Reverse Channel Overview

Reverse Channel Overview

Last updated: July 23, 2008

The Reverse Channel consists of the Access Channel and the Reverse Traffic Channel according to the state of the data connection. The Access Channel is used by the access terminal to initiate communication with the access network or to respond to an access terminal directed message. The Reverse Traffic Channel is used by the access terminal to transmit user-specific traffic or signaling information to the access network.

Along with the C.S0024 standard evolving from the release 0 to release A, the new Subtype 2 Physical Layer is introduced to support higher data rate transmission. This results in some modifications on both the forward channel and reverse channel structure. For details on the forward channels, see Forward Channel Overview . This section will discuss the key differences of reverse channel structure between subtype 2 physical layer and subtype 0 physical layer.

Subtype 0 Reverse Channel Overview

The following figure shows the reverse channel structure as defined by subtype 0 physical layer protocol.

The following table summarizes the walsh code, spread factor and duty cycle for each subtype 0 reverse channel. For more description on spreading factor (SF), see Spread Factor Description .

Subtype 0 Reverse Channel Summary
Reverse Channel Walsh Code

Spread Factor

Duty Cycle Description
R-Pilot Pilot Channel I:0 16 Last 7/8 of a slot Is time-division multiplexed (TDM) with R-RRI channel.
R-RRI Reverse Rate Indicator I:0 16 First 1/8 of a slot Indicates the data rate of the Data Channel being transmitted on the Reverse Traffic Channel (so the access network does not have to perform blind detection). It is time-division multiplexed (TDM) with R-RRI channel
R-DRC Data Rate Control Q:8 16 Full slot (but offset by a 1/2 slot) Indicates the requested Forward Traffic Channel data rate and serving sector.
R-ACK

ACK Channel

I:4 8 First 1/2 of a slot Informs the access network whether a data packet transmitted on the Forward Traffic Channel was received.
R-Data Data Channel Q:2 4 Full slot

Subtype 2 Reverse Channel Overview

The subtype 2 reverse traffic channel has many changes in the definitions of the R-Pilot, R-RRI, R-ACK, and R-Data channels, and introduces two new reverse channels (R-DSC and R-Aux Pilot). Also, the R-Data channel uses a new timing structure (see Subtype 2 Reverse Link Frame Structure ).

The following figure shows the reverse channel structure as defined by subtype 2 physical layer protocol.

The following table summarizes the walsh code, spread factor and duty cycle (transmission duration) for each subtype 2 reverse channel. For more description on spreading factor (SF), see Spread Factor Description .

Subtype 2 Reverse Channel Summary
Reverse Channel Walsh Code

Spread Factor

Duty Cycle Description
R-Pilot Pilot Channel I:0 16 Full slot
R-RRI Reverse Rate Indicator I:4 16 Full slot Indicates the payload size and the sub-packet identifier of the physical layer packet transmitted on the Reverse Traffic Channel (so the access network does not have to perform blind detection).
R-DRC Data Rate Control Q:8 16 Full slot (but offset by a 1/2 slot) Indicates the requested Forward Traffic Channel data rate and serving sector. The serving sector is the sector that the access terminal selects for receiving the Forward Traffic Channel.
R-ACK

ACK Channel

I:12 32 First half of a slot (time-division multiplexed with R-DSC) Informs the access network whether a data packet transmitted on the Forward Traffic Channel was received.
R-DSC Data Source Control I:12 32 Second half of a slot (time-division multiplexed with R-ACK) Indicates the desired forward link serving cell which minimizes service interruption due to server switching on the forward link.
R-Data Data Channel Depending on modulation type as shown in the following table.
R-Aux Pilot Auxiliary Pilot Channel I:28 32 Full slot Is transmitted only if R-Data payload size is equal to or greater that the specified minimum payload threshold. Aids AN for reverse link channel estimation when operating at high data rates. It is used instead of adding power to the R-Pilot so that other reverse link channels will not scale their power as the power energy increases.
Subtype 2 R-Data Channel Code Channel

R-Data Modulation Type

Modulation Name

Walsh Code

Spreading Factor

Duration

B4

BPSK

Q:2

4

Full slot

Q4

QPSK

Q:2

4

Full slot

I:2

4

Full slot

Q2

QPSK

Q:1

2

Full slot

I:1

2

Full slot

Q4Q2

QPSK

Q:2

4

Full slot

I:2

4

Full slot

Q:1

2

Full slot

I:1

2

Full slot

E4E2

8-PSK

Q:2

4

Full slot

I:2

4

Full slot

Q:1

2

Full slot

I:1

2

Full slot

Subtype 2 Reverse Link Frame Structure

In subtype 2 operation, the R-Data channel uses a new timing structure. The frame and slot timing is the same as subtype 0 (each 26.67 ms frame consists of 16, 1.667 ms slots), but the concept of the subframe is introduced. Each subframe consists of four contiguous slots (6.667ms). Also new is the concept of a subpacket. A subpacket is defined to be the smallest unit of a reverse traffic channel transmission that can be acknowledged at the physical layer by the access network. A subpacket is transmitted over four contiguous slots, thus a subpacket is transmitted in one subframe.

Each physical layer packet can be transmitted in up to a maximum of four sub-packets. When more than one sub-packet is transmitted, the transmitted sub-packets use a three sub-frame interlacing. That is, the transmitted sub-packets of a physical layer packet is separated by two intervening sub-frames, and sub-packets of other physical layer packets may be transmitted in the sub-frames between those transmitted sub-packets.

The following figure shows how the four sub-packets from a single physical layer packet (a 4096 bit physical layer packer was used) can be transmitted on the R-Data channel. Note that not all of the sub-packets will be sent if the AN acknowledges the packet as received before all sub-packets have been transmitted (early termination).

Spread Factor Description

Spread Factor (SF) is a term which indicates which Walsh code set the Walsh code belongs to. For example, Walsh code 00000000 belongs to Walsh code set 8, and thus is said to have a spread factor of 8. The spread factor also indicates the length of the Walsh code (spread factor 8 indicates the Walsh code is 8 digits in length).

The diagram below shows an example of the Walsh code distribution of the subtype 0 reverse channels:

Subtype 0 Reverse Channel Walsh Code Distribution

Related Topics


Forward Channel Overview

Expected Power

Waveform Quality + Code Domain Measurement Description