15-Jan-2013 Fanny Mlinarsky

1. Wireless for Miniaturized Consumer Electronics Part II: MIMO or SISO? Wireless Design Considerations and Trade-offs 15-Jan-2013 Fanny Mlinarsky

2. Outline • Multipath in the wireless channel • What’s MIMO? • MIMO vs. SISO performance • Cost/performance/power consumption trade-offs of different design approaches

3. Wireless Channel Multipath clusters Per path angular spread Composite angular spread Line of sight Multipath and Doppler fading in the channel

4. Cyclic Prefix ↔ Guard Interval Guard interval > delay spread in the channel Useful data • The OFDM symbol is extended by repeating the end of the symbol in the beginning. This extension is called the Cyclic Prefix (CP) or Guard Interval (GI). • CP is a guard interval that allows multipath reflections from the previous symbol to settle prior to receiving the current symbol. CP has to be greater than the delay spread in the channel. • CP minimizes Intersymbol Interference (ISI) and Inter Carrier Interference (ICI) making the data easier to recover. TS copy

5. Wireless Channel • Frequency and time variable wireless channel • Multipath creates a sum of multiple versions of the TX signal at the RX Channel Quality … … Frequency-variable channel appears flat over the narrow band of an OFDM subcarrier. Frequency OFDM = orthogonal frequency division multiplexing

6. Antenna Diversity RX During preamble, receiver recovers data and determines whether the preamble is valid. If it is valid, RX uses RSSI from both antennas to select the one that has a stronger signal. Preamble needs to be long enough for the receiver to decode and evaluate the signal from both antennas. RSSI TX Comparator Antenna selection RSSI preamble data RSSI = receive signal strength indicator

7. NxM TX TX 2x2 MIMO radio channel MIMO systems are typically described as NxM, where N is the number of transmitters and M is the number of receivers. RX RX TX TX RX RX 2x2 radio 2x2 radio

8. MIMO vs. SISO Throughput Measured by octoScope Vendor 1 Vendor 2 MIMO = multiple input multiple output SISO = single input single output

9. Multiple Antenna Techniques • SISO (Single Input Single Output) • Traditional radio • MISO (Multiple Input Single Output) • Transmit diversity (STBC, SFBC, CDD) • SIMO (Single Input Multiple Output) • Receive diversity, MRC • MIMO (Multiple Input Multiple Output) • SM to transmit multiple streams simultaneously; can be used in conjunction with CDD; works best in high SNR environments and channels de-correlated by multipath • TX and RX diversity, used independently or together; used to enhance throughput in the presence of adverse channel conditions • Beamforming SM = spatial multiplexing SFBC = space frequency block coding STBC = space time block coding CDD = cyclic delay diversity MRC = maximal ratio combining SM = Spatial Multiplexing SNR = signal to noise ratio

10. MIMO Signal Propagation • MIMO systems provide more than one way for the signal to be received and thus improve probability of packets being received without errors, increasing throughput and range • TX and RX diversity can increase MIMO gains Peak Null MIMO = multiple input multiple output

11. MIMO Channel Capacity Approaching 2x gain at low correlation and high SNR Variation due to antenna correlation MIMO gain is made possible by low correlation and high SNR. Under average channel conditions MIMO gain may be only ~ 20%. Typical 2-stream MIMO channel Typical SISO channel Credit: Moray Rumney Agilent Technologies Inc. SNR = signal to noise ratio α = TX antenna correlation β = RX antenna correlation

12. MIMO Gains • MIMO gains in throughput are a function of MIMO channel correlation • Lower correlation gives higher throughput • Correlation is a function of • Transmit antenna correlation • Receive antenna correlation • Channel correlation (e.g. multipath lowers correlation) • TX diversity techniques (e.g. time offsetting of two TX transmissions to emulate multipath, reduce correlation)

13. Modern Radio DSP Radio 1 Radio 2 Multiple streams? MRC? TX diversity? IP IP MAC MAC Baseband Baseband DSP DSP RF RF Decisions on signaling are made by Baseband DSP based on channel conditions, such as SNR and correlation. DSP = digital signal processing MRC = maximal ratio combining MAC = medium access control IP = internet protocol

14. Beamforming Beamforming increases network capacity by enabling spatial multiplexing. Focused RF beam forms by combining radiation from multiple phase-locked antenna elements Beamforming increases transmission range by focusing the energy in one direction. Typically base stations and APs (not UEs) have antenna arrays for beamforming. UE = user equipment

15. Beamforming and Beam Steering • Beamforming is a feature of 802.11ac and central to 802.11ad • Optimizes the range by focusing the energy between transmitting and receiving nodes

16. Channel Emulation for DSP Development Development and testing of complex DSP algorithms requires channel emulation to create a variety of channel conditions an motion scenarios in a controlled environment.

17. Wireless Channel Emulation A wireless channel emulator is a ‘black box’ that connects to antenna ports of two or more radios and inside emulates a wireless channel. Wireless channel emulation involves emulating multipath reflections and Doppler fading due to mobile reflectors or moving radios. Channel models Radio 1 Radio 2 IP IP MAC MAC Baseband Baseband Wireless channel emulator RF RF

18. Channel Emulator for Wireless Test An example of a MIMO 4x4 channel emulator that interconnects multiple SISO and MIMO devices as a system Azimuth ACE 4x4 channel emulator Source: http://www.azimuthsystems.com/products/wifi-performance-test-suite/

19. 802.11n Channel Models - Summary * Model A is a flat fading model; no delay spread and no multipath

20. Cluster Example – Model D Profile 3 overlapping clusters of Model D

21. MIMO vs. SISO Design Tradeoffs A compromise is to have a single TX and 2 MIMO RX in a low power client device.

22. Next Session • Part III: Bluetooth • Wednesday, January 16th 2013 • 12 pm EST Visit octoScope publications for more material