unit-6ajaybolar.weebly.com/uploads/1/0/1/0/10106930/wireless_unit_6.pdf · is-95 standard describes...

10EC81-Wireless Communication

Department of ECE, Canara Engineering College

UNIT-6

The first form of CDMA to be implemented is IS-95 , specified a dual mode of

operation in the 800Mhz cellular band for both AMPS and CDMA.

IS-95 standard describes the structure of wideband 1.25Mhz CDMA channels and the

operation necessary to provide power control, call processing, handoff for proper system

operation.

CDMA is a multiple access technology that is based on the use of wideband spread spectrum

digital technology

All signals share the same frequency spectrum simultaneously.

In FDMA the available radio spectrum is divided into narrowband channels and each user is

given a particular band.

In TDMA the allocated spectrum is divided into timeslots.



CDMA Network and System Architecture

The initial reference architecture is similar to the GSM reference architecture.



The CDMA MSC serves as the interface between PSTN and BSS. The MSC performs the

functions necessary for call establishment and provides functionality needed to permit the

subscribers mobility and roaming.

Visitor location register ( VLR) is a database stores the temporary information about the

subscriber.

Interworking function (IWF) is the gateway between wireless network and the packet data

network. It provides a direct connection for packet data calls.

Mobile positioning system (MPS )is based on GPS, It is used for emergency services. The

ability to locate the caller is known as E911 program.



Unified messaging/ Voice mail service It integrates email and voice mail access.

HLR/AC The HLR holds the subscriber information in a database that is used by the system.

It stores the ESN, subscribers service plan etc. AC provides secure database for

authentication.

Base station subsystem

BSS consists of one BSC and all the radio base stations controlled by the BSC.

Base station controller – It interfaces between the MSC and the PCN, other BSS’s

in the system.

It provides routing of data packets between the PCN and the RBS’s , radio resource

allocation, system and timing synchronization.

Radio base station – It interfaces between the BSC and the SD.



Functions include CDMA decoding and encoding of the subscriber, traffic and system

overhead channels and the CDMA radio links to and from the subscriber.

PLMN sub network

It provides mobile wireless communication services to subscribers and consists of several

functional subnetworks.

Circuit core network – CCN provides the switching function necessary to complete

calls to and from the mobile subscriber to the PSTN.

CDMA radio access network – C-RAN provides the interface between the wireless

cellular subscriber and CCN.

Packet core network – It interfaces between the C-RAN and the public data network.

It consists of three hardware nodes.

- AAA server : Authentication, authorization and accounting -It authenticates

authorizes the SD. It manages a database that contains user profile . It receives

accounting information from the PDSN node.

- Home agent : HA has the task of forwarding all packets that are destined for the

SD to the PSTN

- Packet data serving node : It provides the needed IP transport capability to

connect the C-RAN and hence the subscriber to the PDN



Network management system

Basically five functions are performed

Fault management – It is concerned with the detection, isolation and repair of

network problems to prevent network faults.

Performance management – It is concerned with gathering and reporting of relevant

network performance statistics that can be used to continuously analyze network

operation.

Trouble management – These functions allow for the display and subsequent

descriptions of occurrences that have affected the network.

Configuration management – These are used to support the administration and

configuration of the network.

Security management- manage user accounts and provide the ability to control and

set user based levels.



System communications links

Subscriber devices

Each SD has a band or set of band over which it can operate and various modes of possible

operation.

CDMA Channel Concept

Forward logical channels

Forward channels exists between the CDMA B.S and the SD’s.



The digital signal to be transmitted over a particular forward channel is spread by first EX-

OR ing it with a particular Walsh code( Wi 64

)

The signal is further scrambled by Inphase and Quadriphase components

PN spreading code assures that each channel is spread sufficiently spread over the entire

bandwidth.

Pilot channel

Synchronization channel

Paging channel

Traffic/power control channels

Pilot channel

It is used to provide a reference signal for all the subscriber devices within the cell.

The all zero Walsh code Wo64

is used for the initial spreading of the sequence. This results in

a sequence of all zeros that are further spread using the short PN spreading code resulting in a

sequence of 0’s and 1’s. The resulting pilot signal is an un-modulated spread spectrum signal.



Synchronization channel

It is used by the system to provide initial time synchronization

In this case W3264

is used to spread the synchronization channel message, Short spreading

sequence further scrambles the signal.



Paging channel

Paging channels are used to page the SD’s when there is a mobile terminated call and to send

control messages to the SD’s when call setup is taking place.

Walshcode W164

through W764

are used for this purpose, The paging channels undergo an

additional scrambling through the use of long PN sequence.

Reverse logical channels

Reverse logical channels exists between the SD’s and the CDMA B.S.

The encoding of digital information is performed differently than forward logical channel.

• Access channels

• Traffic/power control channels

Access channel

The CDMA reverse channels are used by the mobile to answer to pages and transmit control

information for the purpose of call setup and tear down.



