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MIMO OFDM Testbed for future mobile networks
Scalable, adaptive MIMO-OFDM Wireless Communication System
The projects aim is the development and realisation of testbeds for the evaluation of future mobile standards based on OFDM transmission technology. The 3G-LTE Standard developed within the Third Generation Partnership Project (3GPP) provides MIMO/OFDMA transmission in the downlink. The currently proposed uplink uses SC-FDMA multiplexing.
Because of a high degree of scalability and flexibility the MIMO/OFDMA method allows optimal utilization of the available frequency resources.
The major system parameter of the downlink:
- Number of transmit antennas:
- 2
- Number of receive antennas:
- 2
- RF frequency:
- 2.6 GHz
- RF bandwidth:
- 20 MHz
- Scalable RF bandwidth:
- 1.5 … 20 MHz
- Max. datarate:
- 173 MBPS
- Number of subcarriers:
- 1200
- Modulation schemes:
- 64QAM, 16QAM, QPSK
- FEC coding:
- convolutional code CR ½, ¾
- Interfaces:
- Gigabit Ethernet, USB
Using OFDMA allows flexible assignment of frequency resources to individual users. The available spectrum is therefore divided into resource blocks. Each resource block contains 12 neighboured subcarriers in frequency direction for the durability of one subframe (1ms). Ten subframes make one radioframe.
Adaptive modulation and coding of each resource block can be carried out for several users. The resource scheduling at the basestation is based on the channel quality information (CQI) given as feedback from the UEs (User Equipment).
The figure depicts the method of adaptive modulation depending on real channel characteristics. Fading refers to the distortion of parts of the disposable spectrum. According to the associated resource blocks the modulation is downshifted or completely switched off.
The illustration is an example for a 2x2 MIMO transmission downlink for two users with link adaptation. The scheduler at the basestation allocates the resource blocks to several users on the basis of CQI, data rate needs for certain applications (e.g. Internet, Voice, Videostreaming) as well as Quality of Service aspects.
Testbed Realisation
The baseband signal processing was implemented on the in-house FPGA platform FFP Basic+. The major part of the programming was done in VHDL, but floating point operations have been realised with the help of additional DSPs. Using programmable hardware allows continuously adaptation and extension of the testbed as well as realisation of new modes of operation.
The testbed is used both for measurement in real broadcast channels and demonstration issues. IP-based applications e.g. Video-Streaming can be transmitted mobile at high data rates.
The figure shows the testbed during a presentation at 3GSM Exhibition in Barcelona February 2007.
A custom-built analysis software permits access to measurement data and realtime monitoring of significant system parameter. The Microsoft Windows based tool reads out the data periodically from an integrated USB interface.
The figure illustrates the measurement results of a 2x2 MIMO transmission with link adaptation.
The two constellation diagrams show the reconstructed receive signals of all resource blocks. Note that due to the overlayed display of all resource blocks different modulation schemes are visible in the diagram (16 QAM and 64QAM).
The diagrams on the right side show the channel transmission functions of the four existing signal components in the case of a 2x2 MIMO transmission.