Discriminating Among Forward Code Channels

2

Discriminating Among ForwardCode Channels

SyncPilot

FW Traffic(for user #1)

Paging

FW Traffic(for user #2)

FW Traffic(for user #3)

3

Pilot Channel (Walsh Code 0)- The Pilot is “structural beacon” which does not contain a character stream

- Allows Mobile to Acquire the System

- Reference Signal for System Acquiring, Timing, Coherent Modulation

- Provides Mobile with Signal Strength Comparison during handoffs

- Transmitted Constantly

- Non-Modulated Spread Spectrum Signal (Transmit Short PN Code)

- Has Unique PN Offset(512) for each Cell or Sector

- Approximately 20% of radiated BTS power is in the pilot

All 0's

W0 I PN

Q PN

1.2288Mcps

Forward IS-95B Channel Structure

4

Sync Channel (Walsh Code 32)

- Used by Mobile to Synchronize with System

- Carries a data stream of system identification and Parameter information used

by MS during system acquisition

- Pilot PN Offset - System Time - Long PN Code

- System ID - Network ID - Paging Channel Data Rate

- Tx at 1200 bps

Forward IS-95B Channel Structure

1200 bps

W32 I PN

Q PN

1.2288Mcps19.2 kbpsBlock

Interleaver

ConvolutionalEncoder andRepetition

5

Paging Channel (Walsh Code 1 up to 7)

- Used by Base Station to :

- Page Mobile - Transmit Overhead Information

- MS Control - Assign Mobile to Traffic Channel

- Provides Mobile with:

- System parameter Message - Neighbor List Message

- Access Parameter Message - CDMA Channel List Message

- Tx at 9600 or 4800 bps

Forward IS-95B Channel Structure

9600 bps4800 bps

W1 I PN

1.2288Mcps

R = 1/219.2ksps

Paging ChannelAddress Mask Decimator

1.2288Mcps

19.2ksps

Q PN

BlockInterleaver

LongPN Code

Generator

ConvolutionalEncoder andRepetition

6

Traffic Channel (any remaining Walsh codes)- Used to:- Pass voice, commands, and requests from the Base Station to the Mobile

- Tx up to 9600bps on Rate set 1 and up to 14400bps on Rate set 2

Forward IS-95B Channel Structure

7

Access Channel- Used by Mobiles not yet in a call to transmit :

- Registration Requests - Call Setup Requests

- Page Responses - Order Responses

- other Signaling information

- Be Paired to Paging Channel (Each Paging Channel can have up 32 access channels)

- Tx at 4800 bps, 20ms frame length

Reverse IS-95B Channel Structure

307.2 Kbps307.2 Kbps4800 bps

I PN

1.2288McpsConvolutional

Encoder andRepetition

R = 1/328.8kbps

Access ChannelAddress Mask

1.2288Mcps

Q PN

28.8kbps

LongPN Code

Generator

WalshCover

Block Interleaver

8

Traffic Channel

Reverse IS-95B Channel Structure

- Be used by individual users during their actual calls to transmit traffic to the

BTS

- Be really just a user-specific public or private Long Code Mask

- there are many reverse Traffic channels as there are CDMA phones in

the world

9

10

11

12

The Long PN Sequence

• Each mobile station uses a unique User Long Code Sequence generated by applying a mask, based on its 32-bit ESN and 10 bits from the ysytem, to the 42-bit Long Code Generator which was synchronized with the CDMA system during the mobile station initialization.

• Generated at 1.2288 Mcps, this sequence requires 41 days, 10 hours, 12 minutes and 19.4 seconds to complete.

• Portions of the User Long Codes generated by different mobile stations for the duration of a call are not exactly orthogonal but are sufficiently different to permit reliable decoding on the reverse link.

