cdma technology overviewfebruary, 2001 - page 4-1 cdma technology overview lesson 5 – power...
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CDMA Technology Overview February, 2001 - Page 4-1
CDMA Technology CDMA Technology OverviewOverview
Lesson 5 – Power Control, Registration, and Handoffs
CDMA Technology Overview February, 2001 - Page 4-2
Power Control
CDMA Technology Overview February, 2001 - Page 4-3
CDMA is an interference-limited system based on the number of users
Unlike AMPS/TDMA, CDMA has a soft capacity limit
Each user is a noise source on the shared channel
The noise contributed by users is cumulative
This creates a practical limit to how many users a system will sustain
Precise power control of the mobile stations is critical if we want to
Maximize system capacity
Increase battery life of the mobile stations
The goal is to keep each mobile station at the absolute minimum power level necessary to ensure acceptable service quality
Ideally the power received at the base station from each mobile station should be the same (minimum signal to interference)
Mobile stations which transmit excessive power increase interference to other mobile stations
CDMA Power Control
CDMA Technology Overview February, 2001 - Page 4-4
Reverse Open Loop Power Control
• The mobile station makes a coarse initial estimation of the required transmitpower, based upon the total received power.
• Problems with Reverse Open Loop Power Control:
• Assumes same exact path loss in both directions; therefore, cannotaccount for asymmetrical path loss
• Estimates are based on total power received; therefore the power receivedfrom other cell sites by mobile station introduces inaccuracies
• The mobile station makes a coarse initial estimation of the required transmitpower, based upon the total received power.
• Problems with Reverse Open Loop Power Control:
• Assumes same exact path loss in both directions; therefore, cannotaccount for asymmetrical path loss
• Estimates are based on total power received; therefore the power receivedfrom other cell sites by mobile station introduces inaccuracies
BTSMobile
Reverse Open LoopPower Control
BTS
BTS
CDMA Technology Overview February, 2001 - Page 4-5
Estimated Reverse Open LoopOutput Power
mean output power (dBm) = - mean power input (dBm)+ K+ NOM_PWR - 16 x NOM_PWR_EXT+ INIT_PWR
Power output level for the initial probe during open loop probing on the Access Channel (with closed loop correction inactive):
Subsequent probes in the sequence are sent at increased power levels(each probe is incremented by a value equal to the parameter PWR_STEP)
The “turn around constant” K is calculated assuminga nominal cell Effective Radiation Power (ERP) of 5 W
and a nominal cell loading of 50%.
Its value is -73 for cellular systems and -76 for PCS systems
CDMA Technology Overview February, 2001 - Page 4-6
Estimated Reverse Open LoopOutput Power
Power output level for the initial transmission on the Reverse Traffic Channel:Power output level for the initial transmission on the Reverse Traffic Channel:
mean output power (dBm) = - mean power input (dBm)+ K+ NOM_PWR - 16 x NOM_PWR_EXT+ INIT_PWR+ the sum of all access probe corrections (dB)
CDMA Technology Overview February, 2001 - Page 4-7
Reverse Closed Loop Power Control
• Compensates for asymmetries between the forward and reverse paths
• Consists of power up (0) and power down (1) commands sent to the mobilestations, based upon their signal strength, measured at the Base Station andcompared to a specified threshold (setpoint)
• Each command requests a 1dB increment or decrement of the mobilestation transmit power
• Transmitted 800 times per second, always at full power
• Allows to compensate for the effects of fast fading
Mobile BTS
Signal StrengthMeasurement
Setpoint
or
Reverse Closed LoopPower Control
CDMA Technology Overview February, 2001 - Page 4-8
Power Output Estimations (Summary)
The mean input power is defined as the power received at the mobile station for the 1.25 MHz RF channel bandwidth being used
The access parameter values can vary between base stations
They accommodate differing forward power levels and antenna gains
They are specified in the Access Parameters Message, sent over the Paging channel
Open Loop Probing on the Access Channel
Reverse Traffic ChannelInitial transmission
After First Power ControlBit Has Been Received
Based on: 1. mean input power measured at mobile station2. access parameters provided by the base station
(Subsequent probes are adjusted by access parameters provided by the base station)
Based on:1. mean input power measured at mobile station2. access parameters provided by the base station3. sum of all the access probe adjustments
Based on:1. mean input power measured at mobile station2. access parameters provided by the base station3. sum of all the access probe adjustments4. sum of all the power control bits received
CDMA Technology Overview February, 2001 - Page 4-9
Reverse Outer Loop Power Control
• Not part of IS-95A or J-STD-008.
