It looks like 3GPP member companies are moving quickly to standardize data rate enhancements for current 3.5G UMTS / HSPA networks as promised in diverse marketing slides floating around at conferences. The two main ideas to further increase transmission speeds are to use a higher modulation scheme (64QAM) and MIMO (Multiple Input Multiple Output) to send several data streams via different spacial paths simultaneously.
3GPP TS 25.306-800 has an interesting table (table 5.1a) that specifies current and future HSPA terminal categories for downlink. Today, mobile devices typically use on of the following terminal categories:
- Category 12: Speeds up to 1.8 MBit/s (first HSDPA cards for notebooks)
- Category 6: Speeds up to 3.6 MBit/s (most HSDPA cards sold for notebooks in 2007 and pretty much all mobile phones supporting HSDPA)
- Category 7/8: Speeds up to 7.2 MBit/s (the latest HSDPA cards for notebooks)
I’ve tried all three types of HSDPA cards in practice and could reach around 1.4 MBit/s with a Cat-12 device, around 2.5 MBit/s with a Cat-6 device and around 4.2 MBit/s with a Cat-7/8 device.
For HSPA+, Release 8 of the 3GPP standards now introduce terminal categories 13 to 18. In Category 18, a terminal can receive up to 27952 Bits per 2ms TTI per MIMO channel (I assume). That would result in a speed of around 28 MBit/s. Interesting to see that in 2×2 MIMO mode, only 16QAM and not 64QAM is used. Category 14 terminals can receive 42.192 bits per 2ms TTI in non-MIMO mode. That’s still an impressive 21 MBit/s, even without MIMO.
In order not to get too excited about these numbers have a close look at the physical realities of a radio channel in the real world. Even though specified on paper such speeds are only achievable under the very best of radio conditions with little to no interference from neighboring cells. Having said that I nevertheless believe that we will see nice speed enhancements in practice. I am looking forward to see just how much!
Beside the pure technicalities I have the following questions that puzzle me.
Why are companies suddenly so eager to define and implement HSPA+? As far as I understood LTE/SAE is defined to overcome certain restrictions of the existing technology and to solve the architectural issues operators are facing today. If HSPA+ promises to deliver the same thing (or at least close to what LTE is targeting) will that cause LTE to be pushed to a later point in time?
I don’t know much about LTE but I tend to think that the timeline for LTE is much later than what HSPA+ can provide. We already have HSDPA. Rollout of HSUPA is the next step and may happen soon. The natural evolution is to move to HSPA+. I guess rollout of HSPA+ would happen in phases which would give it a leverage over LTE.
HSPA+ is clealy an evolution over what we have. What about LTE? Is it an evolution or something completely new? If it is the latter it will take time.