Long Code Register (@ 1.2288 MCPS)

Public Long Code Mask (STATIC)

User Long CodeSequence

(@1.2288 MCPS)

1 1 0 0 0 1 1 0 0 0 P E R M U T E D E S N

AND

=S U M

Modulo-2 Addition

13

IS-95 CDMA Channels

14

Discriminating Among ForwardCode Channels

SyncPilot

FW Traffic(for user #1)

Paging

FW Traffic(for user #2)

FW Traffic(for user #3)

15

Pilot Channel (Walsh Code 0)- The Pilot is “structural beacon” which does not contain a character stream

- Allows Mobile to Acquire the System

- Reference Signal for System Acquiring, Timing, Coherent Modulation

- Provides Mobile with Signal Strength Comparison during handoffs

- Transmitted Constantly

- Non-Modulated Spread Spectrum Signal (Transmit Short PN Code)

- Has Unique PN Offset(512) for each Cell or Sector

- Approximately 20% of radiated BTS power is in the pilot

All 0's

W0 I PN

Q PN

1.2288Mcps

Forward IS-95B Channel Structure

16

Sync Channel (Walsh Code 32)

- Used by Mobile to Synchronize with System

- Carries a data stream of system identification and Parameter information used

by MS during system acquisition

- Pilot PN Offset - System Time - Long PN Code

- System ID - Network ID - Paging Channel Data Rate

- Tx at 1200 bps

Forward IS-95B Channel Structure

1200 bps

W32 I PN

Q PN

1.2288Mcps19.2 kbpsBlock

Interleaver

ConvolutionalEncoder andRepetition

17

Paging Channel (Walsh Code 1 up to 7)

- Used by Base Station to :

- Page Mobile - Transmit Overhead Information

- MS Control - Assign Mobile to Traffic Channel

- Provides Mobile with:

- System parameter Message - Neighbor List Message

- Access Parameter Message - CDMA Channel List Message

- Tx at 9600 or 4800 bps

Forward IS-95B Channel Structure

9600 bps4800 bps

W1 I PN

1.2288Mcps

R = 1/219.2ksps

Paging ChannelAddress Mask Decimator

1.2288Mcps

19.2ksps

Q PN

BlockInterleaver

LongPN Code

Generator

ConvolutionalEncoder andRepetition

18

Traffic Channel (any remaining Walsh codes)- Used to:- Pass voice, commands, and requests from the Base Station to the Mobile

- Tx up to 9600bps on Rate set 1 and up to 14400bps on Rate set 2

Forward IS-95B Channel Structure

19

Access Channel- Used by Mobiles not yet in a call to transmit :

- Registration Requests - Call Setup Requests

- Page Responses - Order Responses

- other Signaling information

- Be Paired to Paging Channel (Each Paging Channel can have up 32 access channels)

- Tx at 4800 bps, 20ms frame length

Reverse IS-95B Channel Structure

307.2 Kbps307.2 Kbps4800 bps

I PN

1.2288McpsConvolutional

Encoder andRepetition

R = 1/328.8kbps

Access ChannelAddress Mask

1.2288Mcps

Q PN

28.8kbps

LongPN Code

Generator

WalshCover

Block Interleaver

20

Traffic Channel

Reverse IS-95B Channel Structure

- Be used by individual users during their actual calls to transmit traffic to the

BTS

- Be really just a user-specific public or private Long Code Mask

- there are many reverse Traffic channels as there are CDMA phones in

the world

21

22

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Pilot sets• The term pilot refers to a pilot channel identified by a

pilot sequence offset. • A pilot is associated with the forward traffic channels

in the same forward CDMA link.• Each pilot is assigned a different offset of the same

short PN code. • In a particular position of MS, it may detect many

pilot carriers from various cells. • Depending upon the received strength of these pilots

are categorized as Active set Candidate set Neighbor set Remaining set

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Pilot Sets• Active set: It contains those pilots whose paging or traffic channels are

actually being monitored or used.– If MS is in idle condition, it can have only one pilot set. The active pilot will be

the one whose Ec/Io is highest among the candidate set.– If the MS is using traffic channel (conversation), then it can have up to six

pilots.– Simply, the active set contains the currently serving pilots of Cells.

• Candidate set: This set contains the pilots that are not currently in the active set. – However, these pilots have been received with sufficient signal strength to

indicate that the associated forward traffic channels could be successfully demodulated.