• Most gradual form of reverse link error control
• Setpoint is varied according to the FER on the Reverse TrafficChannel (determined at the Base Station Controller)
• Sampled at a rate of 50 frames per second (20 ms / frame)
• Setpoint adjusted every 1-2 seconds
FER
Mobile BTS BSC
Reverse Outer Loop Power
Control
Signal StrengthMeasurement
Setpoint
or
Reverse Closed LoopPower Control
CDMA Technology Overview February, 2001 - Page 4-10
Forward Traffic Channel Power Control
• The base station slowly decreases power to each mobile station
• As the FER (determined at the mobile station) increases, the mobilestation requests a Forward Traffic Channel power increase
FER
Mobile BTS BSC
Adjust Fwd.power
Forward Link Power Control
CDMA Technology Overview February, 2001 - Page 4-11
Summary of All Power Control Mechanisms
• All types of power control work together to minimizes powerconsumption at the mobile stations, and increases the overall capacityof the system transmit power
FER FER
Mobile BTS BSC
Signal StrengthMeasurement
Setpoint
or
Adjust Fwd.power
Reverse Outer Loop Power
Control
Reverse Closed LoopPower Control
Forward Link Power Control
Reverse Open LoopPower Control
CDMA Technology Overview February, 2001 - Page 4-12
Registration
CDMA Technology Overview February, 2001 - Page 4-13
Roaming
A mobile station may be in any of the following roaming states: Home: mobile station is at its home location (not roaming) NID roaming: mobile station is within a foreign NID but in the home SID SID roaming: mobile station is within a foreign SID
A mobile station maintains a list of one or more “home pairs” These are SID/NID combinations defining the mobile station’s home
location They are stored in semi-permanent memory
The identity of current SID/NID is contained in the System Parameters Message (sent on the Paging Channel)
HomeSID/NID List
(2, 3)(2, 0)(3, 1)
SID = 2SID = 4
SID Roaming
NID = 7 NID =3NID = 0
NID Roaming Not RoamingRoaming Status
CDMA Technology Overview February, 2001 - Page 4-14
HLR & VLR
Contains permanent subscriber data provisioning information service information features available to the
subscriber Contains dynamic information
mobile station’s current location Supports call routing Queried by the MTX when
subscriber information is needed, regardless of the mobile station’s current location
Stores a subset of the HLR information pertaining to the mobile stations currently registered in the VLR’s service area
HLR VLR
CDMA Technology Overview February, 2001 - Page 4-15
CDMA Registration
Registration is the means by which a mobile station notifies the cellular system of its location, status, identification, and other characteristics
Balance is required between paging and registration
Infrequent registration results in a high rate of paging
Frequent registration places a high load on access channels
Proper system design allows a base station to efficiently page the mobile station when establishing a mobile-terminated call
Registration also provides
The mobile station’s SLOT_CYCLE_INDEX and SLOTTED_MODE
The mobile station class mark and protocol revision number so that the base station will know the mobile station’s capabilities
Two types of mobile registration
Non-Autonomous: explicitly requested by the base station, or implied based on other types of messages received by the mobile station
Autonomous: triggered by some event other than the reception of an explicit or implicit request from the base station
CDMA Technology Overview February, 2001 - Page 4-16
Forms of CDMA Registration
Power-up registration
Power-down registration
Timer-based registration
Zone-based registration
Distance-based registration Parameter-change registration
Implicit registration
Ordered registration
Traffic channel registration
AutonomousRegistration
AutonomousRegistration
Registration Types NOTSupported by Nortel
Registration Types NOTSupported by Nortel
Non-AutonomousRegistration
Non-AutonomousRegistration
All types of registration can be enabled or disabled by means of the System Parameters Message
CDMA Technology Overview February, 2001 - Page 4-17
Power-Up Registration
Mobile station registers when Directed to power-on by the user Switched to an alternate serving system Switched from using an analog system
Delays 20 seconds Preventing multiple registrations whenever power is quickly
turned on and off
ON
Access Channel
CDMA Technology Overview February, 2001 - Page 4-18
Power-Down Registration
Mobile station registers when directed to power-down by the user Mobile station will not power down until attempt is completed Mobile station will not do power down registration if
