CH 4. Principle of Code Division Multiple Accessmercury.kau.ac.kr/hrpark/lecture/2012년 1학기/이동통신공학... · Korea Aerospace University 3 Mobile Communications Lab. What

1Korea Aerospace University Mobile Communications Lab.

CH 4. Principle of Code Division Multiple Access


Contents

What is CDMA? Basic Concept of Spread Spectrum Techniques Spreading Codes Used for CDMA

Walsh Codes Pseudo-random Noise (PN) Codes, etc.

Classification of CDMA Systems CDMA Features Summary: CDMA System Benefits


What is CDMA?

CDMA (Code Division Multiple Access) is a type of multiple access schemes in which each user expands the signal bandwidth in excess of minimum necessary to send the information and shares the same spectrum at the same time.

In CDMA, each user can be differentiated by its own unique spreading code.

Spreading a signal in CDMA is, in usual, accomplished by means of PN (pseudo-random noise) codes, which are generated by using a linear feedback shift register (LFSR).


Basic Concept of Spread Spectrum Techniques [4]

Definition Spread-spectrum is a radio transmission technology in which the signal

bandwidth is expanded in excess of the minimum necessary to send the information.

Primarily used for military purpose due to its intrinsic natures such as low probability of detection and robustness to jamming, etc.

Since 1990s, applied to commercial mobile communication systems called CDMA.

Types of spread-spectrum systems; direct-sequence spread spectrum (DS-SS), frequency-hopping spread spectrum (FH-SS), hybrid type spread-spectrum, and chirp spread-spectrum systems.


Basic Concept of Spread Spectrum Techniques (cont.)

Fig. 4.1 Transmitter side of a DS-SS system.

22

1


Basic Concept of Spread Spectrum Techniques (cont.)

Fig. 4.2 Receiver side of a DS-SS system.

( ) ( )1(SNR) SNR SNRo obaseband RF RF

o o o

A B A A WN SFN B N N B

= = = ⋅ = ⋅ = ⋅

22

Noise


Spreading Codes for CDMA [1],[2]

Traditional way for multiple access is to separate signals either in the time domain (TDMA) or in the frequency domain (FDMA).

However, in CDMA, different users occupy the same bandwidth at the same time but are separated from each other via some code sequences assigned to each user.

Two representative code sequences used for CDMA: Orthogonal codes Pseudo-random noise (PN) codes

Orthogonal codes are sometimes used in CDMA systems to isolate different channels in either forward or reverse link.

Orthogonality Two random processes X(t) and Y(t) are called orthogonal if they satisfy

Two periodic functions x(t) and y(t) are called orthogonal if they satisfy

Two periodic sequences x and y are called orthogonal if xTy =0.

( ) ( )0

1(0) 0, : period TxyR x t y t dt T

T= =∫

[ ]( ) ( ) ( ) 0 for all . XYR E X t Y tτ τ τ= + =


Spreading Codes: Walsh Code [1]

In the IS-95 CDMA system, the Walsh code with a dimension of 64 x 64 is used to isolate forward link channels.

The 64 x 64 Walsh code is generated by following recursive procedures.

[ ]2 1

, 0

N N

NN N

= =

H HH H

H H

2 24

2 2

0 0 0 00 1 0 10 0 1 10 1 1 0

= =

H HH

H H2

0 0

0 1

=

H

4

1 1 1 11 -1 1 -1

, 0 1, 1 11 1 -1 -11 -1 -1 1

= ⇒ ⇒ −

H


Spreading Codes: Walsh Code (cont.)

