Dual Carrier HSDPA – The Push Beyond 5 MHz

Over at LinkedIn, Eiko Seidel recently published a link to a whitepaper by Nomor research on Dual Carrier HSDPA (or Dual Cell HSDPA operation as it is called in the standards), a new feature currently worked on in 3GPP. I've been waiting for this feature to come out for quite some time now as HSPA+ has already added 64QAM modulation and MIMO to HSPA. Consequently, not much can be done anymore to improve performance in a 5 MHz channel.

In practice, dual carrier (DC) HSDPA means that two adjacent 5 MHz carriers can be bundled by the network and DC capable HSPA mobiles can be assigned resources simultaneously on both carriers. In addition to the higher throughput, the 10 MHz bandwidth can also be used to schedule mobiles more efficiently around fading conditions, which according to the paper, results in an efficiency gain of up to 7% with 32 users and up to 25% with 2 users.

By increasing transmission speeds the round trip delay time is also further reduced, good news for online gamers. I have to note, however, that current round trip delay times of around 100ms are hardly distinguishable anymore from the delay of a DSL line. What's still distinguishable are the longer delay times caused by state changes after some time of inactivity. That's addressed by another feature, though, the enhanced Cell-FACH.

The enhancements also brings a number of new terminal categories. In addition to HSDPA terminal categories 1-20 which exist today (most people these days have a category 6 (3.6 MBit/s) or a category 7/8 (7.2 MBit/s) device), category 21-24 terminals will be able to use two adjacent carriers. The conserve energy on the mobile device side, the network can dynamically instruct such terminals to only listen to a single carrier if the amount of data to be transferred is low and doesn't warrant the use of two simultaneous carriers which requires more energy for decoding.

For the moment, multicarrier HSDPA is only for the downlink direction and while 64QAM is included, MIMO is not. Theoretical peak throughput in the combined 10 MHz carrier is around 42 MBit/s. But I guess this is not the end of the story yet, I think it is quite likely that in 3GPP Release 9, uplink dual carrier and MIMO is added to the feature list. The authors go a step further and speculate that in the future the standard could include further enhancements to go beyond two simultaneous carriers and to even include simultaneous transmission on carriers not adjacent to each other, even in different bands (e.g. 900 + 2100 MHz simultaneously).

While it looks good on paper, it remains to be seen which operators will go for it in practice. Some operators are determined to squeeze out as much as possible of their 3G networks before going to LTE. By the time these features are market ready, I'd say two to three years down the line, it's quite likely that many 3G base stations will already be used with two carriers per sector. If the feature can be done in software only, I guess it could become quite popular. In that time frame, however, many of today's 3G base stations will be end of live and might be replaced with triple mode GSM, UMTS and LTE base stations. If the feature is required when LTE is also in the cabinet, well, that remains to be seen.

But one way or the other, this new round of enhancements show that there is still a lot of life left in HSPA.

And here's some background as to where the technical details can be found in the specifications: First, 3GPP TR 25.825 contains an overview of the feature. Nomor's whitepaper lists a number of Change Requests (CR) to add the functionality to the relevant specification documents (TS 25.211, 25.212 and 25.214). I've had a look at the latest versions in 3GPP Release 8 and those CRs have been approved and are part of the specs now. So it looks like Dual Carrier HSDPA will be part of Release 8 which will be finalized in this quarter.

Let's see if there is already talk about this at the MWC in Barcelona in just a couple of days from now.

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