Not registered in the current system Prevents unnecessary attempts to reach a user
Can be unreliable (v.gr., user powers down in garage)
OFF
Access Channel
CDMA Technology Overview February, 2001 - Page 4-19
Timer-Based Registration
Mobile station registers when a timer expires Registration period is determined by the base station Allows system to de-register mobile stations that fail to register on
power-down
Access Channel
CDMA Technology Overview February, 2001 - Page 4-20
Distance-Based Registration
Mobile Station MS registers whenever it does an “Idle Handoff” (handoff when not in a call) into a cell which lays outside a circle with REG_DIST radius and centered at the base station where MS last registered At position “a” MS registers with Base Station BS-1. BS-1 transmits its latitude and
longitude, and the REG_DIST parameter on its paging channel At position “b” MS does an idle handoff into BS-2 and reads the latitude and longitude
of this base station. MS then calculates the distance between BS-2 and BS-1, and if the result is less than REG_DIST it does not have to re-register
At position “c” MS is still listening to BS-2 (no need to re-register yet) At position “d” MS does an idle handoff into BS-3. MS reads the latitude and
longitude of BS-3 and calculates the distance between BS-3 and BS-1. As this distance exceeds REG_DIST, MS re-registers
a
b
cd
BS-1
BS-2
BS-3
REG_DIST Paging Channel Access Channel
Idle Handoff
Idle Handoff
CDMA Technology Overview February, 2001 - Page 4-21
Zone-Based Registration
The mobile station registers when it enters a new zone A zone is a subset of the base stations within a network The mobile station keeps a list of the zones where it has registered, up to a
maximum determined by the base station Each zone is uniquely identified by the registration zone number parameter
(REG_ZONE) plus the SID and the NID to which it belongs The mobile station activates a timer for every zone where it has registered,
except the active one, and de-registers when the timer expires The mobile station will not re-register if it enters a zone which is already in its list
1 2
3
4 5
NOTE: These are registration zones, not TMSI zones!
CDMA Technology Overview February, 2001 - Page 4-22
Parameter-Change Registration
The mobile station registers after it modifies any of the following parameters (stored in the mobile station): the preferred slot cycle index the slotted mode indicator the call termination enabled indicators
or the following capabilities supported by the mobile station: the band classes the power classes the rates the operating modes
Access Channel
SLOT_CYCLE_INDEXSLOTTED_MODE
MOB_TERM_HOMEetc.
SLOT_CYCLE_INDEXSLOTTED_MODE
MOB_TERM_HOMEetc.
CDMA Technology Overview February, 2001 - Page 4-23
Implicit Registration
Occurs when the mobile station and base station exchange messages not directly related to registration Messaging conveys sufficient information to identify mobile
station and its location Considered successful whenever mobile station sends an
Origination Message or Page Response Message Compatible with AMPS and IS-54 methods Effectiveness considered adequate to preclude use of ordered
registration
Access Channel
Origination Message
CDMA Technology Overview February, 2001 - Page 4-24
Ordered and Traffic Channel Registration
Ordered Registration Allows the base station to order a mobile station to register
mobile station can be idle or in an active call Traffic Channel Registration
Allows the base station to obtain registration information about a mobile station that has been assigned to a Traffic Channel
Information exchange occurs on the Traffic Channel Suggested use is on inter-system handoffs
Neither one is supported by Nortel’s CDMA system
Paging Channel Access Channel Forward Traffic Channel Reverse Traffic Channel
Registration Message
Traffic Channel Registration
Registration Request Order
Registration Request Order
CDMA Technology Overview February, 2001 - Page 4-25
Handoffs
CDMA Technology Overview February, 2001 - Page 4-26
What is Ec/Io?
Ec/Io
Measures the “strength” of the pilot
Foretells the readability of the associated traffic channels
Guides soft handoff decisions
Is digitally derived as the ratio of good to total energy seen by the search correlator at the desired PN offset
Never appears higher than Pilot’s percentage of serving cell’s transmitted energy
Can be degraded by strong RF from other cells, sectors
Can be degraded by noise
Ec/Io dB
-25 -15 -10 0
Ec
Io
Energy of desired pilot alone
Total energy received
CDMA Technology Overview February, 2001 - Page 4-27
What’s In a Handset?
