Mobile broadband

Mobile broadband has been growing rapidly since the mobile phone networks began using the HSDPA standard in 2007, which allows much faster data download speeds than 3G provides, and it makes it cheaper for the mobile networks to deliver data to users, which allowed the mobile networks to drastically reduce the cost of their mobile data packages.

However, HSDPA is just the first of a number of advances in mobile phone systems that will see mobile broadband speeds rise sharply over the next few years. The first change will be an upgrade to the existing HSDPA standard, called HSPA+, which will allow HSDPA's maximum theoretical download speed of 14.4 Mbps to increase to 42 Mbps. But the big changes will happen when the 3.9G and later the 4G technologies are launched.

 

3G LTE

The 3G LTE (Long Term Evolution) system is likely to become the main 3.9G technology, and that will increase the maximum theoretical download speed to 326 Mbps for handsets supporting 4x4 MIMO antenna array technology, or 178 Mbps for handsets supporting 2x2 MIMO. These are maximum theoretical download speeds, though, so don't expect to actually be able to download at 178 or 326 Mbps, because it would only be possible to achieve those speeds if you were sat facing a base station transmitter at 4 a.m. with no-one else in the cell using their mobiles. It is fair to say that the download speeds will be far higher than they are at the moment on HSPDA, though, simply because the maximum download speeds are many times that of HSDPA.

 

4G / LTE-Advanced

Although there's growing confusion over the term "4G", because what used to be referred to as 3.9G (e.g. 3G LTE and other similarly-performing systems) is increasingly being referred to as 4G in the media. To be a true 4G system, however, the system is meant to fulfill the following download speed requirements:

  • 1 Gbps when stationary
  • 100 Mbps when travelling at high speed

An example of a 4G system is the Japanese mobile phone network operator NTT DoCoMo's 4G prototype system, which it has demonstrated transmitting at a speed of 5 Gbps (5,000 Mbps) to a mobile receiver travelling at 10 km/h.

However, 3GPP, which is the organisation in charge of the 3G standards, is doing the preliminary work before designing a system referred to as "LTE-Advanced", which will be a far faster version of the 3G LTE system. And with the amount of support for 3G standards around the world, LTE-Advanced will most likely become the main 4G system.

 

MIMO

The key technology that is enabling these mobile systems to achieve ultra-fast download speeds is called MIMO, which stands for 'multiple-input multiple-output', which itself refers to the fact that there are multiple antennas at the transmitting (i.e. the input) end, and multiple antennas at the receiving (output) end.

MIMO systems use a technique called 'spatial multiplexing' which refers to one high data rate signal being split up into lower data rate signals that are then each transmitted on a different transmitting antenna. The receiver receives the signals on its multiple antennas and DSP (digital signal processing) algorithms split up the received signals into the individual data streams that were originally transmitted. For a MIMO system that uses N transmitting and N receiving antennas, there are effectively N different channels all being transmitted simultaneously within the same bandwidth, and the capacity of such an NxN MIMO system is N-times higher than for a system that only uses one transmitting and one receiving antenna.

An example of a system that uses MIMO that you might already have seen is the 802.11n Wi-Fi standard, which uses two antennas for transmitting and receiving. And the reason why NTT DoCoMo's 4G prototype system mentioned above achieved its 5 Gbps transmission speed was because it used 12 transmitting and 12 receiving antennas — which increased the capacity by a factor of 12 compared to if just one antenna were used at either end.

For the mathematically minded amongst you, the capacity, CMIMO, of a MIMO system is given by the following equation in terms of the capacity of an equivalent SISO system (SISO stands for single-input single-output, so SISO refers to a system that uses one antenna at each end) and the number of transmitting antennas, N, and the number of receiving antennas, M:
 

CMIMO = CSISO min (N, M)
 

That equation shows that in order to achieve X-times the capacity of a single-antenna system, you have to use X antennas at both ends — i.e. if you used 200 transmitting antennas but there was only one receiving antenna, the capacity would be no greater than if there was one transmitting antenna.