The CDMA Physical Layer

 
 

1. Physical Channel in IS-95A

1.1 Forward channels:

The Forward link consists of:

Ø       One Pilot Channel

Ø       One Sync Channel

Ø       One to seven Paging Channels

Ø        Traffic Channels assigned as needed

 

 
 
In the Forward link, channels are divided by assigning each channel a unique Walsh code:
Ø      Walsh code 0 — reserved for Pilot Channel.
Ø      Walsh codes 1 through 7 — reserved for Paging Channel. Unused Walsh codes may be assigned to Traffic Channels.
Ø      Walsh code 32 — reserved for Sync Channel.
Ø       Remaining Walsh codes — assigned to subscriber Traffic Channels as required.
 
 
1.1.1 Pilot Channel
Every cell or sector must transmit a Pilot Channel for each frequency that is supported. Pilot signals act as beacons to notify potential users of the existence of a CDMA Base Station. Forward pilot channel is spread over W0 and modulated with short code directly
Ø      Assist mobile station to be connected with CDMA network
Ø      Handles multi-path searching
Ø      Provide the phase reference for coherent demodulation and help the mobile station estimate the transmission power
Ø      The mobile station measures and compares the pilot channel powers from the base stations during the handoff
Ø      BTS transmits the pilot channel continuously
 

1.1.2 Sync Channel

The Sync Channel is used by mobiles to obtain timing and cell specific information. Mobiles must acquire the Sync Channel and decode its message in order to synchronize with the system. The Sync message is spread with Walsh code 32 and is broadcast continuously.

The Sync message includes the following information:

Ø       Pilot PN offset

Ø      System time

Ø       State of the Long PN code

Ø      Common Air Interface Revision Level

Ø      System ID

Ø       Network ID

Ø       Paging Channel data rate

lHere note that, sync channel rate is 1200bps

 

 

1.1.3 Paging Channel

A Paging Channel is used by the CDMA system to transmit overhead messages and mobile directed messages. Walsh codes 1 through 7 may be used for Paging Channels. A system operator may choose to support less than seven Paging Channels. In this case, unused codes may be assigned to Traffic Channels.

 

A Paging Channel transmits the configuration messages:

_ System Parameters message

_ Neighbor List message

_ Access Parameters message

_ CDMA Channel List message

_ Global Service Redirection message

 

1.1.4 Forward Traffic Channels

The Forward Traffic Channels are used to transmit user data and signaling information. The Forward Traffic Channels are separated by their unique Walsh code assignments. Once the mobile is assigned a Walsh code in an omni cell (or sector) the code cannot be assigned to any other mobile in that omni cell (or sector) for the entire duration of the call. A Forward Traffic Channel can be composed of a Fundamental Channel and Supplemental Code Channels.

 

1.1.4.1 The Fundamental Channel

The Fundamental Forward Channel is used to transmit user data, signaling, and the Power Control Sub-channel.

 

1.1.4.2 The Supplemental Code Channel

Supplemental Code Channels may be used to provide the subscriber with a high speed data capability. The bit rate of a single Fundamental Channel is limited by the Rate Set frame formats.

A Forward Traffic Channel may include several Supplemental Code Channels to provide the required bit rate. Each Supplemental Code Channel requires an additional unique Walsh code assignment. The Supplemental Code Channels always transmit at the maximum rate for the rate set in use and do not carry any signaling or power control sub-channel information. Supplemental Code Channels are a TIA/EIA-95 capability and are not defined in IS-95A.

 

 

 

1.1.4.3 Forward Traffic Channel Generation:

 

 

Symbol repetition makes symbol streams at different rates adapted to those at the rate of 19.2k. The original symbol stream at the rate of 19.2Kbps will not be repeated, 9.6Kbps symbol stream will be repeated once, and 4.8Kbps symbol stream will be repeated twice…. Then, after the interleaving, long code scrambling, spreading, etc., the transmitting power of repeated bits is reduced during the transmission. Thus, variable rates can be achieved and the system capacity be increased. During the long code scrambling, one chip will be sampled from every 64 long code chips for exclusive-or with a scrambled chip.

 A forward traffic channel includes a power control sub-channel and a power control bit is used to instruct an MS to increase or decrease the transmitting power. Each forward traffic channel frame (20ms) can be divided into 16 power control groups (each group is 1.25ms long). Each power control group contains a power control bit , therefore the rate of reverse fast power control is 16* (1s/20ms)=800bps. And the power control bit is embedded right during the long code scrambling.
 