ReceiverRF SectionIF, Detector
TransmitterRF Section
Vocoder
Digital Rake Receiver
Traffic CorrelatorPN xxx Walsh xx
Traffic CorrelatorPN xxx Walsh xx
Traffic CorrelatorPN xxx Walsh xx
Pilot SearcherPN xxx Walsh 0
ViterbiDecoder
CPUDuplexer
TransmitterDigital Section
Long Code Gen.
Op
en L
oo
p Transmit Gain Adjust
Messages
Messages
Audio
Audio
Bit Packets
Symbols
SymbolsChips
RF
RF
AGC
Bit Packets
CDMA Technology Overview February, 2001 - Page 4-28
CDMA Handoffs
Handoff is the process by which a mobile station maintains communications with the Mobile Services Switching Center (MSC/BSC), when traveling from the coverage area of one base station to that of another
While in theIdle State Idle Handoff
CDMA-to-CDMA Hard Handoff
Softer HandoffDuringa Call
Soft Handoff
CDMA-to-Analog Hard Handoff
CDMA Technology Overview February, 2001 - Page 4-29
CDMA Soft Handoff Mechanics
CDMA soft handoff is driven by the handset Handset continuously checks available pilots Handset tells system pilots it currently sees System assigns sectors (up to 6 max.), tells handset Handset assigns its fingers accordingly All messages sent by dim-and-burst, no muting!
Each end of the link chooses what works best, on a frame-by-frame basis! Users are totally unaware of handoff
Handset Rake Receiver
RFPN Walsh
PN Walsh
PN Walsh
SearcherPN W=0
Voice,Data,
Messages
Pilot Ec/Io
BTS
BSCMTX
BTS
Sel.
CDMA Technology Overview February, 2001 - Page 4-30
Softer Handoff
Each BTS sector has unique PN offset & pilot Handset will ask for whatever pilots it wants If multiple sectors of one BTS simultaneously serve a
handset, this is called Softer Handoff Handset is unaware, but softer handoff occurs in BTS
in a single channel element Handset can even use combination soft-softer handoff
on multiple BTS & sectors
Handset Rake Receiver
RF
PN Walsh
PN Walsh
PN Walsh
SearcherPN W=0
Voice,Data,
Messages
Pilot Ec/Io
BTS
BSCMTX
Sel.
CDMA Technology Overview February, 2001 - Page 4-31
Overall Handoff Perspective
Soft & Softer Handoffs are the best but a handset can receive BTS/sectors
simultaneously only on one frequency all involved BTS/sectors must connect to a single
BSC (the BSC must choose packets each frame) must be same on all BTS/sectors
If above not possible, handoff still can occur but will be “hard” like AMPS/TDMA/GSM intersystem handoff: hard change-of-frequency handoff: hard CDMA-to-AMPS handoff: hard, no handback
auxiliary trigger mechanisms available
CDMA Technology Overview February, 2001 - Page 4-32
CDMA-to-CDMA Hard Handoff
Between cells operating on different frequencies
Between cells with traffic channels whose frames are staggered differently
A(ƒ1)
MTX
BSC
PSTN
B(ƒ2)
A(1)
MTX
BSC
PSTN
B(2)
CDMA Technology Overview February, 2001 - Page 4-33
CDMA-to-CDMA Hard Handoff
Between cells that could be on the same frequency and have the same frame alignment, but which are subordinated to different BSCs which are not interconnected. This type of hard handoff would become a soft handoff if the frames received
at both cells could be delivered quickly to the same BSC for comparison, by interconnecting both BSCs with a high-speed link (see Inter BSC Soft Handoff / Inter System Soft Handoff)
MTX
PSTN
BSC
MTX
BSC
BA
CDMA Technology Overview February, 2001 - Page 4-34
Pilot Detection Trigger – CELL_PILOT_BEACON sectors
The mobile station has no knowledge of the “beacon sector” concept. As far as the mobile station is concerned,
a beacon sector is just the same as any other standard CDMA sector.
Handoff Trigger
CELL_STANDARD CELL_PILOT_BEACON
BTS BTS
This trigger utilizes the existing soft handoff algorithm in the mobile station to facilitate the hard handoff.
