MIMO is applicable to all kinds of wireless communication technologies. However, the combination of MIMO and OFDM (Orthogonal Frequency Division Multiplex) has the following advantages. OFDM is adapted for multi-path propagation in wireless systems. The length of the OFDM-frames is determined by the Guard Interval (GI). This Gurad Interval restricts the maximum path delay and therefore the expansion of the network area. MIMO also uses the multi-path propagation.
OFDM is a wideband system with many narrowband sub-carriers. The mathematical MIMO channel model is based on a narrow band non-frequency selective channel. The latter is supported by OFDM as well. Fading effects in wideband systems normally occur only at particular frequencies and interfere with few sub-carriers. The data is spread over all carriers, so that only a small amount of bits get lost, and these can be
repaired by a forward error correction (FEC). OFDM provides a robust multi-path system suitable for MIMO. At the same time OFDM provides high spectral efficiency and a degree of freedom in spreading the time dimension of Space-Time Block Codes over several sub-carriers. This results in a stronger system based on the principle described previously
MIMO Standards:
Table 1 gives an overview of all current MIMO standards and their technologies. It is clear to see, that with the exception of 3GPP Release 7, all standards work with OFDM. The advantages of OFDM can obviously be linked to MIMO.
Table 1 MIMO Standards and the corresponding technology :
Standard Technology
WLAN 802.11n OFDM
WiMAX 802.16-2004 OFDM/OFDMA
WiMAX 802.16e OFDMA
3GPP Release 7 WCDMA
3GPP Release 8 (LTE) OFDMA
802.20 OFDM
802.22 OFDM
MIMO and OFDM
The 3G Long-Term Evolution RADIO INTERFACE CONCEPTS
The ability to provide high bit rates is a key measure for LTE. Multiple parallel data stream transmission to a single terminal, using multiple-input-multiple-output (MIMO) techniques, is one important component to reach this. Larger transmission bandwidth and at the same time flexible spectrum allocation are other pieces to consider when deciding what radioaccess technique to use. The choice of adaptive multi-layer OFDM, AML-OFDM, in downlink will not only facilitate to operate at different bandwidths in general but also large bandwidths for high data rates in particular. Varying spectrum allocations, ranging from 1.25 MHz to 20 MHz, are supported by allocating corresponding numbers of AML-OFDM subcarriers. Operation in both paired and unpaired spectrum is possible as both time-division and frequency-division duplex are supported by AML-OFDM.
A. Downlink – OFDM with Frequency-Domain Adaptation
The AML-OFDM-based downlink has a frequency structure based on a large number of individual sub-carriers with a spacing of 15 kHz. This frequency granularity facilitates to implement dual-mode UTRA/E-UTRA terminals. The ability to reach high bit rates is highly dependent on short delays in the system and a prerequisite for this is short sub-frame duration. Consequently, the LTE sub-frame duration is set as short as 0.5 ms in order to minimize the radio-interface latency. In order to handle different delay spreads and corresponding cell sizes with a modest overhead the OFDM cyclic prefix length can assume two different values. The shorter 4.7 ms cyclic prefix is enough to handle the delay spread for most unicast scenarios. With the longer cyclic prefix of 16.7 ms very large cells, up to and exceeding 120 km cell radius, with large amounts of time dispersion can be handled. In this case the length is extended by reducing the number of OFDM symbols in a sub-frame.
OFDM is suitable for broadcast services. To support such services the same information is transmitted from several (synchronized) base stations to the terminal. The total signal the terminal receives from the base stations will appear as multipath propagation and thus implicitly be exploited by the OFDM receiver. The longer cyclic prefix allows combining
broadcast signals from a large number of distant base stations. Techniques to exploit channel variations in the time domain have been successfully implemented for HSDPA. This has resulted in a substantial increase in spectral efficiency. For EUTRA, the channel-based adaptation can be extended to also include transmission adaptation in the frequency domain thanks to the use of OFDM. When the radio channel varies significantly over the system bandwidth, large performance gains can be achieved.
