MIMO-OFDM Design using LabVIEW
Orthogonal Frequency Division Multiplexing (OFDM) is one of the most promising physical layer technologies for high data rate wireless communications due to its robustness to frequency selective fading, high spectral effciency, and low computational complexity. OFDM can be used in conjunction with a Multiple-Input Multiple-Output (MIMO) transceiver to increase the diversity gain and/or the system capacity by exploiting spatial domain. Because the OFDM system effectively provides numerous parallel narrowband channels, MIMO-OFDM is considered a key technology in emerging high-data rate systems such as 4G, IEEE 802.16, and IEEE 802.11n.
MIMO communication uses multiple antennas at both the transmitter and receiver to exploit the spatial domain for spatial multiplexing and/or spatial diversity.
Spatial multiplexing has been generally used to increase the capacity of a MIMO link by transmitting independent data streams in the same time slot and frequency band simultaneously from each transmit antenna, and differentiating multiple data streams at the receiver using channel information about each propagation path.
In contrast to spatial multiplexing, the purpose of spatial diversity is to increase the diversity order of a MIMO link to mitigate fading by coding a signal across space and time so that a receiver could receive the replicas of the signal and combine those received signals constructively to achieve a diversity gain.
LabVIEW Resources for MIMO and OFDM simulation
LabVIEW MIMO-OFDM simulator with variable pilot-to-data power ratio (PDPR)
In MIMO-OFDM systems, channel state information (CSI) is essential at the receiver in order to coherently detect the received signal and to perform diversity combining or spatial interference suppression. The channel is very important to the performance of diversity schemes, and more variable channels give more diversity. Thus, in order to attain accurate CSI at the receiver, pilot-symbol-aided or decision-directed channel estimation must be used to track the variations of the frequency selective fading channel. Among the various resources in MIMO multicarrier systems the power assignment is related to the accuracy of the channel estimation. Pilot symbols facilitate channel estimation, but in addition to consuming bandwidth, they reduce the transmitted energy for data symbols per OFDM symbol under a fixed total transmit power condition. This suggests a tradeoff between the system capacity and the accuracy of the channel estimation in MIMO-OFDM systems according to the power allocation when the total transmit power is fixed.
In order to model the MIMO-OFDM system in a graphical and fast simulation, we have developed the MIMO-OFDM simulator in the LabVIEW simulation package from National Instruments. By using this simulator, one can see the bit errror rate (BER) performance of the system and the channel capacity lower bound according to a given PDPR with three different types of pilot patterns.
Click here to download the LabVIEW library (mimoofdm.llb). You can access the tutorial (pdf version) of this LabVIEW simulator by clicking here. The simulation library contains about 20 sub VI's which can be reused for similar applications.
Types of VIs contained in the simulation library are briefly explained:
Other LabVIEW resources for MIMO and OFDM systems
There are some other LabVIEW simulation toolkits for MIMO and OFDM systems.
You can download the LabVIEW MIMO toolkit develped by Prof. Heath's research group. This tool kit offers a flexible tool to simulate MIMO systems.
Also, there is an IEEE 802.16a simulator developed by Prof. Evans' research group. It provides a graphical user interface and basic functionality of an IEEE 802.16a 256-subcarrier OFDM system. It is a discrete baseband simulation of the necessary modulation and demodulation functions, that operate over the Stanford University Interim (SUI) fixed broadband wireless access channel models. You can find more information from the link.