user data signal with a fast code sequence, which mostly is independent of the transmitted data message. In the case that multiple users share the same portion of the radio spectrum but use different codes to distinguish their transmissions, we speak of Code Division Multiple Access (CDMA)

2

Advantages of Spread Spectrum − As the signal is spread over a large frequency band, the Power Spectral Density becomes very small. Other communications systems may not suffer from this kind of communications. − Confidentiality: without knowing the spreading code, it is difficult to recover the transmitted data. Moreover, as the spectral density is small, the signal may remain undetected. − Spreading and despreading makes the signal robust against interference. This also holds for Multipath self interference. 3

Advantages of Spread Spectrum

− Fading rejection: as the bandwidth can be made much larger than the coherence bandwidth of the channel, the system is less susceptible to deep fades at particular frequencies.

4

Advantages of Spread Spectrum

− Originally developed navigation purposes

for

military

and

 Hard to be intercepted  Anti-jamming

− Nowadays feasible for commercial applications especially for mobile communication systems

5

Classification of Spread Spectrum Techniques

− Various spread-spectrum techniques have been proposed:  Direct-Sequence  Frequency-Hopping

6

Direct-Sequence Spread Spectrum (DSSS)

7

Direct-Sequence Spread Spectrum (DSSS)

The duration of an element in the code is called the "chip time".

8

Direct Sequence CDMA

9

DSSS Transmission

Data A

0

Tb

Transmitter

chip

1

0

0

Spreading Code B

0 1 1 0 1 0 0 1 0 1 1 0 1 0 1 1

Tc Spread Signal C = A + B 0 1 1 0 0 1 1 0 0 1 1 0 1 0 1 1

Spreading

10

DSSS Reception

11

DSSS Reception

Received Signal C

0 1 1 0 0 1 1 0 0 1 1 0 1 0 1 1 Locally Generated Spreading Code B

Receiver

0 1 1 0 1 0 0 1 0 1 1 0 1 0 1 1 Data output A = C + B

0

1 Despreading

0

0 12

Processing Gain

13

Processing Gain

− The ratio between the user symbol time and the chip time is called the spread factor. The transmit signal occupies a bandwidth that equals the spread factor times the bandwidth of the user data

14

Code Division Multiple Access (CDMA)

15

Orthogonal Spreading & DeSpreading Example

16

Orthogonal Spreading

− Orthogonal Spreading  As the principle behind spreading and despreading is that when a symbol is XORed with a known pattern and the result is again XORed with the same pattern, the original symbol is recovered. In other words, the effect of an XOR operation if performed twice using the same code is null.  In orthogonal spreading, each encoded symbol is XORed with all 64 chips of the Walsh code.

17

Orthogonal Sequences –Channelization Using Orthogonal Spreading

18

Orthogonal Sequences –Channelization Using Orthogonal Spreading

− Example of Channelization Using Orthogonal Spreading − By spreading, each symbol is XORed with all the chips in the orthogonal sequence (Walsh sequence) assigned to the user. − The resulting sequence is processed and is then transmitted over the Physical Channel along with other spread symbols. − In this figure, a 4-digit code is used. The product of the user symbols and the spreading code is a sequence of digits that must be transmitted at 4 times the rate of the original encoded binary signal. 19

Orthogonal Sequences – Recovery of Spread Symbols

20

Orthogonal Sequences – Recovery of Spread Symbols

−Recovery of Spread Symbols −The receiver despreads the chips by using the same Walsh code used at the transmitter. −Notice that under no-noise conditions, the symbols or digits are completely recovered without any error. −In reality, the channel is not noise-free, but CDMA2000 systems employ Forward Error Correction (FEC) techniques to combat the effects of noise and enhance the performance of the system. 21

Orthogonal Spreading & DeSpreading Example (With 3 Users)

22

An Example of Spreading with 3 Users

− In this example, three users, A, B, and C are assigned three orthogonal codes for spreading purposes  User A signal = 00, Spreading Code = 0101  User B signal = 10, Spreading Code = 0011  User C signal = 11, Spreading Code = 0000

− The analog signal shown on the bottom of the figure is the composite signal when all of the spread symbols are summed together.