B. Uplink – Single-Carrier FDMA with Dynamic Bandwidth
A key requirement for uplink transmission is that the transmission should allow for power-efficient user-terminal transmission to maximize coverage. To reach this, single-carrier frequency-division multiple access (FDMA) with dynamic bandwidth is a good choice. In order to achieve intra-cell orthogonality, the base station assigns a unique time-frequency interval to the terminal for the transmission of user data. This is done for each time interval. The users are separated primarily by time-domain scheduling; however, if the terminal has a limited transmission power or not enough data to transmit, also frequency-domain scheduling is used.
C. Multi-Antenna Solutions
Advanced multi-antenna techniques will play an important role in fulfilling the 3G LTE requirements on increased data rates and improved coverage and capacity. This includes both beamforming and multi-layer transmission solutions to better exploit the potential of using the spatial domain. This potential is large and not always fully exploited in existing radio access technologies. Increasing data rates can be achieved by transmitting multiple parallel streams or layers to a single user. This Multi-layer transmission is often referred to as MIMO. The preferred use for MIMO is in conditions with favorable signal-to-noise ratio and rich scattering in the radio channel, e.g., small cells or indoor deployments. Multi-layer transmission may be applied for downlink as well as uplink transmission. The receiver has the possibility to separate the multiple data streams by using the channel properties and knowledge of the coding scheme. In order for the receivers to solve this task it is necessary to standardize the multi-layer transmission scheme selected for the long-term 3G. Selective per-antenna rate control (S-PARC) is an interesting technique where the number of layers and the data rate per layer, is adapted to the instantaneous channel conditions.
Beamforming implies that multiple antennas are used to form the transmission or reception beam and, in this way, increase the signal-to-noise ratio at the receiver. This technique can both be used to improve coverage of a particular data rate and to increase the system spectral efficiency. The increased signal-to-noise ratio is not only due to a larger gain in the direction of the desired user, but also due to a better control of the spatial interference distribution in the cell. Beamforming can be applied both to the downlink and the uplink. It is possible to make beamforming transparent to the terminal, which would eliminate the need to standardize a particular solution. Instead, the exact algorithms can evolve over time and be tailored to particular needs. Alternatively, one could include some explicit support for a specific beamforming solution, especially if that would increase the efficiency of the system and enable low complexity implementations.
It is also possible to combine multi-layer transmission and beamforming. An example of this would be to transmit two data streams with two groups of antennas, where beamforming is employed within each group. Beamforming is then used to increase the received signal-to-noise ratio and multi-layer transmission is applied to convert the increased signal-to-noise ratio into a higher data rate.
Smart Antenna Technology for Mobile WiMAX
Smart antenna systems refer to a class of antenna technologies designed to improve the received signal strength in a wireless access network. The intent is to improve the carrier to-interference plus noise ratio (CINR). The use of ‘smart’ antenna technologies can both increase received signal strength and decrease interference levels to greatly enhance performance in a mobile communication network
Smart antenna technologies involve complex vector or matrix operations on signals due to the use of multiple antennas. OFDMA allows smart antenna operations to be performed on vector-flat sub-carriers. Mobile WiMAX supports a full range of smart antenna technologies to enhance system performance. The smart antenna technologies supported are:
- Space-Time Code (STC)
- Beam forming
- Spatial Multiplexing (SM)
1 Space-Time Coding
Space-time coding (STC) is a family of techniques for implementing transmission diversity. Mobile WiMAX uses transmit diversity in the downlink direction to provide spatial diversity that enhance the signal quality to a specific subscriber located anywhere within the range of the antenna beam. Although providing less signal gain than beam-forming, transmit diversity is more robust for mobile users since it does not require prior knowledge of the path characteristics of a subscriber’s particular frequency channel. One such STC technique, known as the Alamouti Code, was published in 1998 and has been incorporated in the WiMAX standard
2 Beam-Forming
The transmission of signals from several antennas at specific relative phases can be used to create a much narrower antenna beam giving rise to the name ‘beam-forming’. Beam-forming provides substantial improvement in the link budget in both the downlink and uplink directions by increasing the effective antenna gain in addition to reducing fade margin requirements due to interference. Beam-forming does require knowledge of a subscriber’s location making it more challenging to implement for subscribers moving at high speeds. According to cellular network statistics however, the majority of subscribers are either stationary or only moving at pedestrian speeds thus enabling beam-forming to provide significant benefits for most usage models.
3 Spatial Multiplexing (SM)
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