Microwave Engineering Europe - April 2008 - (Page 30)

30 MIMO Advanced receiver design boosts performance By Wen Tong, Nortel M ultiple input/multiple output (MIMO) is the foundational technology underlying all current 4G mobile broadband standards, including WiMax LTE and UMB. An optimum spatial-multiplexing receiver based on the exhaustive-search maximumlikelihood decoder. To achieve higher throughput and a richer user experience, especially for mobile access to high-bandwidth applications such as YouTube, video gaming and movie downloads, devices must provide optimal reception performance for MIMO signals. MIMO implementations on most current devices, however, offer only suboptimal reception. Using advanced DSP techniques and ASIC technology, the maximum-likelihood decoder (MLD) receiver for spatial multiplexing of the MIMO signal can improve performance. MIMO antenna-processing technology has been adopted by all mobile broadband standards for 4G macrocellular networks, as well as by IEEE 802.11n for Wireless LANs. By creating multiple parallel, independent data streams among multiple transmit and receive antennas, MIMO increases the transmission rate without increasing the spectrum bandwidth or transmit power. The increase in data rate is proportional to the number of transmit antennas used, and the total transmit power is divided among the antennas. Therefore, if n transmit antennas are used, n independent streams of the data are transmitted, and data throughput is increased n times. The technique is called spatial multiplexing. On the receive side, multiple antennas and receive radios are employed. The transmit signals propagate through a MIMO channel, and signals from all of the transmit antennas are mixed at each receive antenna. The concurrent receptions of the mixed signals are processed at the receivers to recover the parallel transmitted data streams. Because of cost and space limitations on the device, a two-transmit and two-receive spatial multiplexing conﬁguration for downlink transmission will be widely used for MIMO applications. The mass market of 4G-capable devices will employ two receive antennas because of inherent power and space constraints. The pivotal design challenge for creators of MIMO receivers is leveraging the advanced DSP algorithms and architectures to achieve the best performance while reducing power consumption. MIMO receiver technologies The major constraints in implementing the spatial multiplexing MIMO technology are the cost of multiple transmit and receive radio chains and the form-factor limitation of multiple antennas for handheld devices. By making use of advances reﬂective of Moore’s Law, sophisticated signalprocessing techniques can be applied at the baseband processing stage to achieve high-performance MIMO transmission. These can be implemented in the ASIC to reduce the cost and power consumption of the device. A number of MIMO receiver algorithms can be used, depending on the receiver complexity: Zero-Forcing (ZF), Minimum Mean Square Error (MMSE) and MIMO MLD. Fundamentally, the MIMO channel is a broadcast channel. In this case, the individual data streams transmitted from each antenna are mixed with other transmit data streams at each receive antenna. To separate the mixed data streams at the receiver, designers can multiply the mixed-stream received signals to a matrix that is the inverse of the MIMO channel matrix. This is called a ZF receiver. It is a simple MIMO receiver that often suffers performance loss at higher noise and interference levels at the receivers. Microwave Engineering Europe ● April 2008 ● www.mwee.com 030-031-032-033_MWEE.indd 30 27/03/08 15:57:00 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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