Adapting EV-DO to Limited Link Budgets

EV-DO (Evolution – Data Optimized) is a popular name of 3GPP2’s 3G air interface for high-speed data. The specification calls it High Rate Packet Data, or HRPD. At the December 3GPP2 meeting, a new framework was proposed that would modify HRPD to allow it to perform better in limited link-budget environments, such as in satellite and machine-to-machine (M2M) applications. The new system is called EXtended Cell High Rate Packet Data, or xHRPD. Contributing companies include TerreStar, DBSD North America (ICO), Skyterra, and Qualcomm.

For hybrid satellite/terrestrial networks, xHRPD is intended to allow a handset to operate with the greater path losses of long satellite links yet have the same form factor as a typical smartphone. For M2M, it will improve reliability in cases of increased building attenuation and more-distant telemetry devices. Targeted services include 2 kbps VoIP and low-rate data, down to 640 bps.

The biggest physical-layer change from today’s HRPD is that the reverse link is channelized in a new, narrowband manner. The typical 1.25 MHz spread-spectrum bandwidth is changed to 192 narrowband FDM channels, each channel 6.4 kHz wide. A terminal is assigned one or two of these channels. These channels are orthogonal, reducing intra-cell interference and improving the link-budget. Narrowband channels reduce search time and need less link-margin than wide-bandwidth channels. New coding and reduced overhead improves link efficiency for small packets. Changes to the MAC layer also help.

In HPRD, forward link transmission relies on a Channel Quality Indicator sent periodically by the terminal. xHRPD can send a constant CQI value for a longer period of time to allow coherent combining of long-delayed, weak CQI symbols at the access network. This means the data rate does not change as often on the forward link.

To accommodate long satellite path delays and large satellite cells, a new narrowband Access Channel uses the Slotted Aloha protocol. The modified reverse link power-control channel operates at 50 bps instead of the typical 150 bps. Hybrid ARQ is disabled in xHRPD protocol due to the long path delays.

This framework remains open in 3GPP2. Discussions are to continue at the next meeting in late January.