Discriminating Among Forward
Code Channels
Pilot
Sync
FW Traffic
(for user #1)
FW Traffic
(for user #2)
Paging
FW Traffic
(for user #3)
2
 Forward IS-95B Channel Structure
 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
W0
All 0's
I PN
1.2288
Mcps
Q PN
3
 Forward IS-95B Channel Structure
 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 ID
- System Time
- Network ID
- Long PN Code
- Paging Channel Data Rate
- Tx at 1200 bps
W32
1200 bps
Convolutional
Encoder and
Repetition
Block
Interleaver
19.2 kbps
I PN
1.2288
Mcps
Q PN
4
 Forward IS-95B Channel Structure
 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
I PN
W1
R = 1/2
9600 bps
4800 bps
Convolutional
Encoder and
Repetition
Block
Interleaver
Paging Channel
Address Mask
Long
PN Code
Generator
1.2288
Mcps
19.2
ksps
1.2288
Mcps
Decimator
19.2
ksps
Q PN
5
 Forward IS-95B Channel Structure
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
6
 Reverse IS-95B Channel Structure
Access Channel
- Used by Mobiles not yet in a call to transmit :
- Registration Requests
- Page Responses
- Call Setup Requests
- 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
I PN
R = 1/3
4800 bps
Convolutional
Encoder and
Repetition
28.8
kbps
Block
Interleaver
Access Channel
Address Mask
28.8
kbps
Walsh
Cover
Long
PN Code
Generator
307.2 Kbps
1.2288
Mcps
1.2288
Mcps
Q PN
7
 Reverse IS-95B Channel Structure
 Traffic Channel
- 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
8
9
10
11
The Long PN Sequence
Long Code Register
(@ 1.2288 MCPS)
AND
1 1 0 0 0 1 1 0 0 0
P E RMU T E D
=
S UM
E SN
Public Long Code Mask
(STATIC)
User Long Code
Sequence
(@1.2288 MCPS)
Modulo-2 Addition
•
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.
12
IS-95 CDMA Channels
13
Discriminating Among Forward
Code Channels
Pilot
Sync
FW Traffic
(for user #1)
FW Traffic
(for user #2)
Paging
FW Traffic
(for user #3)
14
 Forward IS-95B Channel Structure
 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
W0
All 0's
I PN
1.2288
Mcps
Q PN
15
 Forward IS-95B Channel Structure
 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 ID
- System Time
- Network ID
- Long PN Code
- Paging Channel Data Rate
- Tx at 1200 bps
W32
1200 bps
Convolutional
Encoder and
Repetition
Block
Interleaver
19.2 kbps
I PN
1.2288
Mcps
Q PN
16
 Forward IS-95B Channel Structure
 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
I PN
W1
R = 1/2
9600 bps
4800 bps
Convolutional
Encoder and
Repetition
Block
Interleaver
Paging Channel
Address Mask
Long
PN Code
Generator
1.2288
Mcps
19.2
ksps
1.2288
Mcps
Decimator
19.2
ksps
Q PN
17
 Forward IS-95B Channel Structure
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
18
 Reverse IS-95B Channel Structure
Access Channel
- Used by Mobiles not yet in a call to transmit :
- Registration Requests
- Page Responses
- Call Setup Requests
- 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
I PN
R = 1/3
4800 bps
Convolutional
Encoder and
Repetition
28.8
kbps
Block
Interleaver
Access Channel
Address Mask
28.8
kbps
Walsh
Cover
Long
PN Code
Generator
307.2 Kbps
1.2288
Mcps
1.2288
Mcps
Q PN
19
 Reverse IS-95B Channel Structure
 Traffic Channel
- 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
20
21
22
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
23
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.
24
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.
25
Search Windows
•
Search window is centered on the earliest arriving
usable signal (direct path).
•
We have three search windows
1. SRCH_WIN_A: size for active and candidate set
2. SRCH_WIN_N: size for neighboring sets.
3. SRCH_WIN_R: size for remaining sets.
26
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.
27
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.
28
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
29
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.
30
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.
31
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.
32
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.
33
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 scheme
o 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
34
Soft Handoff
Inter-sector or softer
Inter-cell or Soft
Soft-softer
35
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 .
36
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.
37
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.
38
The following example shows a soft
hand off process of a MS between
two cells A and B.
39
40
Hand off Contd…
1.
Served by ‘A’ only.
1.
2.
3.
2.
Active set contains only pilot A.
The mobile measures pilot B Ec / I0 and finds it to be greater than
T_ADD.
The mobile sends a pilot strength measurement message and moves
pilot B from the neighbor set to the candidate set.
The mobile receives a handoff direction message from cell
A.
1.
2.
The message directs the mobile to start communicating on a new
traffic channel with cell B
The message contains the PN offset of cell B and the Walsh code of
the newly assigned traffic channel.
41
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.
42
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.
43
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
44
45
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.
46
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.
47
48
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.
49
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)
50
51
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.
52
Power Control Types
• Reverse Power Control
– Open-loop Power Control
– Closed-loop Power Control
• Forward Power Control
53
Reverse Open-loop Power Control
Reverse Open Loop
Power Control
Mobile
BTS
 Reverse open loop power is mobile station
controlling its transmit power .
54
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…..
55
Reverse open loop power control
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
56
Reverse Closed Loop Power Control
Mobile
BTS
or
Reverse Closed Loop
Power Control
Signal Strength
Measurement
Set point
 It take cares of path losses in both forward and
reverse directions
57
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.
58
Forward Power Control in IS-95
Mobile
BTS
BSC
Adjust Fwd.
power
FER
Forward Link Power Control
The base station continually and slowly decreases power
to each mobile station(each user’s forward traffic channel)
59
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.
60
Summary of Power Control
Reverse Open Loop
Power Control
Mobile
BSC
BTS
or
Reverse Closed Loop
Power Control
FER
Signal Strength
Measurement
Setpoint
Adjust Fwd.
power
FER
Forward Link 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
61
THANK YOU
62