Continuous Packet Connectivity (CPC) Is Not Sexy – Part 3

In a previous post I’ve given a broad overview of a 3GPP release 7 work item called "Continuous Packet Connectivity" (CPC).
This feature or rather this set of features aim to improve user
experience by enhancing battery lifetime, reaction time after idle
times and to increase network bandwidth in situations with many
simultaneous voice over IP and other real time service users. Rather
than introducing a bold new concept, CPC very much works "under the
hood" by improving functions that are already present. Part 2 of this mini series has started to look at a first set of features and this part now finishes by looking at the remaining ones:

Discontinuous reception (DRX) in Downlink at the UE (based on section 4.5 of 3GPP TR 25.903):

While a mobile is in activate high speed (HSDPA) mode it has to monitor one or more high speed shared control channels (HS-SCCH) to see when packets are delivered to it on the high speed shared channels. This monitoring is continuous, i.e. the receiver can never be switched off.

For situations when no data is transmitted or the average data transfer rate is much lower than what could be delivered over the high speed shared channels, the base station can instruct the mobile to only listen to selected slots of the shared control channel. The slots which the mobile does not have to observe are aligned as much as possible with the uplink control channel gating (switch off) times. Thus there will be times when the terminal can power down the transmitter unit to conserve energy.

Once more data arrives from the network than what can be delivered with the selected DRX cycle the DRX mode is switched off again and the network can once again schedule data in the downlink continuously.

HS-SCCH-less operation which includes an HS-SCCH less initial transmission  (based on section 4.7 and 4.8 of TR 25.903):

This feature is not intended to improve battery performance but to increase the number of simultaneous real time VoIP users in the network.

VoIP service e.g. via IMS requires relatively little bandwidth per user and thus the number of simultaneous users can be high. On the radio link, however, each connection has a certain signaling overhead. Thus, more users mean more signaling overhead which decreases overall available bandwidth for user data. In the case of HSDPA, the main signaling resources are the high speed shared control channels (HS-SCCH). The more active users, the more they proportionally require of the available bandwidth.

HS-SCCH-less operation aims at reducing this overhead. For real time users which require only limited bandwidth, the network can schedule data on high speed downlink channels without prior announcements for the terminal on a shared control channel. This is done as follows: The network instructs the mobile not only to listen to the HS-SCCH but in addition to all packets being transmitted on one of the high speed downlink shared channels. The terminal then attempts to blindly decode all packets received on that shared channel. To make blind decoding easier, packets which are not announced on a shared control channel can only have one of four transmission formats (number of data bits) and are always modulated using QPSK. These restrictions are not an issue for performance since HS-SCCH-less operation is only intended for low bandwidth real time services.

The checksum of a packet is additionally used to identify for which terminal the packet is intended for. This is done by using the terminal’s MAC address as an input parameter for the checksum algorithm in addition to the data bits. If the terminal can decode a packet correctly and if it can reconstruct the checksum the packet is intended for the terminal. If the checksum does not match then either the packet is intended for a different terminal or a transmission error has occurred. In both cases the packet is discarded.

In case of a transmission error the packet is automatically retransmitted since the terminal did not send an acknowledgement (HARQ ACK). Retransmissions are announced on the shared control channel which requires additional resources but should not happen frequently as most packets should be delivered properly on the first attempt.

And for more on HSDPA and HSUPA…

I hope that this introduction to Continuous Packet Connectivity (CPC) answers more questions than it raises. In case some fundamental things remain unclear consider taking a look at my book on mobile communication systems which covers HSDPA and HSUPA from the ground up.