The primary characteristic that differentiates an EV-DO channel from a 1xRTT channel is that it is Time Multiplexed on the forward link (from the tower to the mobile). This means that a single mobile has full use of the forward traffic channel within a particular geographic area (a sector) during a given slot of time. Using this technique, EV-DO is able to modulate each user’s time slot independently. This allows the service of users that are in favorable RF conditions with very complex modulation techniques while also serving users in poor RF conditions with simpler and more redundant signals.
The forward channel is divided into slots, each being 1.667 ms long. In addition to user traffic, overhead channels are interlaced into the stream. These include the Pilot which helps the mobile find and identify the channel, the Media Access Channel (MAC) which tells the mobiles when their data is scheduled, and the Control Channel, which contains other information that the network needs the mobiles to know.
The modulation to be used to communicate with a given mobile is determined by the mobile itself. It listens to the traffic on the channel, and depending on the receive signal strength along with the perceived multi-path and fading conditions, makes its best guess as to what data-rate it can sustain while maintaining a reasonable frame error rate of 1-2%. It then communicates this information back to the serving sector in the form of an integer between 1 and 12 on the "Digital Rate Control" (DRC) channel. Alternatively, the mobile can select a "null" rate (DRC 0), indicating that the mobile either cannot decode data at any rate, or that it is attempting to hand off to another serving sector.
The DRC values are as follows:
| DRC Index | Data rate (kbit/s) | Slots scheduled | Payload size (bits) | Code Rate | Modulation | SNR Reqd. |
|---|---|---|---|---|---|---|
| 1 | 38.4 | 16 | 1024 | 1/5 | QPSK | -12 |
| 2 | 76.8 | 8 | 1024 | 1/5 | QPSK | -9.6 |
| 3 | 153.6 | 4 | 1024 | 1/5 | QPSK | -6.8 |
| 4 | 307.2 | 2 | 1024 | 1/5 | QPSK | -3.9 |
| 5 | 307.2 | 4 | 2048 | 1/5 | QPSK | -3.8 |
| 6 | 614.4 | 1 | 1024 | 1/3 | QPSK | -0.6 |
| 7 | 614.4 | 2 | 2048 | 1/3 | QPSK | -0.8 |
| 8 | 921.6 | 2 | 3072 | 1/3 | 8-PSK | 1.8 |
| 9 | 1228.8 | 1 | 2048 | 2/3 | QPSK | 3.7 |
| 10 | 1228.8 | 2 | 4096 | 1/3 | 16-QAM | 3.8 |
| 11 | 1843.2 | 1 | 3072 | 2/3 | 8-PSK | 7.5 |
| 12 | 2457.6 | 1 | 4096 | 2/3 | 16-QAM | 9.7 |
Another important aspect of the EV-DO forward link channel is the scheduler. The scheduler most commonly used is called "proportional fair". It's designed to maximize sector throughput while also guaranteeing each user a certain minimum level of service. The idea is to schedule mobiles reporting higher DRC indices more often, with the hope that those reporting worse conditions will improve in time.
The system also incorporates Incremental Redundancy Hybrid ARQ. Each sub-packet of a multi-slot transmission is a turbo-coded replica of the original data bits. This allows mobiles to acknowledge a packet before all of its sub-sections have been transmitted. For example, if a mobile transmits a DRC index of 3 and is scheduled to receive data, it will expect to get data during four time slots. If after decoding the first slot the mobile is able to determine the entire data packet, it can send an early acknowledgement back at that time; the remaining three sub-packets will be cancelled. If however the packet is not acknowledged, the network will proceed with the transmission of the remaining parts until all have been transmitted or the packet is acknowledged.
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