To illustrate the isolation of different channels using Walsh codes, suppose that there are three different users and that each user tries to send a separate message,

Each of the three users is assumed to be assigned a Walsh code as follows,

[ ] [ ] [ ]1 1 1 ,1 1 1 ,1 1 1 321 ++−=−++=+−+= mmm

[ ][ ][ ]1111

11111111

3

2

1

+−−+=−−++=−+−+=

www



After spreading, the composite spread-spectrum signal is given by

In order to separate out the original messages m1(t), m2(t), and m3(t) from the composite signal C(t), the receiver multiplies C(t) by the assigned Walsh code and integrate over one bit duration as follow,

Since the decision rule is given by

( ) ( ) ( ) ( ) ( ) ( ) ( )twtmtwtmtwtmtC 332211 ++=

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ){ } ( )

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ){ } ( )

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ){ } ( ) 4 4 4

4 4 4

4 4 4

0 33322110 33

0 23322110 22

0 13322110 11

−⇒++==

−⇒++==

−⇒++==

∫∫∫∫∫∫

bb

bb

bb

TT

TT

TT

dttwtwtmtwtmtwtmdttwtCM



( )( )

ˆ 1, if 0ˆ 1, if 0

i i

i i

m t M

m t M

= >

= − <



The estimated messages are as follows,

It should be noted that the orthogonality between Walsh codes is maintained only in case where there is no shift between the codes. Compare w2 and w3.

Therefore the Walsh code is mostly used in case where all channels are aligned on time, as in the forward link of IS-95.

( )( )( )

1

2

3

ˆ 1 1 1 ˆ 1 1 1ˆ 1 1 1

m t

m t

m t

= −

= −

= −


Spreading Codes: PN Code [1],[2]

The most representative spreading code used for CDMA is the PN code that may be generated by a linear feedback shift register.

Since there are exactly 2n-1 non-zero states in a linear n-stage shift register, the period of the sequence generated by a linear n-stage shift register cannot exceed 2n-1.

Fig. 4.3 An n-stage linear feedback shift register.

( ) .1or 0 ,1 11

221 =+++++= −

− inn

n cxxcxcxcxf


Spreading Codes: PN Code (cont.)

If the period of the PN sequence generated by an n-stage linear shift register is 2n-1, the PN sequence is called the maximal-length pseudo-random noise sequence or m-sequence.

The normalized autocorrelation function of a maximal-length pseudo-random noise sequence is two-valued and periodic, which is given by

where l is any integer with n denoting the number of stage of the shift register and N = 2n-1.

( )( )( )

1.0 2 1

1 2 1

n

n

k lR k

k lN

= −= − ≠ −



Fig. 4.4 A 3-stage linear feedback shift register and its output.

1x 2x 3x

( ) 321 xxxf ++=



Fig. 4.5 Autocorrelation function of 3-stage maximal-length linear feedback shift register.



PN codes can be used for multiple access. Suppose again the same three users wish to send three separate messages,

Also assume that each of the three users is assigned a PN code, respectively,

Then the composite signal is given by

In order to separate out the original messages m1(t), m2(t), and m3(t) from the composite signal C(t), the receiver multiplies C(t) by the assigned PN code and integrates it.

[ ] [ ] [ ]1 1 1 ,1 1 1 ,1 1 1 321 ++−=−++=+−+= mmm

[ ][ ][ ]

0

3

6

1 1 1 1 1 1 1

1 1 1 1 1 1 1 0 1, 1 1

1 1 1 1 1 1 1

P

P

P

= − + − − − + +

= − − + + − + − ⇒ → → −

= + − + − − − +

( ) ( ) ( ) ( ) ( ) ( ) ( )tptmtptmtptmtC o 63321 ++=



It should be noted that the cross-correlation of different PN codes is not zero, which becomes a source of other-user interference in CDMA.

The PN codes are mostly used to isolate different users in case where orthogonal codes cannot be employed as in the CDMA reverse.

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ){ } ( )

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ){ } ( )

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ){ } ( ) 7 7 9

9 7 7

7 9 7

0 66332010 63

0 36332010 22

0 06332010 01

−⇒++==

−⇒++==

−⇒++==

∫∫∫∫∫∫

bb

bb

bb

TT

TT

TT

dttPtPtmtPtmtPtmdttPtCM




Classification of CDMA Systems

CDMA systems can be classified into the synchronous CDMA system and the asynchronous CDMA system.