– Maximum size of the candidate set is six pilots. – The candidate pilot can become an active pilot any time.

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Pilot Sets Contd….

• Neighbor set: – This set consists of pilots that are not currently in the active or the

candidate set but they are likely candidates for Handoff being in close vicinity.

– The neighbor list is sent to the mobile in the system parameter message on the paging channel.

– The maximum size of the neighbor set is 20.– The neighbor list should be updated according to the geographical

locations of cells and the corresponding H/O between these cells.

• Remaining set:– This set contains all possible pilots in the current system, excluding

pilots in the active, candidate, or neighbor sets.

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Search Windows

• Search window is centered on the earliest arriving usable signal (direct path).

• We have three search windows1. SRCH_WIN_A: size for active and candidate set2. SRCH_WIN_N: size for neighboring sets.3. SRCH_WIN_R: size for remaining sets.

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Search Windows• While searching for a pilot, the mobile is not limited to the

exact offset of the short PN code. • The short PN offsets associated with various multipath

components arrive a few chips later relative to the direct path component.

• The mobile uses the search windows to accommodate such multipath components and add them constructively.

• Search window sizes are defined in number of short PN chips. (1 chip = 244.14 meters)

• Search window defines size of sets to include pilot on the basis of distance of BTS or shifted version of a signal due to multi-path effect.

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SRCH_WIN_A

• Mobiles uses to track the active and candidate set pilots.

• It defines Handoff region.• It should be large enough to capture all usable

multi-path signal components of a base station at the same time it should be as small as possible in order to maximize searcher performance.

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SRCH_WIN_A Contd…

• Path A = 1 Km– 4.1 chips

• Path B = 4 Km– 16.4 chips

• Distance traveled between two path is(16.4 – 4.1 = 12.3 chips)

• Search window size– 12.3 χ 2 = 24.6 chips

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SRCH_WIN_A Contd…• The above fig. shows multipath situation. • The direct path (path A) travels 1 km to the mobile, while the multipath

(path B) effectively travels 4 km before reaching the mobile. • Since one chip corresponds to a propagation distance of 244.14m, the

direct path travels a distance of 4.1 chips.• And the multipath travels a distance of 16.4 chips.• Therefore, the difference in distance traveled between the two paths is

16.4chips – 4.1 chips = 12.3 chips• Note that the direct path (path A) arrives the earliest and is thus at the

center of the search window, while the multipath (path B) arrives 12.3 chips later.

• In order for the search window to simultaneously capture these two paths, the window must be at least (2 X 12.3) chips, or 24.6 chips wide.

• In general, an RF engineer must set SRCH_WIN_A according to his or her knowledge of multipath conditions within the cell.

31

SRCH_WIN_N• This is the search window that the mobile uses to monitor

the neighbor set pilots. • The size of this window is typically larger than that of

SRCH_WIN_A. • The window needs to be large enough not only to capture all

usable multipath of the serving base stations signal, but also to capture the potential multipath of neighbor’s signals.

• The maximum size of this search window is limited by the distance between two neighboring base stations ie maximum size of SRCH_WIN_N is given by distance between two neighbor cell in number of chips.

32

SRCH_WIN_R

• SRCH_WIN_R monitors all the remaining set of PNs.

• A typical requirement for the size of this window is that it is at least as large as SRCH_WIN_N.

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Pilot Search

• Different pilot signals can arrive at the mobile at different times, and a multipath component of one pilot may arrive a few chips later than its direct-path component.

• Therefore, search windows are provided to search for pilots that are in the active, candidate, neighbor, and remaining windows.

• The parameter SRCH_WIN_A defines the search-window width used to search for pilots in the active and candidate sets.

• The parameter SRCH_WIN_N defines the search-window width used to search for pilots in the neighbor sets.

• The parameter SRCH_WIN_R defines the search-window width used to search for pilots in the remaining sets.

• The mobile should center the search window for each pilot in the active and candidate sets around the earliest arriving usable multipath component of the pilot.

