Microwave Engineering Europe - April 2008 - (Page 31)

MIMO 31 To minimize performance loss, the inversion of the MIMO channel matrix operation is adjusted according to the interference or noise level. This is called an MMSE receiver. But both the ZF and MMSE receivers typically have up to 2 dB performance loss compared with MLD receivers. Improved reception of spatial multiplexing MIMO transmission requires an exhaustive search of MIMO signal constellation combinations. The MIMO transmission process is emulated at the receiver in such a way that a speciﬁc complex modulation constellation is generated for each transmit antenna stream. The constellations are then applied to the MIMO channel input. At the MIMO channel output, the corresponding MIMO reception signal is generated for each receive antenna. At this point, the Euclidean distance is computed for the emulated MIMO output signal against the received signal. In this way, the different modulation constellations at MIMO channel input constitute different hypothesis tests. If all possible combinations of constellations at MIMO channel input can be emulated, their associated MIMO channel outputs can be generated, and the Euclidean distance can be computed against the received signal. The minimum Euclidian distance associated with the constellation combination hypothesis provides the most likely decoding. Such an exhaustive constellation search at MIMO input is the optimal MIMO receiver. Optimal receiver performance, however, comes at the cost of receiver complexity. Performing all available, such as the sphere decoder algorithm. MLD-based MIMO receivers have many advantages compared with ZF and MMSE receivers. The MLD receiver’s performance advantages are signiﬁcant indeed when the MIMO channels are correlated; small-form-factor user devices require a compact package of multiple antennas, causing a high level of antenna correlation as well as a correlated MIMO channel. The ZF and MMSE receivers, conversely, can both suffer sharp performance losses when MIMO channels are correlated. In addition, because of the limitations of small-form-factor user devices, it is difﬁcult to achieve the same antenna gain for the two receive antennas embedded in the device. In the case of 4-dB antenna gain imbalance, the performance loss for the MLD receiver is negligible. Another advantage is diversity gain. The MLD receiver not only can separate the transmit data streams but also can achieve the receive diversity gain for multiple receive antennas. Further, the receiver can support a very high mobility environment, including reception at speeds exceeding 120 km/hour. Reception of the spatial multiplexing MIMO transmissions can be optimized by selecting the best receiver operation, based on the MIMO channel condition and the modulation constellation, to reduce the average processing power required. This Microwave Engineering ● April 2008 ● www.mwee.com 030-031-032-033_MWEE.indd 31 27/03/08 15:57:12 http://www.lairdtech.com/wirelesssystems http://www.lairdtech.com/wirelesssystems http://www.mwee.com

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Microwave Engineering Europe - April 2008 News Contents Comment Test and Measurement: Comprehensive WiMAX and Wi-Fi Product Design Demands Effective Channel Emulation Military/Aerospace Focus: Hardware Needs Limit Software Radio Interview — Mitsubishi Electric Europe: GaAs Technologies Spanning High-End Space and Radar Through to Cost-Sensitive Handset and LNB Applications How Do You Test ZigBee Transmitters? Advanced Receiver Design Boosts Performance CMOS PAs Pave the Way for One-Chip Phones Products Calendar

Microwave Engineering Europe - April 2008

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