Traffic/power control channels

CDMA reverse traffic/power control channel support both voice and data at the two rate sets.

CDMA frame format

Forward channel frame formats

Of the four forward logical channels only pilot channel does t support any frame format. It

consists of continuous transmission of Rf signal.



CDMA System (Layer 3) Operations

Initialization/registration

The mobile may be either in attached or detached mode.

When turned on the mobile goes through a power up state during which it selects a CDMA

system and then acquires the pilot and sync channels which allow it to synchronize its timing

to CDMA system. When attached the mobile can be in : Idle, system access or the mobile

station control on the traffic channel state.

CDMA system supports 10 different type of registration

Power up registration

Power down registration

Timer based registration

Distance based registration

Zone based registration

Power change registration

Ordered registration

Implicit registration



User zone registration



Call establishment

It requires various system tasks including mobile initialization, Idle, system , traffic channel

communication and call termination.

Initialization state – When the mobile is powered on it enters into mobile-

initialization task.

During this process the mobile searches for a pilot channel, once a valid pilot channel

is acquired the mobile synchronizes with it .

When the mobile locates the CDMA pilot signal it switches to Walsh code 32 and

looks for the sync channel message. Sync message channel contains information

about system time and the PN code needed for synchronization.

Idle state – Once the mobile has achieved the initialization it moves to the idle state.

While in the idle state , the mobile is waiting to receive calls or data messages or is

ready to originate call or some form of data transfer.

Access state – The CDMA mobile will enter the access state when it receives a

mobile directed requiring an ack originates a call or is required to perform



registration . When in access state the mobile will randomly attempt to access the

system.

Access channel probing- The access channel probing process consists of the M.S

sending a series of sequences is formed by the repeated transmission of additional

access probes until either mobile has received an Ack over the paging channel.

Traffic state

Mobile-originated call

Mobile-terminated call

Call termination

Mobile-originated call



To originate a call mobile sends a system access message on the access channel and then

monitors the paging channel for response from the system, If the access is successful a

forward traffic channel is assigned that corresponds to a particular Walsh code and a B.S

receiver is assigned for the reverse traffic channel long PN code.

Mobile-terminated call

For a mobile terminated call the B.S sends a message to the mobile on a paging channel. If a

attached to the system the mobile sends an ACK configures a FTC and assigns a receiver to

the mobile s RTC. The B.S begins to send null traffic on the FTC and sends a message

containing Walsh code and RTC information

Call termination

It occurs at the end of a call and can be initiated by either the mobile or B.S. If the mobile

initiates the call termination it sends a call termination message to the B.S stops transmitting

on the RTC and returns to the M.S initialization task.



Call handoff

Idle/access handoff- If the mobile is in the idle state and moves from the coverage

area of one sector/cell into another sector/cell an idle handoff can occur.

Soft handoff – soft handoff occurs when the mobile is able to communicate

simultaneously with several new cells over a forward traffic channel while still

maintaining communications over the FTC of the current cell

Hard handoff

Hand-down



– Due to intercarrier handoff

– Due to disjointed regions

– Border and transition cells

• See Figure 6-32 and Figure 6-33

Answers to Problems and Questions

Chapter 6



Section 6.1

1. CdmaOne is the trademarked term used to describe the first CDMA systems (i.e. IS-95A).

2. The unique aspect of CDMA technology is its ability to share the same frequency

assignment or portion of the frequency spectrum.

3. The ultimate limit for the number of users of a CDMA cellular wireless system is the total

noise created by all the users of the system. As more users become active, the background

noise increases; eventually, the noise level increases to the point where the system can not

function.

4. Not all of the numbered channels are useable by operators. For instance, channels 0-24 are

not valid channel numbers for frequency block A. Channel 25 is however. Use of Channel 25

would yield a bandwidth that would extend down to the band edge but not beyond.

5. Additional CDMA system capacity can be achieved by using additional new channels

within a cell.

Section 6.2

6. The primary function of the InterWorking Function (IWF) node in a 2G CDMA system is

to serve as a gateway between the wireless network and the packet data and circuit-switched

network.

7. The function of a CDMA mobile positioning system (MPS) is to facilitate the operation of

Enhanced 911 (E911). The MPS is based on the GPS system. Information provided by the

MPS is used to locate the CDMA cell phone user.

8. In a cdma2000 system, the function of the packet core network is to connect the radio

portion of the system to the PDN. Its main components are the AAA server, the Home Agent

(HA), and the packet data serving node (PDSN).



9. The function of the Home Agent node in a cdma2000 system is to facilitate mobile IP. This

system element has the task of forwarding all packets that are destined for the subscriber

device to the PSDN over an IP network.

10. The function of the packet data serving node (PDSN) is to connect the C-RAN and hence

the subscriber to the public data network.

Section 6.3

11. Walsh codes are special orthogonal spreading codes used to perform channel encoding.

The typical Walsh code has a length that is a power of 2 and contains an equal number of 0s

and 1s.