23

24

DeSpreading Example At the Receiver of User A

−At the receiver of user A, the composite analog signal is multiplied by the Walsh code corresponding to user A and the result is then averaged over the symbol time. This process is called correlation. −Note that the average voltage value over one symbol time is equal to 1. −Therefore, the original bit transmitted by A was “0”. −You may try to decode the symbols for users B or C in the same manner. This process occurs in the CDMA mobile for recovering the signals. 25

26

CDMA Power Control

27

Power Control − Power control can substantially impact the capacity and perceived quality in cellular wireless systems. − High Speed Quality , High Capacity and Low Power Consumption are major goals in cellular radio Communication System. − Power Control is one of several Techniques used to achieve these goal. Power Control regulates the signal strength to reduce the overall interference

28

Power Control and MAI

− Interference limited multiple access system  Regardless of the mode of multiple access -- be it frequency, time or code division -- power control is necessary to combat the intercell, or co-channel, interference that arises from frequency reuse. − In systems with multiple users, strong nearby transmitters may completely block weak signals from remote transmitters − The power control problem arises due to multiple access interference (MAI)  Each user looks like random noise to other users and causes unnecessary interference to the system 29

Power Control and Near-Far Problem − Users near the base station are received with high power − Users far from the base station are received with low power − Nearby users will completely swamp far away users − Power control is implemented to overcome the near-far problem reduce MAI, and to maximize the capacity of CDMA system

30

Why power control is needed − If all mobiles transmitted at the same power level, the base station would receive unnecessarily strong signals from mobiles nearby and extremely weak signals from mobiles that are far away. This would reduce the capacity of the system. − This problem is called the near-far problem . A major

difficulty in Direct Sequence transmission is the Near-Far effect.

− In cellular CDMA systems, (adaptive) power control is needed to avoid this problem; otherwise, the link

performance will suffer from the near-far effect, a condition where the transmissions received from distant MSs experience excessive interference from nearby MSs. 31

Summary --Power Control

32

Summary --Power Control

33

Summary --Power Control

34

Summary --Power Control

35

Summary --Power Control

− The IS-95 reverse link employs a fast closedloop power control algorithm to combat variations in the received signal power due to path loss, shadowing, and fast envelope fading (at low Doppler frequencies). − A large number of power control algorithms have been suggested

36

Summary --Power Control − Reverse link power control consists of two processes:  Open loop  Closed loop

− Open loop is an initial estimate of the power the mobile needs to transmit to the BTS. Closed loop is a refinement of the open loop estimate

37

Open loop power control − Open loop is the mobile's estimate of the power at which it should transmit. The open loop estimate is based on the strength of the pilot signal the mobile receives. − As the pilot signal gets weaker or stronger, the mobile adjusts its transmission strength upwards or downwards.

38

Closed loop power control

− In closed loop, the BTS sends a command to the mobile to increase or decrease the strength at which it is transmitting. − The BTS determines this command based on the quality of the signal it receives from the mobile.

39

Summary Power Control

40

Summary Power Control

41

CDMA Frequency Reuse

42

Cell Interference • If cell A and B were on the same frequency in a conventional cellular systems, area C would have a frequency conflict and interference. • With the deployment of a FDMA network channel (frequency) reuse is required. • In the FDMA system there is a conflict when adjacent cells use the same channel (frequency).

43

FDMA/TDMA Frequency Reuse

44

45

FDMA/TDMA Frequency Reuse − Frequency Reuse of 7  To avoid conflict between cells, FDMA and TDMA systems use a reuse factor of seven (six cells surrounding each cell cannot use the same frequency).  Adjacent cells will be assigned to separate channels (frequencies).  As capacity requirements increase additional cells will be added to the network creating a reworking of the frequency plan in the network. 46

FDMA/TDMA Frequency Reuse − Cell Separation  A channel (frequency) can be used again within the network but cells using the same channel must be separated by an appropriate distance.

47

CDMA Frequency Reuse

48

CDMA Frequency Reuse

49

CDMA Frequency Reuse − CDMA Universal Frequency Reuse  CDMA has a frequency reuse of one.  Each BTS in the network uses the same frequency eliminating the need for frequency planning.

50

51

CDMA Rake Receiver

52

Multipath

−What is multipath?  Signals sent over the air can take a direct path to the mobile, bounce off objects,and arrive at the mobile’s antenna at different times.  These different paths are referred to as multi-paths.

