Сымсыз деректерді тарату стандарты: EV-DO

Telecommunications standard for the wireless transmission of data through radio signals

Evolution Data Optimized (EV DO, EVDO, etc.) 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 (IS 2000) 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 (CDMA) as well as time division multiplexing (TDM) 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. EV DO service was discontinued in much of Canada in 2015. An EV DO channel has a bandwidth of 1.25 MHz, the same bandwidth size that IS 95A (IS 95) and IS 2000 (1xRTT) 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.

There have been several revisions of the standard, starting with Release 0 (Rel. 0). This was later expanded upon with Revision A (Rev. A) to support quality of service (to improve latency) and higher rates on the forward and reverse link. In late 2006, Revision B (Rev. B) was published, whose features include the ability to bundle multiple carriers to achieve even higher rates and lower latencies (see TIA 856 Rev. B below). 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.

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 (i. e., moving cellular phone service). Initially, the standard was called High Data Rate (HDR), but was renamed to 1xEV DO after it was ratified by the International Telecommunication Union (ITU) under the designation TIA 856. Originally, 1xEV DO stood for "1x Evolution Data Only", referring to its being a direct evolution of the 1x (1xRTT) 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 (lowercase) 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.

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 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, which include the 'pilot', which helps the mobile find and identify the channel, the Media Access Channel (MAC) 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" (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. 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. 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 50 kbit/s.