Reverse Link

Overview

The EV-DO RL consists of fixed size physical layer packets, each 26-2/3 ms in duration, which is equivalent to 16 consecutive FL slots. Note that the RL is not time division multiplexed like the FL and here, slot/frame is just a convenient unit of time.

EV-DO uses a pilot-aided coherently demodulated RL (i.e. the AT transmits a pilot on the RL in order to aid demodulation and decoding at the AN). As in IS-95, transmissions from each AT are identified by the AT's unique long PN code offset. The sectors access channel is mapped to a unique public long PN code offset, used by all ATs in that sector which wish to establish a radio link with the AN. The RL data rate may vary between 9.6 to 153.6 Kbps.

Channels

The RL is logically partitioned into the following set of channels:

1. Access
   1. Pilot
   2. Data
2. Traffic
   1. Pilot
   2. Medium Access Control
   3. Reverse Rate Indicator
   4. Data Rate Control
   5. ACK
   6. Data

Access

The access channel is used by the AT to initiate communication with the AN or to respond to a page. The AT's first transmits an access probes and waits for a response from the AN. If there is no response, the AT transmits another access probe, at a higher power level. Each access attempt thus consists of a series of such access probes. If one access attempt fails, the AT waits for a certain period of time before initiating a new access attempt. The wait period between successive access probes in the same access attempt, the power step increase for successive access probes, etc. are all system parameters that are broadcast by the AN on the control channel. The situation is depicted pictorially below:

Each access probe consists of a preamble followed by an access channel data packet. The preamble duration is 1 frame, or 16 slots. During the preamble, only the pilot channel is transmitted. The access channel datapacket is transmitted over 4 frames. During the access channel data packet, both the pilot and access data packets are transmitted using code division multiplexing.

Traffic

The RL traffic channel structure at the physical layer is as follows:

As we see from the above two diagrams, the RL traffic channel consists of four code division multiplexed information streams, two streams per I and Q phases. As already mentioned, the user data is a fixed size 16 slot-duration packet, on the Q phase. Also on the Q phase is the DRC information stream (more on this later). On the I phase, the ACK information stream is transmitted on each slot, for half a slot. The RRI and pilot streams are time division multiplexed onto the I phase: the first 256 chips of each slot contain the RRI information and the rest of the chips are dedicated to the pilot.

For identification, each information stream is covered by a (known to the AN) Walsh cover. Since the Walsh covers are only used to identify the information streams within an AT's traffic channel, 16-ary and 8-ary Walsh covers are used instead of 64-ary ones. The pilot/RRI information stream is covered by 16_{W_0}. The ACK information stream is covered by 8_{W_4}. The DRC information stream is covered by 16_{W_8}and the user data stream is covered by 4_{W_2}.

Reverse Rate Indicator

The RL MAC channel contains two streams of information: the RRI and the DRC. The RRI indicates to the AN the rate at which the AT is transmitting on the RL. The RRI is included as the preamble for RL frames. Essentially this boosts the RL capacity as it aids coherent decoding and demodulation of RL frames at the AN. An 8-ary orthogonal code is used to indicate the rate.

Data Rate Control

The DRC MAC sub-channel is used by the AT to indicate to the AN the desired rate of transmit on the FL as well as the specific AN sector on which to transmit [Cui02]. The data rate is requested using 8-ary bi-orthogonal coding. The desired AN sector is requested using an 8-ary Walsh cover. Each DRC slot contains 1024 chips and the data is centered around the middle of the slot so as to minimize the delay between SINR estimation and the start of AN transmission

ACK

The ACK information stream is related to the use of HARQ on the EV-DO FL in order to improve system throughput under changing channel conditions (more on this later). It indicates whether the last transmitted packet was received and decoded correctly at the AT or not.