12. The use of spreading codes increases signal bandwidth by increasing the number of bits

required to encode a symbol. If the symbol rate remains constant and the number of bits per

symbol increases, the bit times are shorter (pulse width is smaller) and the bandwidth is

therefore greater.

13. The CDMA short PN spreading codes are of length 32,768.

14. Base stations in a CDMA system are able to be differentiated by mobile stations based on

their use of a particular timing offset for the short PN code. All the CDMA base stations in a

system use the same short PN code but each base station uses a different time offset.

15. The generation of the IS-95 CDMA pilot channel is achieved by signal spreading an all 0s

sequence with the all 0s Walsh code. The resulting all 0s sequences are then spread by the

short I and Q PN code sequences. These sequences are applied to a QPSK modulator to raise

them to a radio frequency for transmission.

16. The CDMA traffic channel is able to provide power control information to the

subscriber’s mobile device by multiplexing power control information into the transmitted bit

stream every 1.25 ms or at a rate of 800 bps.



17. The reverse logical channel signals are generated by applying the bit stream to an

orthogonal modulator. This modulator takes 6 bits at a time, applies them to a lookup table

and encodes them as 64-bit Walsh codes.

18. The CDMA system differentiates uplink signals transmitted by different mobiles by the

use of a unique long PN code used to spread the signal during signal processing. This unique

long code is derived from the cell phone’s electronic serial number (ESN).

19. The CDMA frame format is used to facilitate a slotted paging operation that increases

battery life. The mobile is instructed by the system to “wake up” periodically to listen to a

particular paging timeslot.

20. The use of power control groups on the CDMA reverse traffic channel facilitates better

CDMA system performance. If a traffic rate of less than 9600 bps can be transmitted, the

system randomly transmits data on half of the power control groups. This decreases overall

system noise. If even lower bit rates can be used, the system uses even fewer control groups

to transmit the message. Again, this process lowers overall system noise.

Section 6.4

21. The three states that a CDMA mobile may be in while in the attached mode are: the

mobile station idle state, the system access state, or the mobile station control on the traffic

channel state. Certain system operations are performed by the mobile station in each one of

these states.

22. The normal steps a CDMA mobile goes through in the initialization state are: entering the

power up state during which the mobile selects a CDMA system and then acquires the system

pilot and sync channels.

23. The three circumstances that put the CDMA mobile into the access state are: the mobile

receives a mobile directed message requiring an acknowledgement, originates a call, or is

required to perform registration.



24. The CDMA mobile’s operation that is known as access channel probing consists of the

mobile transmitting a series of access probe sequences (i.e. successively higher power

signals) until the system either responds on a paging channel or the number of allowed probe

sequences is exceeded.

25. A CDMA soft handoff is when the CDMA mobile is able to simultaneously communicate

with several cells. When handoff is required, no loss of communications occurs since a

connection is already in place.

26. The difference between the CDMA soft handoff and the CDMA softer handoff is that the

soft handoff is between two cells, while the softer handoff is between two sectors of the same

cell.

27. The measurement used by the CDMA mobile to implement a soft/softer handoff is the

pilot RSS. A CDMA mobile will continuously scan for pilots and establish communications

with any sector or cell (up to a maximum of 3) that has a pilot RSS that exceeds a certain

threshold value.

28. The CDMA forward traffic channel power level is controlled in the following fashion.

The base station starts transmitting on the FTC at a nominal power level and then starts to

continually lower its output power. The mobile station transmits the frame error rate (FER) to

the base station. When the FER reaches a certain threshold, the base station adjusts its power

output level to lower the FER below the threshold value.

29. The assumption of equal path loss is made (between base station and mobile) when the

CDMA mobile determines its initial output power level as it attempts a system access.

30. The fast closed-loop power control system used over the CDMA reverse link consists of

the transmission of a power control bit that is transmitted over the FTC every 1.25 ms. The

mobile either raises or lowers its output power as directed by the power control bit. The base

station transmits a 1 to lower the mobile’s output power and a 0 to raise the mobile’s output

power. The decision to transmit a 1 or a 0 depends upon the base station’s measurement of

received signal-to-interference ratio for the mobile’s RTC.



Section 6.5

31. The function of the CDMA supplementary code channels in IS-95B and cdma2000 is to

provide a means by which the overall system data transfer rate may be increased.

32. A CDMA 3X spreading rate refers to the use of either three 1.2288 mcps direct-sequence

spread carriers or a single 3.6864 mcps direct-sequence spread carrier. This technique

provides increased bandwidth for higher data transfer rates.

33. Up to seven additional supplemental code channels may be supported by IS-95B.

34. The function of the reverse pilot channel in cdma2000 is to enhance radio link

performance (i.e. lower bit or frame error rate).

35. The most probable conditions for the use of frequency division duplexing CDMA is by

license holders with paired frequency bands.

unit-6ajaybolar.weebly.com/uploads/1/0/1/0/10106930/wireless_unit_6.pdf · is-95 standard describes...

Documents