CDMA2000 1xEV-DO Air Interface

- details of the CDMA2000 1xEV-DO air interface or radio interface with details of the forward link and reverse link.

The CDMA2000 1x EV-DO air interface obviously has many similarities with other systems in the CDMA2000 family.

However CDMA2000 1x EV-DO has been optimised for data transmission. It utilises the spectrum more efficiently and also allows high speed data to be transmitted, making it fully competitive with other 3G cellular systems.

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.

As with all other CDMA2000 systems, CDMA technology forms the basis of CDMA2000 1x EV-DO.

Note on CDMA:

CDMA, Code Division Multiple Access, is a multiple access scheme used by many 3G cellular technologies, and other forms of wireless technology. It uses a process called Direct Sequence Spread Spectrum where spreading codes are used to spread a signal out over a given bandwidth and then reconstituting the data in the receiver by using the same spreading code. By supplying different spreading codes to different users, several users are able to utilises the same frequency without mutual interference.

Click on the link for a CDMA tutorial

EV-DO 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.

CDMA2000 1x EV-DO 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.

By Ian Poole

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