Synchronous CDMA System All base stations are synchronized with each other by using the time

information obtained from GPS satellites. All base stations use the same PN code with different phase shifts so that the

initial synchronization at the mobile is relatively easy. IS-95 and cdma2000 systems

Asynchronous CDMA System Unlike synchronous systems, GPS satellites are not used for synchronization

while base stations are roughly synchronized with each other by using a network clock.

Each base station should employ a different PN code ==> Initial synchronization requires a long time and a special synchronization circuit so called the matched filter should be used.

WCDMA system


CDMA Features: High Frequency Reuse Efficiency [1],[2]

The frequency reuse factor is defined for AMPS and TDMA systems as the actual bandwidth assigned to each base station divided by the total bandwidth: F = 1/7

In a CDMA system, the frequency reuse factor is redefined as the self-cell interference over total interference.

1 2 3

self-cell interference 1total interference 1

1 0.721 6 12 18

Ff

k k k

= =+

= ≈+ + + +

k k k1 2 30 06 0 002 0 0003= = =. , . , . 100 %

6 %

6 %

6 %

6 %

6 %

6 %

0.2%

0.2%

0.2%

0.2%

0.2%

0.2%

0.2%

0.2%

0.2%

0.2%

0.2%

0.2%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

0.03%

Fig. 4.6 Typical interference in hexagonal cellular patterns.

==> High frequency reuse efficiency!

other-cell interference : loading factorself-cell interference

f =


CDMA Features: Sectoring Gain [1],[2]

The interference at the base station can be reduced significantly if the cell is sectored.

Three sector antenna is typically used so that each sector receives the signals coming over only 120 degrees as shown in Fig. 4.7.

This arrangement decreases the interference by a factor of approximately 1/3, meaning that sectoring gain Ga is 3.

If the cell is sectored to six sectors, then the interference is decreased by a factor of approximately 1/6.

In actual, however, the sectoring gain becomes about 2.5 for three-sector antenna and 5 for six-sector antenna due to overlapping of antenna beam-pattern.


CDMA Features: Sectoring Gain (cont.)

Fig. 4.7 Sectored antenna pattern.


CDMA Features: Voice Activity Gain [1],[2]

The voice activity is defined as the actual talking time normalized by the user’s overall holding time in voice communications.

CDMA systems can reduce interference effectively by using the voice activity in such a way that the system does not transmit any signal while a user is not actually talking.

The voice activity is about 0.4 so that the channel capacity can be increased up to 2.5 times by simply utilizing the voice activity.


CDMA Features: Power Control [1],[2]

There are two problems in cellular environments, near-far problem and fading, which can be solved by power control.

Especially in CDMA, all users share the same spectrum so that each user looks like interference to other users.

Therefore, the power of each user must carefully be controlled so that no user is unnecessarily interfering with others.

Fig. 4.8 Near-far problem.


CDMA Features: Power Control (cont.)

There are three power control schemes in CDMA: closed-loop, open-loop, and outer-loop power controls.

We herein investigate them mainly for the reverse link.

Closed-loop power control The closed-loop power control is used to compensate for rapid signal

fluctuation due to multipath fading. As shown in Fig. 4.9, a base station measures Eb/No or received power from

a mobile station. If the measured Eb/No (or power) exceeds a certain threshold, then the base

station commands the mobile to decrease its transmit power. If the measured Eb/No (or power) is below a certain threshold, then the base

station commands the mobile to increase its transmit power. In IS-95 reverse link, the transmit power at mobile is adjusted every 1.25 ms

(power control rate: 800 Hz) with a step size of 1 dB. The power control command is transmitted by puncturing forward traffic

channel with power-up or power-down command every 1.25 ms, as shown in Fig. 4.10.



Fig. 4.9 Reverse link closed-loop and outer-loop power control.