• For example, if SRCH_WIN_A is defined to be 40 chips, then the mobile searches 20 chips around the earliest arriving multipath component of the pilot.

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Handoff• Handoff is a process in which a mobile station changes its

serving BTS or moves to a new traffic channel.• In cellular communication, H/O is must to continue a call in

progress even though the subscriber moves from the coverage area of one cell to another and so on.

• H/o should occur as fast as possible and this operation must be successful.

• Handoff has two schemeo Hard handoff (break before make scheme)o Soft handoff (make before break scheme)

» Inter-sector or softer Handoff» Inter-cell or soft Handoff » Soft softer Handoff

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Soft Handoff

Inter-sector or softer Inter-cell or Soft

Soft-softer

36

Soft handoff Contd….• Soft handoff is also known as “make before break scheme”

handoff.• The various types of soft handoff are as follows.• Softer handoff:

– If H/O is occurring two sectors of the same BTS (Cell), then such type of H/O is called softer H/O.

– Frequently occurred in macro cell (BTS having multiple sectors).• Soft H/O:

– Soft H/O process occurring between different sectors of different cell (BTS).

• Soft/softer H/O:– The mobile communicates with two sectors of one cell and one sector

of another cell ie combination of soft and softer The mobile communicates with two sectors of one cell and one sector of another cell .

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Benefits of Soft Handoff

• Less call drop because mobile set continuously monitor multiple pilots. Hence the quality of service is increased.

• Soft handoff reduces transmission power of forward and reverse traffic channels.

• Less transmission power from mobile results in longer battery life and reduces the overall interference hence the capacity also increased.

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Handoff parameters• Pilot detection threshold (T_ADD)

– T_ADD defines a threshold value above which the pilot can be considered as active or candidate set.

– T_ADD must be large enough to quickly add useful pilots and high enough to avoid false alarm due to noise.

• Comparison threshold (T_COMP)– T_COMP also has effect similar to T_ADD.

• Pilot drop threshold (T_DROP)– T_DROP defines a boundary below which the pilot signal is considered

weak.• Drop timer threshold (T_TDROP)

– As soon as pilot falls below T_DROP, T_TDROP counter starts and after finished counting the active or candidate pilot is moved to neighbor set.

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The following example shows a soft hand off process of a MS between

two cells A and B.

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Hand off Contd…

1. Served by ‘A’ only.1. Active set contains only pilot A. 2. The mobile measures pilot B Ec / I0 and finds it to be greater than

T_ADD. 3. The mobile sends a pilot strength measurement message and moves

pilot B from the neighbor set to the candidate set.

2. The mobile receives a handoff direction message from cell A.

1. The message directs the mobile to start communicating on a new traffic channel with cell B

2. The message contains the PN offset of cell B and the Walsh code of the newly assigned traffic channel.

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Hand off Contd…3. The mobile moves pilot B from the candidate set to the active

set. Now the active set contains two pilots.4. The mobile detects that pilot A has now dropped below

T_DROP. The mobile starts the drop timer.5. The drop timer reaches T_TDROP. The mobile sends a pilot

strength measurement message.6. The mobile receives a handoff direction message. The

message contains only the PN offset of cell B. The PN offset of cell A is not included in the message.

7. The mobile moves pilot A from the active set to the neighbor set, and it sends a handoff completion message.

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CDMA Power Control

• Power control is essential to the smooth operation of a CDMA system. Because all users share the same RF band through the use of PN codes, each user looks like random noise to other users.

• The power of each individual user, therefore, must be carefully controlled so that no one user is unnecessarily interfering with others who are sharing the same band.

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CDMA Power Control

CDMA is an interference-limited system based

on the number of users, the interference comes

mainly from nearby users

Each user is a noise source on the shared

channel, this creates a practical limit to how

many users a system will handle

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46

CDMA Power Control

• The above fig shows the near far effect due to absence of power control.

• If there is no power control, both users would transmit a fixed amount of power pt.

• Because of the difference in distance, the received power from user 2, or pr2, would be much larger than the received power from user 1, or pr1.

