Mobile access has recently got a turbocharger thanks to a 3G/UMTS technology called HSDPA (High Speed Downlink Packet Access), first developed in Japan and now available in many countries all over the world. This technology is able to increase the download speed of 3G phones, laptops, and PDAs by a factor between 5 and 8: e-mails with large attachments can be retrieved instantly, high quality videos can be downloaded at a very fast rate, and, why not, on-line gaming is possible wherever you want, even on the bench of your favorite park.
But what is HSDPA?
It is a new 3G standard specified by the 3GPP (3rd Generation Partnership Project), an authority in charge of defining new standards for the mobile industry. The purpose of HSDPA is to increase the downlink speed from the previous end-user value (384 Kbps, with a theoretical upper limit of 2 Mbps), to a new maximum rate of 14.4 Mbps. That’s really an awesome improvement! Downlink speed is dramatically boosted by HSDPA, whereas uplink speed is not affected; the reason of this fact is very simple: internet traffic goes mainly in one direction, from the provider to the end-user, so there is no actual need to increase the speed in the opposite direction (nevertheless, some providers has also increased to 2 Mbps the uplink speed from the original 384 Kbps value). HSDPA offers two kinds of systems, each of which has its own modulation type: the first one achieves a maximum downlink speed of 3.6 Mbps and it has to be always guaranteed, whereas the second one offers a higher downlink data rate, with a maximum value of 14.4 Mbps. Moreover, HSDPA allows a better usage of the available transmission bandwidth and increases also the quality of service (QoS, an important index related to packet data transmission systems), thus satisfying the strict 3G quality standard levels.
HSDPA technology has been already added to some models of mobile phones (an example is shown in the following picture):
A huge contribution to the success of this 3G technology, however, is due to the wide spread of the so called “internet key”, a USB device for laptop (but can be used also on desktop) computers, with a shape quite similar to that of a regular pen drive, and with all the hardware required to establish a HSDPA connection with the service provider (see an example in the following picture).
The downlink speeds supported by the current HSDPA deployments are 1.8, 3.6, 7.2 and 14.4 Mbps, but HSPA+ technology will offer a further improvement, providing a downlink speed of up to 42 Mbps.
It should be noticed that all the above mentioned data rates correspond to the theoretical values: actual speed rate might assume lower values, depending also on the traffic and on the number of end-users connected to the service.
HSDPA offers today a high downlink speed, overcoming the UMTS theoretical limit of 2 Mbps, a feature that some time ago was possible only with an ADSL (Asymmetric Digital Subscriber Line) connection. HSDPA is the natural evolution of a growing technology started with the GSM (a second generation, or 2G, mobile standard), and then followed by GPRS (2.5G), EDGE (2.75G), and UMTS (3G). Nevertheless, it is not going to be the last step of this evolution: HSUPA, a HSDPA improvement, and LTE are already ready for the next rush.
High downlink speed is achieved in HSDPA applying three main features, which are:
- HARQ (hybrid automatic repeat request): it implements an incremental redundancy mechanism transmitting the user data more times with different coding. If at the end-user side (the receiving node) a corrupted packet is detected, it is not thrown away, but stored in memory and later combined with re-transmitted packets to achieve a high level of efficiency. It should be noticed that an error free packet might be reconstructed even combining two corrupted packets: in other worlds, HARQ is a very performing algorithm.
- Fast packet scheduling at the base station: this is another interesting feature inside HSDPA. First of all, the downlink channel is shared among several end-users, and a scheduling algorithm is executed at the base station in order to transmit data to the end-users depending on the available bandwidth and on the signal quality reported by each user. In fact, each end-user node transmit five hundreds time a second the signal quality of the channel at the receiving side. Combining all these information from all the receiving nodes, the base station can decide which user will receive the data and how much data will be delivered to it; this decision is performed periodically every two milliseconds. Normally, users who have the best channel quality levels have a higher priority in the base station scheduler; in other words, fast downlink speed reserved to the best quality users is preferred to lower downlink rates for all the users.
- AMC (Adaptive modulation and coding). The base station, processing the signal quality information received by each user, can also decide which modulation scheme and coding shall be adopted. Adaptive modulation means that the modulation, and a results also the maximum bandwidth that can be achieved with it, are selected basing on how good the signal is (that is the power level at the receiving node): if the climatic conditions are favorable, a 16 QAM modulation scheme shall be selected, thus providing a high radio capacity. In theory, a combination of modulation scheme and coding can deliver a downlink rate of 14.4 Mbps.