Demod



Distance or time Distance or time

Rec.

sig

nal pow

er

Rec.

sig

nal pow

er

No power control Closed-loop power control

Distance or time

Tra

ns.

sig

nal pow

er

Closed-loop power control

Fig. 4.11 Power control effect against short-term fading.

Fig. 4.10 Puncturing the power control bit in IS-95 systems.



Open-loop power control The open-loop power control is used to compensate for a large fluctuation of

the signal due to long-term fading where there is a high correlation between the forward and reverse links.

As shown in Fig. 4.12, a mobile station continuously monitors the received power from the base station and adjusts transmit power in response to the change in received power from base station.

The term open-loop is used in that it is purely a mobile-controlled operationand does not involve the base station at all.

Outer-loop power control In CDMA systems, the quality of link is finally determined by the FER (frame

error rate) or BER (bit error rate). The outer-loop power control is the process to adjust target Eb/No (or

threshold) in response to the FER measurements at the base station, as shown in Fig. 4.9.

That is, if the measured FER at the base station is larger than a certain threshold (10-2 FER for voice for example) for a given target Eb/No (or threshold), then the base station increases the target Eb/No.

Otherwise, the base station decreases the target Eb/No.



Fig. 4.12 Reverse link open-loop power control.


CDMA Features: Soft Handoff [1],[2]

Cell A Cell B

Fig. 4.13 Handoff in cellular mobile systems.

Handoff (or handover) is the process that a mobile changes the call connection from the current base station to the target base station with better channel conditions in order to continuously communicate with the other party in cellular networks.

There are three handoff schemes used in CDMA systems; hard handoff, soft handoff, and softer handoff.


CDMA Features: Soft Handoff (cont.)

Hard handoff: A mobile station starts to communicate with a target base station after interrupting communications with a current base station (break-before-make). It occurs between CDMA systems with different frequency assignments in

current DCN (digital cellular network) and PCS systems. It also occurs between analog and CDMA systems or between analog

systems. Soft handoff: A mobile station starts to communicate with a target base

station without interrupting communications with a current base station (make-before-break). It occurs between CDMA systems with the same frequency assignments. During soft handoff, the mobile station receives the signals from both target

and current base stations and both base stations also receive the signal from the mobile station.

By using soft handoff, a significant increase of cell coverage can be obtained in addition to the dissolution of call-drop problems.

Softer handoff: A mobile station changes the sector inside a base station by the same scheme as the soft handoff (soft handoff between sectors).



Fig. 4.14 Combining of received signals at mobile station during soft handoff.



Fig. 4.15 Selection diversity during soft handoff at base station controller (BSC).


CDMA Features: Rake Receiver [1],[2]

A type of receiver consisting of parallel demodulators to utilize various diversity techniques in CDMA systems is called a rake receiver.

A three-finger rake receiver is used at Qualcomm’s MSM (mobile station modem) chip, while a four-finger rake receiver is employed at Qualcomm’s BSM (base station modem) chip.

Fig. 4.16 Rake receiver.


Summary: CDMA System Benefits [1],[2]

High Capacity: Theoretically 20 times larger than analog systems due to utilizing voice activity, various diversity techniques, and high frequency reuse efficiency, sectoring gain, etc. In actual, about 10 times larger than analog systems

High Privacy: The user private long code and data scrambling provide high degree of privacy

High Quality of Service: Robust to fading by using various diversity techniques due to wideband nature. Reduced call-drop by soft hand-off

Easy Cell Planning and System Deployment: frequency assignment unnecessary

Increased Battery Life Time: Reduced transmit power due to low required Eb/No ==> Smaller handset size


References

1. Samuel C. Yang, CDMA RF System Engineering, Artech House, 1998.

2. Qualcomm, CDMA System Training Handbook-vol. 1, 1993.

Documents

CH 4. Principle of Code Division Multiple Accessmercury.kau.ac.kr/hrpark/lecture/2012년 1학기/이동통신공학... · Korea Aerospace University 3 Mobile Communications Lab. What