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CDMA Power Control

• User 2 has a much higher SNR and thus enjoys great voice quality, but user 1’s SNR is barely making the required SNR.

• This inequity is known as the classic near-far problem in a spread-spectrum multiple access system.

• Moreover, absence of power control also reduces the capacity of the system as shown in the following example.

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Power control contd….• Let us suppose that the minimum SNR required to establish reliable

communication link be 1/10.• In the above example, the respective distances of users 1 and 2 are such

that the received power from user 2 is 10 times greater than that from user 1.

• ie,received power from user 1 = 1 unit.• received power from user 2 = 10 unit.• Thus, the SNR of user 2 is (10), which is very higher than the required SNR

(1/10). Hence it enjoys greater voice quality. • Where as, the SNR of user 1 is (1/10), which is the minimum required

value. Hence user 1 is barely making a communication link.• Such inequity effect is called classical near far problem.• Moreover, the capacity is also limited to only 2 users with this scenario.

Because any user that tries to come into the system either does not get enough SNR (1/10) to establish reliable link or it has to kick out the existing users.

• In any case the capacity of the system is limited.

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Power Control contd….

The goal is to keep each MS at the absolute minimum

power level necessary to ensure acceptable service

quality

MS with excessive transmit power increase

interference to other Mobile stations.

Ideally the power received at the base station from

each mobile station should be the same(minimum

signal to interference)

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Power control contd….

• In above fig. there is a perfect power control mechanism.

• Thus, the power received from each user to the BTS is same and that value is equal to the minimum value required for reliable link ie (SNR = 1/10).

• Hence, the capacity of the system has increased to 11 as shown in above fig.

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Power Control Types

• Reverse Power Control– Open-loop Power Control– Closed-loop Power Control

• Forward Power Control

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Reverse Open-loop Power Control

Reverse open loop power is mobile station controlling its transmit power .

Mobile BTS

Reverse Open LoopPower Control

55

Reverse open loop power control

Reverse open loop power control Estimates how strong the mobile station should transmit based on a

coarse measurement of how much power it is receiving from the base station.

The transmit power from the MS has inverse relationship with the receiving power from the BTS.

The Reverse open loop method of power control provides a quick response to changes in signal conditions.

Hence this type of power control has faster response has does not create burden (load) to the system.

However, the problems with reverse open loop power control are…..

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Problems with Reverse Open Loop Power Control:– Assumes same path loss in both directions

(Forward and Reverse) and doesn’t look for asymmetrical path loss

– The frequency deviation of forward and reverse link is different and hence the path losses as well.

– Estimates are based on total power received; therefore the power received from other cell sites by mobile station introduces inaccuracies

Reverse open loop power control

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Reverse Closed Loop Power Control

It take cares of path losses in both forward and reverse directions

Mobile BTS

Signal StrengthMeasurement

Set point

or

Reverse Closed LoopPower Control

58

Reverse Closed Loop Power Control

This mechanism consists of power up (0) & power down (1) commands sent from BTS to the mobile stations, based upon their signal strength measured at the Base Station and compared to a specified threshold (set point).

If the received power to the BTS is greater than the set point the power down command (1) is sent to the MS and vice-versa.

Thus, the MS is kept at the equilibrium condition ie well power controlled.

Each command requests a 1dBm increase or decrease of the mobile station transmit power

Transmitted 800 times per second by puncturing.

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Forward Power Control in IS-95

The base station continually and slowly decreases power to each mobile station(each user’s forward traffic channel)

FER

Mobile BTS BSC

Adjust Fwd.power

Forward Link Power Control

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Forward Power Control in IS-95

• As the FER (determined at the mobile station) increases, the mobile station requests a Forward Traffic Channel power increase and vice-versa.

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Summary of Power Control

All types of power control work together to minimizes power consumption at the mobile stations and BS, and increases the overall capacity of the system

FER FER

Mobile BTS BSC

Signal StrengthMeasurement

Setpoint

or

Adjust Fwd.power

Reverse Closed LoopPower Control

Forward Link Power Control

Reverse Open LoopPower Control

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