WiMAX Technology

WiMAX is a wireless access to the Internet technology whose aim is to provide broadband access to the network to end users, mainly in cities. It is based on the American IEEE 802.16 and European standards ETSI HiperMAN. WiMAX is an alternative to wireless networks, especially in areas where telecommunication network structure is poorly developed. WiMAX provides optimal solutions of the so-called last-mile problem, i.e. leading the link to its end user. Both standards (European and American) make creation of many base stations` configurations possible, which, in consequence, may cause a situation in which devices made by different producers may not co-operate with each other. That is why WiMAX`s task is to standardize the method of devices` configuration - to solve this problem. The discussed solutions are supposed to guarantee the possibility of co-operation of the devices both with and without direct optical visibility of the aerials and they also give operators the possibility to extend their range of access to the Internet services with mobility by creating competitive solutions (VoIP) for mobile networks. Solutions described in the hereby article refer to 802.16 standard and that is why it is worth to begin with presentation of the standards in-question. It was created from 1999 and, as one may expect, many versions of it have hitherto been created. At the moment two of them are valid:

- 802.16-2004 finished in 2004, it offers access to the network for terminals that are not on the move.

- 802.16e finished in 2005, it offers access to the network for terminals that are either stationary or on the move. Certification of devices that comply with this standard should begin in 2007. As I have already mentioned, the technology does not require direct visibility of the aerials, therefore it uses the NLOS model of radio waves propagation ( Non Line of Sight). Application of such a model created certain problems for the standard creators, the most important one being extension of reach of the system. Given lack of optical visibility, multiway propagation takes place, which means that many various signals reach the terminal, signals with various delay times, disturbed signals and with polarity different than in the direct signal. These phenomena cause a considerable decrease of the received power. That is why, in order to improve the system parameters, a number of solutions such as those presented below was applied:

OFDM (Orthogonal Frequency Division Multiplexing): It is a multiplexing technique in which every data portion is sent on a separate available subcarrier, into which the available spectrum is divided, i.e. the data stream is divided into many parallel smaller sub-streams. The subcarriers, each of which transmits one sub-stream, are orthogonal towards each other. In other words, they do not disturb each other despite the fact that they overlap. This solution is resistant to multiroad propagation and in case of degradation of one of the subcarriers, the whole data stream is not lost, only the part transmitted on the given subcarrier. Basing on OFDM, a whole OFDMA method can be created, used optionally in the upward connection, in which every user is allocated with a specific number of subcarriers out of the total number of subcarriers for transmission.

Division into sub-channels: such division is optional in the upward link. Without the division into sub-channels, construction of price-effective terminals would be difficult to realize. For example, if the terminal wanted to transmit in the same way as the base station, then it would require great power amounts and complex broadcasting systems which would result in higher expenses. Reduction of terminal`s transmitter`s power to ex. 25% of the base station transmitter`s power can be achieved in two ways: either by using all of the subcarriers and reducing power of each of them by 75% or by using division into sub-channels, i.e. transmit on every fourth subcarrier with the same power as the base station. Both solutions require the terminal to transmit with equal power. The second solution is obviously far better and its only drawback is fourfold reduction of the upward link`s speed. However, thanks to this, the terminal may transmit with far lower power than the base station and therefore, its costs decrease.

Application of aerial techniques:

- AAS: such aerials make it possible to direct the aerial beam in a given way. During the signal transmission light from the reflector may be directed at a given user. During the transmission, AAS can direct beams only in the direction from which the transmission comes.

- MIMO: application of numerous aerials for signal transmission at both ends of the radio connection (i.e. in the terminal and in the base station). This contributes to considerable speed increase.

- Adaptor modulation. This technique consists in the choice of the right modulation, depending on the distance between the terminal and the base station. The greater the distance, the more susceptible the signal is to degradations. Therefore, modulations more resistant to disturbances are used at the expense of lower transmission speed.

Others:

-Separation of transmission and receipt
-Proper detection and transmission errors correction techniques
-Power steering

See also: WiMAX Technology - Architecture