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        MIMO Technology for
Advanced Wireless Local Area Networks
                 Dr. Won-Joon Choi
                  Dr. Qinfang Sun
                Dr. Jeffrey M. Gilbert
              Atheros Communications

 2005 Design Automation Conference – June 15, 2005

   This presentation will give an overview of MIMO
    technology and its future in Wireless LAN:

   Wireless Local Area Networks (WLAN)
           Current standards (11a/b/g)
           Next-generation 11n overview and status
   MIMO fundamentals
           Beamforming
           Spatial Multiplexing
   MIMO scalability
           Bandwidth
           Number of spatial streams
      The Wireless LAN Explosion
  The Wireless LAN / Wi-Fi market has exploded!
  New technology is enabling new applications:
      Office                 Home                “Hot-spots”
Email / Info anywhere   Internet everywhere    Hot-spot coverage
   Voice over IP             Multimedia       Metro-Area Networks
Wireless LAN Technology Advances
Wireless LAN technology has seen rapid advancements
      Standards: 802.11  .11b  .11a  .11g
      Data rates: 2Mbps  100+ Mbps
      Range / coverage: Meters  kilometers
      Integration: Multiple discretes  single chip solutions
      Cost: $100’s  $10’s (sometimes free w/rebates!)

How can this growth continue?
    Previous advances have been limited to a single
       transmitting and receiving radio
      The next generation exploits multiple parallel radios
       using revolutionary class of techniques called
       MIMO (Multiple Input Multiple Output) to send
       information farther and faster
 Existing 802.11 WLAN Standards

                               802.11b       802.11a       802.11g       802.11n

Standard Approved             Sept. 1999    Sept. 1999                      ?

Available Bandwidth            83.5 MHz      580 MHz       83.5 MHz

Frequency Band of Operation    2.4 GHz        5 GHz        2.4 GHz      2.4/5 GHz

# Non-Overlapping Channels
                                  3             24            3           3/24

                                                                         1 – 600
Data Rate per Channel         1 – 11 Mbps   6 – 54 Mbps   1 – 54 Mbps
                                                                        DSSS, CCK,
                                                          DSSS, CCK,
Modulation Type               DSSS, CCK       OFDM                        OFDM,
    What Is Being Proposed for 802.11n?

Main Features


          Beamforming

          Spatial     Multiplexing
       Extended bandwidth (40MHz)
       Advanced coding
   MAC
       Aggregation
       Block ACK
       Coexistence
       Power saving
          Wireless Fundamentals I
In order to successfully decode data, signal strength needs to be
   greater than noise + interference by a certain amount
       Higher data rates require higher SINR (Signal to Noise and
        Interference Ratio)
       Signal strength decreases with increased range in a wireless

                        60                                    Data Rate 1
                        50                                    Data Rate 2

                             1   2   3   4   5    6   7   8    9 10 11 12
           Wireless Fundamentals II

Ways to increase data rate:
     Conventional single tx and rx radio systems
          Increase transmit power
               Subject to power amplifier and regulatory limits
               Increases interference to other devices
               Reduces battery life
          Use high gain directional antennas
               Fixed direction(s) limit coverage to given sector(s)
          Use more frequency spectrum
               Subject to FCC / regulatory domain constraints

     Advanced MIMO: Use multiple tx and / or rx radios!
                  Conventional (SISO)
                   Wireless Systems
  Bits          DSP   Radio                      Radio   DSP        Bits
           TX                                                  RX

Conventional “Single Input Single Output” (SISO)
  systems were favored for simplicity and low-cost
  but have some shortcomings:
        Outage occurs if antennas fall into null
           Switching between different antennas can help

        Energy is wasted by sending in all directions
           Can cause additional interference to others

        Sensitive to interference from all directions
        Output power limited by single power amplifier
                MIMO Wireless Systems

                    Radio                          Radio
                                  channel                  D
  Bits          S                                          S        Bits
                P   Radio                          Radio   P

           TX                                                  RX

Multiple Input Multiple Output (MIMO) systems with multiple
  parallel radios improve the following:
        Outages reduced by using information from multiple antennas
        Transmit power can be increased via multiple power amplifiers
        Higher throughputs possible
        Transmit and receive interference limited by some techniques
                 MIMO Alternatives
There are two basic types of MIMO technology:
     Beamforming MIMO
        Standards-compatible techniques to improve the range of
         existing data rates using transmit and receive beamforming
        Also reduces transmit interference and improves receive
         interference tolerance

     Spatial-multiplexing MIMO
        Allows even higher data rates by transmitting parallel data
         streams in the same frequency spectrum
        Fundamentally changes the on-air format of signals

            Requires new standard (11n) for standards-based operation

            Proprietary modes possible but cannot help legacy devices
    Beamforming MIMO Overview
Consists of two parts to make standard 802.11 signals “better
Uses multiple transmit and/or receive radios to form coherent
802.11a/b/g compatible signals
        Receive beamforming / combining boosts reception of
         standard 802.11 signals

                                                  Radio           D
                                                                  S         Bits
 Bits           Radio
          TX                                                      P

