Technology
Requirements
The requirements imposed on future broadband wireless access systems include support of high data rates, operation in a hostile multipath environment, provision of various QoS levels, and minimal consumption of the limited resources such as bandwidth and transmit power. The most promising technologies that satisfy these demands include Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple Input Multiple Output (MIMO).
OFDMA
OFDMA concept roots back to Orthogonal Frequency Division Multiplexing (OFDM), a transmission scheme that partitions the available bandwidth into a number of narrowband subcarriers. In order to overcome frequency selective fading causing intersymbol interference, the bandwidth of each subcarrier is chosen to be sufficiently smaller than the coherence bandwidth of the channel. Therefore, flat fading subcarriers can be easily corrected in the frequency domain. OFDMA is in fact a form of Frequency Division Multiple Access (FDMA), where a user may be assigned one or more subcarriers (equivalent to FDMA channels) in order to satisfy its traffic requirements. The key advantage of OFDMA is that it exposes the frequency domain for adaptive processing. This may, for example, include exploitation of the multiuser diversity: a piece of spectrum (i.e., a group of subcarriers) that is of low quality to one user, can be of high quality to another user and should be allocated accordingly. This level of adaptivity is inaccessible by any of the CDMA-based radio interfaces, where the goal is to spread the signal in the frequency domain rather than make any adaptations.
MIMO
In MIMO systems, by having multiple antenna elements available at the transmitter and at the receiver, we expose yet another natural domain for adaptive processing, namely the space. We can exploit the spatial domain either to enhance the transmission robustness or to increase the system capacity. Robustness is enhanced by realizing spatial diversity schemes. Capacity can be increased by creating multiple, separate, and distinguishable links within the same time and frequency resources with the use of multiple antennas and appropriate signal construction. This is also referred to as spatial multiplexing. MIMO systems are particularly attractive when combined with OFDM/OFDMA. This is because equalization in case of OFDM/OFDMA is simple and elegant. Moreover, signal processing and adaptive resource allocations can now be realized in the three dimensions, namely: time, frequency and space.
Conclusions
OFDMA-based radio interfaces, after loosing the battle for 3G, appear to be winning the entire war for most of the beyond-3G systems. OFDM/OFDMA is the key element of the majority of the modern communication systems, including cable (xDSL), broadcasting (DAB, DVB), Wireless LAN (IEEE 802.11) and beyond-3G broadband access systems such as 3GPP-LTE and WiMAX together with their future 4G evolutions. In our daily operation we give special attention to the latter two systems.
LTE
The 3GPP Long Term Evolution (LTE) is a project aiming at future evolutions of UMTS. The new, evolved radio access interface is referred to as Evolved UTRA (E-UTRA), while the new radio access network – Evolved UTRAN (E-UTRAN). Often, the terms LTE and E-UTRA or E-UTRAN are used interchangeably. 3GPP is responsible for standardization of E-UTRAN, as well as UTRAN and GERAN. It was formed in 1998 from a group of standard-developing organizations, including ETSI (European Telecommunications Standard Institute). The work on the LTE standards started in November 2004. In March 2009, the specs defining the E-UTRAN within 3GPP Release 8 were frozen.
Release 8 E-UTRA radio interface uses OFDMA and MIMO in the downlink. In the uplink however, yet another transmission scheme is used, namely Single-Carrier FDMA (SC-FDMA). Fortunately, it bears many similarities to OFDMA. Both downlink and uplink utilize the concept of adaptive modulation (QPSK, 16-QAM, 64-QAM). Only packet-switched transmission is supported and the radio resources are dynamically allocated by the resource manager (also referred to as the scheduler). Various channel bandwidths can be used, from 1.4 MHz, up to 20 MHz. The radio access network has a flat architecture, which improves system reactivity to the environmental changes and shortens the delays of packet delivery.
The next 3GPP releases, namely 9 and 10 (LTE Advanced), will still be based on OFDMA, SC-FDMA and MIMO. It is expected that the role of these schemes will increase, since further gains will have to be found in their efficient usage.
WiMAX
The IEEE 802.16 is a family of standards defining the PHY and MAC layers of the wireless metropolitan area network. They are often referred to as WiMAX. The WiMAX system is supported by the WiMAX Forum, which is the industry institution gathering operators, equipment providers, and component manufacturers. There are the two key releases of the WiMAX specs. The first one is the IEEE 802.16d, which was completed in June 2004 and is also known as fixed WiMAX. The second one is the IEEE 802.16e, completed in December 2005, and known as mobile WiMAX. Further, we narrow our consideration to mobile WiMAX only.
Mobile WiMAX radio interface is based on the application of OFDMA and MIMO in both the downlik and the uplink. The IEEE 802.16e allows for bandwidths from the range of 1.25 – 20 MHz, however, the WiMAX Forum limited the channel bandwidths to the range from 3.5 MHz to 10 MHz. Similarly to the LTE, the concept of adaptive modulation and coding is used. Also, only the packet-switched transmission is supported and the radio resources are dynamically allocated by the resource manager (i.e., the scheduler). The radio access network has a flat architecture in this case as well.
The next IEEE 802.16 release, namely 802.16m, similarly to the LTE case, will not introduce significant changes to the basic organization of the radio interface. It will still be based on the same two key techniques as 802.16e that is OFDMA and MIMO.