Because with increased speed comes increased bandwidth. Increase the bandwidth and you increase the speed for everyone. Ever wonder why at night your home internet connection is faster even though the rated speed stays the same? It's because less people are using it. So, more bandwidth = more speed. Same is true for mobile connections.
Ok, I don't know much about this but I want to understand the mechanic behind how it's actually working.
In HSPA, a subframe consists of 2560 chips per timeslot, which is 2/3rds of a millisecond. With a TTI of 2 ms that gives us 3 subframes per TTI (2/ (2/3)) for 7680 chips per 2 milliseconds or 3.84 million chips per second. Presuming the very best error correction rate (ie code rate = 1) - that is to say radio conditions are so good that we're not using error correction at all - something that will never happen in reality lets look at what we have.
Every 2 milliseconds the tower decides how to divide up the bandwidth it has available and it does this via a mechanism called CDMA. You can take any channel and code it with a rate higher than the symbol rate to divide that channel into n number of equal bandwidth, where n is the spreading factor. That is to say, with a spreading factor of 1, all of the bandwidth in the channel goes to a single code. HSPA always uses SF=16 but a given receiver can use a varying number of codes to despread a given transmission.
Basically, that 2 millisecond block that the tower (and handset) are working with is divided up into 16 parts in the code domain. Those 16 parts (codes) are divided up among handsets that are going to be receiving information in that 2 ms slice, if I have 3 handsets that I want to divide bandwidth equally to, I would transmit for 5 codes to handset 1, 5 to handset 2, and 5 to handset 3.
Ok, so given that we have a 2 millisecond time block to work with and there are 7680 chips within that block, and that can be broken up 16 ways via spreading codes, how many bits are being transmitted in that 2 milliseconds within a single code?
Using 16QAM you get 4 bits per chip - there are four possible positions in the constellation (the combination of phase and amplitude) that the signal can be in. So, 7680 chips * 4 = 30720 bits per 2 milliseconds and that is broken up into 16 equal parts for 1920 bits per code per 2 milliseconds. Since there are 500 of those 2 millisecond blocks in a second that's 960,000 bits per second per code.
Early on, a handset could receive (despread) 5 codes of the 16 so that made the maximum possible bandwidth 4.8 megabits / second, then later on 10 codes could be despread for a maximum bandwidth of 9.6 megabits per second and then finally 15 codes could be despread (on the G2 and mytouch 4G) for 14.4 megabits per second.
Note that in reality there's another factor that's being left out, and that's the code rate - how much of a given block of bandwidth goes to error prevention - the lower the code rate (more bits used for error correction) the more immune the transmission is to noise and other problems.
So HSDPA defines several different categories for the user equipment. One of these categories can despread 5 codes and uses a 3/4 code rate which gives the very highest bandwidth that the handset can actually receive (not counting error correction) as 4.8 mbps * 3/4 = 3.6 megabits per second (sound familiar)?
A higher up category can despread 10 codes and uses the same 3/4 code rate and gives 9.6 mbps * 3/4 = 7.2 megabits per second (still sounding familiar)?
And finally we get to where the mt4g and g2 are, despreading 15 codes but with a much higher code rate (35/36 I believe) for 14.4 mbps * 35/36 = 14.0 megabits per second.
A given handset's chipset is pretty fixed in it's capabilities - it can only despread so many codes for example. So for example if you have a 3.6 mbps 3G HSDPA network what that's really saying is that the tower will never try to transmit to any handset using more than 5 spreading codes. If the tower's software is upgraded to allow 10 per handset that doesn't matter for a given handset that is already chipset limited to despreading 5 codes.
So going along with all that, my question is given that a given handset can only despread a certain number of codes at a certain maximum chip rate, how is increasing the tower capability going to increase the performance of that same handset? I'm looking for what part of the handset (say a Nexus S, able to despread 10 codes at 7680 chips / 2 milliseconds with a 3/4 code rate) is being changed to allow for increased download performance?
I think 1-1.5 is good for 3G. HSPA+ I'm guessing 3-5 would be average.
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