−Effects of multipath signals  Multipath signals in a narrow band signal, such as FDMA and TDMA, may cause a loss of the signal through cancellation .  Multipaths in CDMA can be used to increase the quality of the signal.  This is possible because CDMA is a wideband signal. 53

Multipaths

54

Multipaths

55

Multipaths

56

Multipaths

57

Multipaths

58

CDMA Receiver- Rake Receiver − The rake receiver is multiple receivers in one. − There is a rake receiver at both the mobile and BTS. − Each receiver may assigned to a received signal. − The rake receiver is a CDMA feature that turns what is a problem in other technologies into an advantage for CDMA − Multi-paths can cause a loss of signal through cancellation in other technologies

59

CDMA RAKE Receiver

60

CDMA RAKE Receiver − The rake receiver identifies

the multi-path signals &

combines the multi-path signals to produce one very strong signal 61

CDMA RAKE Receiver

62

CDMA RAKE Receiver

Correlator 1 Correlator 2

Combiner

Receive set

The combined signal

Correlator 3 Calculate the time delay and signal strength

Searcher correlator

s(t)

s(t)

t

t

63

CDMA Soft Handoff

64

What is a Handoff?

− As the phone moves through a network the system controller transfers the call from one cell to another, this process is called “handoff”. − Handoffs maybe done with the assistance of the mobile or the system controller will control the process by itself.

65

Handoff

− transfers the call to a new channel belonging to new BS. − Handoff is a process, which allows users to remain in touch, even while breaking the connection with one BS and establishing connection with another BS. − To keep the conversation going, the Handoff procedure should be completed while the mobile station is in the overlap region

66

Handoff

Cell overlap region

G

Old BS

New BS

67

What is a Handoff?

68

Handoff − Handoff detection strategies −Mobile-Controlled handoff (MCHO) −Network-Controlled handoff (NCHO) • Mobile-Assisted handoff (MAHO)

69

Mobile-Controlled Handoff (MCHO) − In this strategy, the MS continuously monitors the radio signal strength and quality of the surrounding BSs. − When predefined criteria are met, then the MS checks for the best candidate BS for an available traffic channel and requests the handoff to occur. − MCHO is used in DECT

70

Network-Controlled Handoff (NCHO) −

In this strategy, the surrounding BSs, the MSC or both monitor the radio signal.

−

When the signal’s strength and quality deteriorate below a predefined threshold, the network arranges for a handoff to another channel.

−

NCHO is used in CT-2 Plus and AMPS.

71

Mobile-Assisted Handoff (MAHO) −

It is a variant of NCHO strategy.

−

In this strategy, the system directs the MS to measure the signal from the surrounding BSs and to report those measurements back to the network.

−

The network then uses these measurements to determine where a handoff is required.

−

MAHO is used in GSM and IS-95 CDMA.

72

Handoff − Break-Before-Make  In a “hard” handoff, the mobile must disconnect (or break) its connection before connecting to the new cell.  As the mobile moves from one coverage area to another, the mobile will be instructed to change to the new network.

− Advantages of Hard Handoff  Continue the call beyond the current network.  Provide expanded service.  Reduce dropped calls. 73

Handoff − Make-Before-Break  A new connection can be made prior to breaking the old connection. This is possible because CDMA cells use the same frequency and the mobile uses a rake receiver.  In a CDMA system, while a call is in progress, the mobile assists the network in making a new connection before breaking the old connection.  As the mobile moves from one coverage area to another, the mobile detects a new pilot and the base station establishes a new connection for the mobile.  A communications link is established with new BTS’ while the old link is maintained. 74

Soft Handoff

75

Soft Handoff

−CDMA uses the mobile to assist the network in the handoff. Soft Handoff is Mobile Assisted` −Soft handoffs occur between cells, sectors in a cell, or combination of cells and sectors. −Different sectors are allowed to use different Walsh sequences when in soft handoff. 76

Soft Handoff

77

Soft Handoff

78

Soft Handoff Advantages

79

Handoffs between CDMA to Analog systems—Hard Handoffs

− Hard handoffs occur between CDMA to Analog systems. − A hard handoff entails a brief disconnection from a current serving cell prior to establishing a connection with the target cell during the handoff. − Hard handoffs can occur for several reasons. Hard handoff occurs when a soft handoff cannot take place (either due to lack of resources or due to the inability to transmit identical frames from both cells). − Hard handoffs can also occur between CDMA cells. CDMA-toCDMA hard handoffs are due to frequency mismatches etc. 80