1.1.4.4 Rate Sets
A Rate Set is a set of Traffic Channel frame formats. Each set is composed of 4 bit rates.
A Rate Set may carry voice, user data, or signaling.
Two Rate Sets are defined for use in cdmaOne systems. All services provided over the air interface must conform to one of these two rate sets:
Ø      Rate Set 1 — supports a maximum of 8550 bps, with an additional 1050 bits of overhead for a total max rate of 9600 bps.
Ø       Rate Set 2 — supports a maximum of 13,300 bps, with additional overhead bringing the TIA/EIA- Total transmission rate to 14,400 bps maximum.
 
1.1.4.5 Service Options
A Service Option is a service capability of the system. Service Options may be applications such as voice, data, or fax. TIA/EIA Telecommunications System Bulletin 58 (TSB-58) defines assigned Service Option numbers:
 
·        1 — 8K voice
·        2—8K Mobile Station Loop back
·        3 — EVRC (Enhanced Variable Rate Coder)
·        4 — Async Data (Rate Set 1)
·        5 — Group 3 Fax (Rate Set 1)
·        6 — Short Message Services (Rate Set 1)
·        7 — etc
 
Categories of Traffic
Ø      Primary Traffic—User voice
Ø      Secondary Traffic—User data other than voice
Ø       signaling—Call control messaging
 
1.1.4.6 TIA/EIA-95 Medium Data Rate (MDR) Option
To help satisfy the growing appetite for wireless data applications, TIA/EIA-95 includes an optional Medium Data Rate (MDR) feature, which may operate on both Forward and Reverse links. In order to support data rates higher than Rate Set 1 or Rate Set 2, there must be some way to combine multiple channels together. Remember that CDMA users are channelized by unique codes. For higher speed data requirements, the transmitter may simultaneously use multiple code channels to deliver data to the receiver. The MDR feature allows up to eight code channels to be bundled together to support up to eight times the maximum data rate of a single channel.
 TIA/EIA-95 defines a Traffic Channel as either a Fundamental Channel or Supplemental Code Channel. The structure of a Supplemental Code Channel is the same as that of a Fundamental Channel, but there are differences in how it is used.
 
Reverse Link
 

 
1.2 Reverse channels
The Reverse cdmaOne link is substantially different than the Forward link. The difference is primarily due to the power control requirements and to the non-coherent nature of the Reverse link.
Reverse link channels are identified by unique time shifts of the long PN code. Recall that time shifted versions of a PN code have very little correlation with each other. Hence, the subscribers in the Reverse direction are channelized using a unique shift of the long PN code.
There are two types of code channels in the Reverse link:
  1. Traffic
  2. Access
Note that a reverse channel is first spread with WALSH to 307.2Kbps and then becomes 1.2288Mcps after long code modulation. The long code here not only spreads spectrum, but implements the function of channelization. “Data burst randomizer” means discarding repeated chips during the transmission according to a certain algorithm and implementing discontinuous transmission so as to reduce the transmission rate and increase the capacity of reverse channels.
In OQPSK reverse modulation, as opposed to the data modulated by I channel PN sequence, the data modulated by Q channel PN sequence has the delay of half a PN chip (406.901ns). Thus, the maximum phase change of four-phase modulation is 90 degrees instead of 180-degree mutation.
 
 
 
1.2.1 Reverse Traffic Channel:
Generation of the Reverse Traffic Channel is different than generation of the Forward Traffic Channel. Convolutional encoding and interleaving are performed as in the Forward direction, but several new processes then follow. An orthogonal modulation scheme is used, followed by a data burst randomizer that determines when to turn off the mobile transmitter to reduce average transmit power.
 
1.2.1.1 Reverse Fundamental and Supplemental Channels
TIA/EIA-95 supports high speed data on the Reverse link as well. Supplemental Channels may be used as necessary to provide the subscriber with the data rate desired. Supplemental Channels are also transmitted at a unique offset of the long PN code.
 
 
 

1.2.2 Access Channel Generation
Generation of the Access Channel is identical to that of the Reverse Traffic Channel, except that there is no data burst randomization. Data burst randomization is used to take advantage of periods of reduced speech activity. The Access Channel is not a speech channel (only messages) and is transmitted at a constant rate.
 
 
 
1.3 Initialization of the MS

 

Synchronous Channel message contains the LC_STATE, SYS_TIME, P_RAT, and synchronizes with the system.

 

Ø      LC_STATE is Long Code State

Ø      SYS_TIME is system time.

Ø      Search for the CDMA carrier, acquire the Pilot Channel and synchronize the short code.

Ø      Receive the Synchronous Channel message containing the LC_STATE, SYS_TIME

Ø      P_RAT, acquire timing and synchronize with the system.

Ø      Monitor the Paging Channel and receive the system message.

Ø      The MS can register on the Access Channel.

 
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