Evolution-Data Optimized

Evolution-Data Optimized is a telecommunications standard for the wireless transmission of data through radio signals, typically for broadband Internet access. EV-DO is an evolution of the CDMA2000 standard which supports high data rates and can be deployed alongside a wireless carrier's voice services. It uses advanced multiplexing techniques including code-division multiple access as well as time-division multiplexing to maximize throughput. It is a part of the CDMA2000 family of standards and has been adopted by many mobile phone service providers around the world particularly those previously employing CDMA networks. It is also used on the Globalstar satellite phone network.
An EV-DO channel has a bandwidth of 1.25 MHz, the same bandwidth size that IS-95A and IS-2000 use, though the channel structure is very different. The back-end network is entirely packet-based, and is not constrained by restrictions typically present on a circuit switched network.
The EV-DO feature of CDMA2000 networks provides access to mobile devices with forward link air interface speeds of up to 2.4 Mbit/s with Rel. 0 and up to 3.1 Mbit/s with Rev. A. The reverse link rate for Rel. 0 can operate up to 153 kbit/s, while Rev. A can operate at up to 1.8 Mbit/s. It was designed to be operated end-to-end as an IP-based network, and can support any application which can operate on such a network and bit rate constraints.

Standard revisions

There have been several revisions of the standard, starting with Release 0. This was later expanded upon with Revision A to support quality of service and higher rates on the forward and reverse link. In late 2006, Revision B was published, whose features include the ability to bundle multiple carriers to achieve even higher rates and lower latencies. The upgrade from EV-DO Rev. A to Rev. B involves a software update of the cell site modem, and additional equipment for new EV-DO carriers. Existing cdma2000 operators may have to retune some of their existing 1xRTT channels to other frequencies, as Rev. B requires all DO carriers be within 5 MHz.

EV-DO Rel. 0 (TIA-856 Release 0)

The initial design of EV-DO was developed by Qualcomm in 1999 to meet IMT-2000 requirements for a greater-than-2 Mbit/s down link for stationary communications, as opposed to mobile communication. Initially, the standard was called High Data Rate, but was renamed to 1xEV-DO after it was ratified by the International Telecommunication Union under the designation TIA-856. Originally, 1xEV-DO stood for "1x Evolution-Data Only", referring to its being a direct evolution of the 1x air interface standard, with its channels carrying only data traffic. The title of the 1xEV-DO standard document is "cdma2000 High Rate Packet Data Air Interface Specification", as cdma2000 is another name for the 1x standard, numerically designated as TIA-2000.
Later, due to possible negative connotations of the word "only", the "DO"-part of the standard's name 1xEV-DO was changed to stand for "Data Optimized", the full name - EV-DO now stands for "Evolution-Data Optimized." The 1x prefix has been dropped by many of the major carriers, and is marketed simply as EV-DO. This provides a more market-friendly emphasis of the technology being data-optimized.

Forward link channel structure

The primary characteristic that differentiates an EV-DO channel from a 1xRTT channel is that it is time multiplexed on the forward link. This means that a single mobile has full use of the forward traffic channel within a particular geographic area 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 in favorable RF conditions with very complex modulation techniques while also serving users in poor RF conditions with simpler 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, which include the 'pilot', which helps the mobile find and identify the channel, the Media Access Channel which tells the mobile devices when their data is scheduled, and the 'control channel', which contains other information the network needs the mobile devices to know.
The modulation to be used to communicate with a given mobile unit is determined by the mobile device 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 a 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" channel. Alternatively, the mobile can select a "null" rate, 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	Slots scheduled	Payload size	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.

Reverse link structure

The reverse link on EV-DO Rel. 0 operates very similar to that of CDMA2000 1xRTT. The channel includes a reverse link pilot along with the user data channels. Some additional channels that do not exist in 1x include the DRC channel and the ACK channel. Only the reverse link has any sort of power control, because the forward link is always transmitted at full power for use by all the mobiles. The reverse link has both open loop and closed loop power control. In the open loop, the reverse link transmission power is set based upon the received power on the forward link. In the closed loop, the reverse link power is adjusted up or down 800 times a second, as indicated by the serving sector.
All of the reverse link channels are combined using code division and transmitted back to the base station using BPSK where they are decoded. The maximum speed available for user data is 153.2 kbit/s, but in real-life conditions this is rarely achieved. Typical speeds achieved are between 20 and 50 kbit/s.

EV-DO Rev. A (TIA-856 Revision A)

Revision A of EV-DO makes several additions to the protocol while keeping it completely backwards compatible with Release 0.
These changes included the introduction of several new forward link data rates that increase the maximum burst rate from 2.45 Mbit/s to 3.1 Mbit/s. Also included were protocols that would decrease connection establishment time, the ability for more than one mobile to share the same timeslot and the introduction of QoS flags. All of these were put in place to allow for low latency, low bit rate communications such as VoIP.
The additional forward rates for EV-DO Rev. A are:

DRC Index	Data rate in kbit/s	Slots scheduled	Payload size	Code Rate	Modulation
13	1536	2	5120	5/12	16-QAM
14	3072	1	5120	5/6	16-QAM

In addition to the changes on the forward link, the reverse link was enhanced to support higher complexity modulation. An optional secondary pilot was added, which is activated by the mobile when it tries to achieve enhanced data rates. To combat reverse link congestion and noise rise, the protocol calls for each mobile to be given an interference allowance which is replenished by the network when the reverse link conditions allow it. The reverse link has a maximum rate of 1.8 Mbit/s, but under normal conditions users experience a rate of approximately 500-1000 kbit/s but with more latency than DOCSIS and DSL.