Codes in CDMA

81

Codes in CDMA

CDMA Codes Orthogonal Codes Walsh Codes Codes Walsh

Pseudo-noise (PN) Codes Long PN Code

Short PN PN Codes Codes Short

82

Walsh Codes

83

Code Basics – XOR Function − XOR Function − The figure depicts a two-input XOR gate and its corresponding truth table. A and B denote the inputs, while Y denotes its output. The XOR operation (or function) is simply defined by the equation:

− The XOR gate produces a one when the two inputs are at opposite levels. − When the total number of ones at the inputs is odd, the result of XORing them is “1”. − This operation is also needed for the upcoming discussion of codes. 84

Walsh Codes

Two codes are orthogonal if the process of “XORing” them results in an equal number of 1’s and 0’s 85

Walsh Codes

XORing

Equal No. of 0s & 1s

86

Walsh Codes Generation

87

Walsh Codes Generation

0

0 0

0 1

0 0

0 1

0 0

0 1

0 0

0 1

1 1

1 0

88

Walsh Codes Generation

89

Walsh Codes Matrix Generation

• Walsh codes are easily generated by starting with a seed of 0 H1 = 0

H 2N =

0

0

0

1

0 0 H4 = 0

0 1 0

0 0 1

0 1 1

0

1

1

0

H2 =

HN HN HN HN 90

Walsh Codes Generation

91

Frequency Spreading Code 4 Spreading Code 3 Spreading Code 2 Spreading Code 1

CDMA

Time

92

Walsh Codes in CDMA2000 1x RC1 & RC2 IS-95A (cdmaone)

93

94

Walsh Codes

95

Important Note -----CDMA2000 Radio Configurations

96

Orthogonal Sequences –Walsh Usage

− RC1 and RC2 use Walsh 64. − RC3 through RC9 use variable length Walsh functions.  1x typically uses 64 and 128 length.

− Length is a function of data rate. − For 1x the Walsh chip rate is always 1.2288 Mcps. 97

Orthogonal Sequences –Walsh Usage − Walsh Usage  In RC1 and RC2 ,only Walsh 64 is used.  RC3 through RC9 use variable length Walsh functions to handle different data rates. For RC3, voice calls use Walsh 64, while for RC4 voice calls use Walsh 128.  The higher the data rate, the shorter the Walsh function used.

98

PN Codes

99

PN Codes

PN Codes

Long PN Codes

Short PN Codes

100

Short PN Code

101

Short PN Code

102

PNc PNb

PNa

103

Short PN Code

104

Short PN Code Offsets

215 / 64 = 32768 / 64 = 512

105

Short PN Code Offsets

106

Short PN Code Offsets

107

PN Planning Analysis – Example PN Offset Reuse

37 * 3 = 111 offsets used in a cluster of 37 cells 17 offsets are available for growth

108

Long PN Code − PN sequences have an important property: timeshifted versions of the same PN sequence have very little correlation with each other − The channelization of users in the Reverse link is accomplished by assigning them different time shifted versions of the long code, thus making them uncorrelated with each other. − This property is then exploited to separate subscriber’s signals in the BTS receivers. 109

Long PN Code

242 -1 = 4400 Billion Chips

110

Long PN Code Offsets Reverse Link

111

What We Learned

112

PN Code Generation & Offsets

113

PN Code Generation

114

PN Code Generation

115

PN Code Generation

116

PN Code Generation

2N-1 In this example, the number of distinct states in the shift registers is 23-1=7

117

PN Code Offsets (Masking) − Sequence Produced by a Masked Generator

− Masking provides the shift in time for PN codes. Different masks correspond to different time shifts.

− A mask produces the same original sequence shifted in time. − Masking is used to produce offsets in both the short codes and the long code. − The offsets of the short PN codes are used to uniquely identify the Forward Channels of individual sectors or cells. − The offsets of the Long PN code are used to separate code channels in the reverse direction.

118

Walsh Code Function in Forward Link Pilot FW Traffic (for user #1)

Sync

FW Traffic (for user #2)

Paging FW Traffic (for user #3)

 A Mobile Station receives a Forward Channel from a sector in a Base Station.  The Forward Channel carries a composite signal of up to 64 forward code channels.  Some code channels are traffic channels and others are overhead channels.  W0= Pilot, W1=Paging, W32= Synchronous, all other for traffic.