•Certain pilots in the region where hard handoff is desired are identified as CELL_PILOT_BEACON in the Pilot Database of the SBS Controller
•The cell on the left serves the mobile station on frequency f1
•The cell on the right operates in frequency f2 and has a Pilot Beacon Unit that generates a pilot on frequency f1 (or this pilot is generated by a standard CDMA cell of an adjacent system/market)
•As the mobile station travels into the region in which hard handoff is desired, soft handoff processing is initiated when the mobile station reports to the network that the signal of the beacon cell is received with sufficient strength
•SBS software determines that the reported pilot corresponds to a beacon cell, and hard handoff processing commences
CDMA Technology Overview February, 2001 - Page 4-35
Hard Handoff using Beacon Pilot Sectors
ƒ1 ƒ1 ƒ2 ƒ2ƒ1
ƒ2 ƒ2
Beacon Sector on ƒ1
ƒ1 ƒ2
Beacon Sector on ƒ2
ƒ1
BEACON
BEACON
CDMA Technology Overview February, 2001 - Page 4-36
Boundary Sector Trigger(“CELL_BORDER” Sectors)
This is a two-stage trigger which indirectly utilizes the existing soft handoff algorithm in the mobile station to facilitate the hard handoff
certain pilots in the region where hard handoff is desired are identified as “CELL_BORDER” in the Pilot Database of the SBS Controller
as the mobile station travels from left to right, it enters into handoff with both sectors and eventually ceases communication with the sector on the left (“CELL_STANDARD”)
when the active set contains only sectors datafilled as “CELL_BORDER,” the first-stage trigger is met and the second-stage trigger is enabled
the SBS starts monitoring the Round Trip Delay (RTD) of the signals between the mobile station and the base station from which it derives its time reference
when the RTD exceeds a certain threshold, the second-stage trigger is met and handoff processing continues with the target selection activity
The mobile station has no knowledge of the “border sector” concept. Asfar as the mobile station is concerned, a border sector is just the same asany other standard CDMA sector.
First-stage trigger
Second-stage trigger
CELL_STANDARD CELL_BORDER
RTDBTS
CDMA Technology Overview February, 2001 - Page 4-37
Hard Handoff Using Border Sectors
ƒ1 ƒ1 ƒ2 ƒ2
ƒ1 ƒ1 ƒ2 ƒ2
Border Sector for ƒ1
Border Sector for ƒ2
CDMA Technology Overview February, 2001 - Page 4-38
CDMA-to-Analog Handoff
The mobile station is directed from a forward traffic channel to an analog voice channel
Radio link continuity is not maintained Two types of handoff:
Inter-system - occurs while the mobile station is traveling into another system that has no CDMA service
Messaging will tell the mobile station to select AMPS Currently, the mobile station cannot handoff back from
AMPS to CDMA (until the end of the call, when the mobile station reacquires the system) because the necessary signaling messages not supported)
Intra-system - occurs while the mobile station is traveling within the system
Load balancing Improve voice quality No CDMA service
CDMA Technology Overview February, 2001 - Page 4-39
Lesson Review
1. What is the purpose of power control?
To maximize system capacity by minimizing noise/interference, and to increase battery life in the mobile stations.
2. What is the ideal situation at the base station regarding power level of the received mobile station signals?
To receive exactly the same (minimum) power level from every mobile station.
3. Define CDMA registration and explain its purpose
CDMA registration is the means in which a mobile station notifies the cellular system of its location, status, identification and other characteristics. The purpose of CDMA registration is to allow the system to efficiently page the mobile station when establishing a mobile terminated call.
CDMA Technology Overview February, 2001 - Page 4-40
Lesson Review
4. Name the forms of registration, indicating whether they are autonomous or not.
Autonomous:Power-up registration,Power-down registration,Timer-based registration,Distance-based registration, andZone-based registration.
Non-autonomous:Parameter-change registration, Implicit registration,
Ordered registration and Traffic Channel registration.
5. Identify the cases of CDMA handoff that can occur when the mobile station is in the Traffic Channel State.
Soft handoff, Softer handoff, CDMA-to-CDMA Hard Handoff, and CDMA-to-Analog Hard Handoff.
6. Identify the message sent by the mobile station to report the strength of the pilots it measures
Pilot Strength The Measurement Message.
CDMA Technology Overview February, 2001 - Page 4-41