After some basic tests to see how HSDPA performs in a number of different networks I’ve now moved to a number of somewhat more complicated scenarios. The last article in this technical series on HSDPA focused on network and terminal HSDPA behavior for an incoming call. This article shows how the network behaves when several mobiles communicate simultaneously in a single cell.
In general, Internet applications usually have bursty traffic requirements. Web surfing is a good example. The time required to load a page is typically much shorter than the reading time during which no data is transfered. When downloading large chunks of data, think YouTube, podcast download, etc. the radio channel is occupied for a much longer time which increases the chances that the radio resources have to be shared with several other users. Sharing the radio channel with other users is a strength of HSDPA as users are not assigned dedicated channels anymore as in UMTS. Instead, the cell assigns short bursts on several High Speed Shared Channels. The duration of a burst is 2 ms and the system can flexibly assign those bursts to different users.
Figure one on the left shows the throughput graph of a file transfer. In the first part, the terminal is the only one in the cell. The throughput is constant and at its maximum value for the current radio conditions at around 1.2 MBit/s. At about the middle of the file transfer, I’ve started a second file transfer from a different notebook with a different HSDPA data card. Immediately the network reacts to the second independent download and reduces the speed for the first user. Quite interesting to see that the throughput is no longer constant but varies quite a bit. It’s likely that this is the effect of changing radio conditions (fading) to which the network reacts by assigning more bandwidth to the user with better radio conditions at a given moment.
Figure two on the left is also quite interesting. It shows the inter-packet spacing graph for the file transfer which was introduced here. In summary, this graph shows the time distance between two packets. HSDPA has typical lines at 10, 20 and 30 ms, explained in more detail here. In the second part of the graph where the second user comes in, inter-packet distance suddenly jumps to 20 and 30 milliseconds. In this case this is not due to a deteriorating radio channel but due to the file transfer of the second user.
The graphs were produced with a HSDPA category 6 Motorola V3xx terminal with a theoretical maximum speed of 3.6 MBit/s. As the top speed reached in the cell at the beginning of the file transfer is 1.2 MBit/s it’s likely that the cell of the Vodafone Germany network used for the test has not yet been upgraded for 16QAM modulation.
So that’s it for today. In case your are looking for an introduction to HSDPA, have a look at chapter 3 of my book. And for more real live measurement scenarios, click on the HSDPA tag next to the date below.