119

119

Short PN Function in Forward Link

Up to 64 Code Channels

Up to 64 Code Channels

A

B

 A mobile Station is surrounded by Base Stations, all of them transmitting on the same CDMA Frequency.  Each Sector in each Base Station is transmitting a Forward Traffic Channel containing up to 64 forward code channels.  A Mobile Station must be able to discriminate between different Sectors of different Base Stations.  Short PN Codes are defined for the purpose of identifying sectors of different base stations.  These Short PN Sequences can be used in 512 different ways in a CDMA system. Each one of them constitutes a mathematical code which can be used to identify a particular sector. 120

120

Long PN Function in Reverse Link

RV Traffic from M.S. #1837732008

System Access Attempt by M.S. #2000071301 (on access channel #1)

UUUUUU

RV Traffic from M.S. #1997061104

RV Traffic from M.S. #1994011508

 The CDMA system must be able to identify each Mobile Station that may attempt to communicate with a Base Station. − A very large number of Mobile Stations will be in the market. − One binary digit sequence called the Long PN Sequence (or Long PN Code) is defined for the purpose of uniquely identifying each possible reverse code channel. − This sequence is extremely long and can be used in trillions of different ways. Each one of them constitutes a mathematical code which can be used to identify a particular user (and is then called a User Long Code) or a particular “user Reverse Traffic channel”.

121

121

Technology choice for WLL

122

Technology choice for WLL

123

WLL Technology Selection Criteria

124

GSM as WLL − GSM has cheaper CPE − GSM is most widely deployed MOBILE SYSTEM − Low spectral efficiency − Low data capability: GSM=9.6kbps, GPRS= 54kbps − The migration path of GSM towards 3G ends up at WCDMA − GSM not available in 450MHz 125

CDMA (CDMA2000 1x) as WLL − Most widely deployed WLL solution in the world − High spectral efficiency to handle Wireline like traffic − Data capability inherent in system (up to 144kbps) − Backward and forward compatibility − Available in 450, 800 and 1900MHz

126

127

CDMA2000 Radio Configurations

128

CDMA Channel or CDMA Carrier or CDMA Frequency − Duplex channel made of two 1.25 MHz-wide bands of electromagnetic spectrum, one for Base Station to Mobile Station communication (called the FORWARD LINK or the DOWNLINK) and another for Mobile Station to Base Station communication (called the REVERSE LINK or the UPLINK) − In 800 Cellular these two simplex 1.25 MHz bands are 45 MHz apart − In 1900 MHz they are 80 MHz apart − CDMA Forward Channel  1.25 MHz Forward Link − CDMA Reverse Channel  1.25 MHz Reverse Link

CDMA CHANNEL CDMA Reverse Channel 1.25

CDMA Forward Channel 1.25 MHz

MHz

45 or 80 MHz

129

CDMA 2000

130

CDMA 2000 Platforms

CDMA2000-1x (1xRTT)

CDMA20001xEV-DO

CDMA20001xEV-DV

CDMA2000-3X (3xRTT)

131

CDMA 2000 1x (1x RTT)

132

CDMA 2000 1xEV-DO

133

CDMA 2000 1xEV-DO

134

CDMA 2000 1xEV-DV

135

CDMA 2000 3xRTT

136

CDMA2000 Radio Configurations

137

Rate Sets − A Rate Set is a set of Traffic Channel frame formats. − A Rate Set may carry voice, user data, or signaling. − Two Rate Sets are defined for use in cdma One systems. All services provided over the air interface must conform to one of these two rate sets:  Rate Set 1 — supports a maximum of 8550 bps, with an additional 1050 bits of overhead for a total max rate of 9600 bps.  Rate Set 2 — supports a maximum of 13,300 bps, with additional overhead bringing the total transmission rate to 14,400 bps maximum. 138

Radio Configurations- Forward Link

− Orthogonal Transmit Diversity splits transmitted symbols into two streams with each stream being transmitted on an antenna 139

Radio Configurations- Forward Link

140

Radio Configurations- Reverse Link

141

Spreading Rate (SR1) & Spreading Rate ( SR3)

142

Spreading Rate (SR1) And Spreading Rate ( SR3)

143

Spreading Rate (SR1) also called 1x

144

Spreading Rate (SR3) also called 3x or MC ( Multi-Carrier)

145