The Basics of CDMA2000 1xEV-DO

- 1x EV-DO, the data optimised 3G cellular telecommunications (cell phone) system for CDMA2000

CDMA2000 1x EV-DO cell phone system is a standard that has evolved from the CDMA2000 mobile phone system and it is now firmly established in many areas of the world. The letters EV-DO stand for Evolution Data Only or Data Optimised. From the title it can be seen that it is a data only mobile telecommunications standard that can be run on CDMA2000 networks.

The EV-DO cell phone system is capable of providing the full 3G data rates up to 3.1 Mbps now that release A of the standard has been issued. The first commercial CDMA2000 1xEV-DO network was deployed by SK Telecom (Korea) in January 2002. Now operators in Brazil Ecuador, Indonesia, Jamaica, Puerto Rico, Taiwan and the USA to mention but a few have all launched networks and more are to follow.

Basics
Th CDMA2000 1xEV-DO cell phone system is defined under IS-856 rather than IS-2000 that defines the other CDMA2000 standards, and as the name indicates it only carries data. In Release 0 of the standard the maximum data rate was 2400 Mbps in the forward (downlink) with 153 kbps in the reverse (uplink) direction, the same as CDMA2000 1X. However in the later release of the standard, Release A, the forward data rate rises to 3.1 Mbps, and 1.2 Mbps in the reverse direction.

Data rates for CDMA2000 1xEV-DO

The forward channel forms a dedicated variable-rate, packet data channel with signalling and control time multiplexed into it. The channel is itself time-divided and allocated to each user on a demand and opportunity driven basis. A data only format was adopted to enable the standard to be optimised for data applications. If voice is required then a dual mode phone using separate 1X channel for the voice call is needed. In fact the "phones" used for data only applications are referred to as Access Terminals or ATs.

Air interface
The EV-DO RF transmission is very similar to that of a CDMA2000 1X transmission. It has the same final spread rate of 1.228 Mcps and it has the same modulation bandwidth because the same digital filter is used. Although 1xEV-DO has many similarities with 1X transmissions, it cannot occupy the same channels simultaneously, and therefore requires dedicated paired channels for its operation. Accordingly new bands, often in the new 3G allocations are being dedicated for EV-DO in some areas.

Forward link
The forward link possesses many features that are specific to EV-DO, having been optimised for data transmission, particularly in the downlink direction. Average continuous rates of 600 kbps per sector are possible. This is a six fold increase over CDMA2000 1X and is provided largely by the ability of 1xEV-DO to negotiate increased data rates for individual ATs because only one user is served at a time.

The forward link is always transmitted at full power and uses a data rate control scheme rather than the power control scheme used with 1X, and the data is time division multiplexed so that only one AT is served at a time.

In order to be able to receive data, each EV-DO AT measures signal-to-noise ratio (S/N) on the forward link pilot every slot, i.e. 1.667 ms. Based on the information this provides the AT sends a data rate request to the base station. The AN receives requests from a variety of ATs, and decisions have to be made regarding which ATs are to be served next. The AN endeavours to achieve the best data transfer, and this is done by serving those ATs offering a good signal to noise ratio. This is achieved at the expense of users at some distance from the AN's antenna.

Accurate time synchronisation is required between the EV-DO Access Nodes. To achieve this time information is taken from the Global Positioning System as this is able to provide an exceedingly accurate time signal.

Forward link channels
A number of channels are transmitted in the forward direction to enable signalling, data and other capabilities to be handled. These channels include the Traffic channel, MAC channel, Control channel and Pilot. These are time division multiplexed.

Traffic Channel - This channels uses Quadrature Phase Shift Keying (QPSK) modulation for data rates up to 1.2288 Mbps. For higher data rates, higher order modulation techniques are used in the form of 8PSK with 3 bits per symbol or 16QAM with 4 bits per symbol. The levels of the I and Q symbols are chosen so that the average power becomes 1.

The Incoming data to be used as the modulation comes from the from the turbo coder and is scrambled by mixing it with a Pseudo Random Number (PN) sequence. The initial state of the PN is derived from known parameters, and is unique for each user. Every packet starts at the same initial value of the PN sequence.

At the beginning of the transmission to each user, there is a preamble that contains the user ID for the data. Its repeat rate is determined by the data rate because lower data rates require higher repeat values. However even at its largest, the preamble will fill no more than half the first slot.

Control Channel - This channel carries the signalling and overhead messages.

Pilot - The differentiator between the cell and the sector is still the PN offset of the pilot channel and the pilot signal is only gated on for 192 chips per slot.

Medium Access Control (MAC) Channel - This channel carries a number of controls including the Reverse Power Control (RPC), the Data Rate Control (DRC) Lock, and the reverse activity (RA) channels.

Reverse Link
The reverse link for 1xEV-DO has a structure similar to that for CDMA2000. In EV-DO all signalling is performed on the data channel and this means that there is no Dedicated Control Channel. The data channel can support 5 data rates which are separated in powers of 2 from 9.6 to 153.6 kbps. These rates are achieved by varying the repeat factor. The highest rate uses a Turbo coder with lower gain. The following channels are transmitted in addition to those used with 1X:

Reverse Rate Indicator (RRI) Channel - This indicates the data rate of the Reverse Data Channel.

Acknowledgement (Ack) Channel - This channel is transmitted after the AT detects a frame with the preamble detailing it to be the recipient of the data.

Data Rate Control (DRC) Channel - This channel contains a four bit word in each slot to allow the choice of 12 different transmission rates.