   Phased array transmit beamforming to focus energy to each
                D       Radio
 Bits           S
                P                                         Radio            Bits
                        Radio                                         RX
           Benefits of Beamforming
     Power gain (applicable only to transmit beamforming)
          Power from multiple PA’s simultaneously
           (up to regulatory limits)
          Relaxes PA requirements, increases total
           output power delivered
     Array gain: “dynamic high-gain antenna”
     Interference reduction
          Reduce co-channel inter-cell interference

     Diversity gain: combats fading effects
     Multipath mitigation
          Per- subcarrier beamforming to reduce spectral nulls
               Multipath Mitigation

   Multiple transmit and receive radios allow compensation of notches on
    one channel by non-notches in the other
   Same performance gains with either multiple tx or rx radios and
    greater gains with both multiple tx and rx radios
Spatial Multiplexing MIMO Concept

Spatial multiplexing concept:
          Form multiple independent links (on same channel) between
           transmitter and receiver to communicate at higher total data rates

                   DSP   Radio                       Radio   DSP
            Bit                                                     Bit
Bits                                                                       Bits
           Split                                                   Merge
                   DSP   Radio                       Radio   DSP
       TX                                                             RX
  Spatial Multiplexing MIMO Difficulties

Spatial multiplexing concept:
           Form multiple independent links (on same channel) between
            transmitter and receiver to communicate at higher total data rates

           However, there are cross-paths between antennas

                    DSP   Radio                        Radio   DSP
             Bit                                                      Bit
Bits                                                                         Garbage
            Split                                                    Merge
                    DSP   Radio                        Radio   DSP
           TX                                                           RX
   Spatial Multiplexing MIMO Reality

Spatial multiplexing concept:
           Form multiple independent links (on same channel) between
            transmitter and receiver to communicate at higher total data rates

           However, there are cross-paths between antennas

           The correlation must be decoupled by digital signal processing

                     DSP   Radio                       Radio   D
              Bit                                              S    Bit
 Bits                                                                        Bits
             Split                                             P   Merge
                     DSP   Radio                       Radio
            TX                                                        RX
    Spatial Multiplexing MIMO Theory
   High data rate
       Data rate increases by the minimum of number of transmit and
        receive antennas
       Detection is conceptually solving equations
        Example of 2-by-2 system:
            Transmitted signal is unknown, x1 , x2
            Received signal is known, y1 , y2
            Related by the channel coefficients, h11, h12 , h21, h22
                           y1  h11x1  h12 x2
                           y2  h21x1  h22 x2
            Need more equations than unknowns to succeed

   High spectral efficiency
       Higher data rate in the same bandwidth
               MIMO Scalability

   Moore’s law
       Doubling transistors every couple of years
   MIMO
       Increases number of streams
       Higher performance/speed
       Higher complexity

MIMO is the bridge to allow us to exploit
 Moore’s law to get higher performance
               MIMO Scalability

   Notation
       R: data rates (Mbps)
       Es: spectral efficiency (bps/Hz)
       Bw: bandwidth (MHz)
       Ns: number of spatial streams
       NR: number of Rx chains
       NT: number of Tx chains
                 MIMO Scalability

   Data Rates
       R = Es * Bw * Ns -> Scales with bandwidth and the
        number of spatial streams
       Example
            11a/g: Es = 2.7; Bw = 20MHz; Ns=1; R = 54Mbps
            Spatial multiplexing MIMO
              Es = 3.75; Bw=40MHz;Ns = 2; R = 300Mbps

   Number of Tx/Rx chains
       At least as many chains as Ns
         Ns = min(NR, NT)
   MIMO Hardware Requirements

MIMO Transmitter (parallelism and data rate scaling)

                       MOD                      IFFT      RF
            Stream                Spatial
            Split                 Mapping
                       MOD                      IFFT      RF

          1*           Ns *           1*          NT*      NT*
      O(Bw*Es*Ns)    O(Bw*Es)   O(Bw*Es*Ns*NT) O(Bw*Es) Analog RF
  MIMO Hardware Requirements

MIMO Receiver (parallelism and data rate scaling)

    RF        FFT                        Demod
                         MIMO                       Stream    DEC
                         Equalizer                  Merge
    RF        FFT                        Demod

   NR*         NR*           1*             Ns*        Ns*       1*
Analog RF   O(Bw*Es)   O(Bw*Es*NR*Ns2)   O(Bw*Es)   O(Bw*Es) O(Bw*Es*Ns)

   The next generation WLAN uses MIMO technology
      Beamforming MIMO technology
            Extends range of existing data rates by transmit and
             receive beamforming

       Spatial-multiplexing MIMO technology
            Increases data rates by transmitting parallel data streams

   MIMO allows system designers to leverage Moore’s law to
    deliver higher performance wireless systems
        Circuit Implications of MIMO
   Crystal
       Common crystal is required
   Synthesizer
       Common synthesizer is preferred
   PA
       Allow additional flexibility
          With total power limit, PA requirements relaxed
          With PA limit, total power increased.

   Cross-talk/ Coupling
       Need to minimize coupling between antennas
        Circuit Impairments/Corrections
   Timing offset
       Common across multiple chains
   Frequency offset
       Common across multiple chains
   Phase noise
       Common with common synthesizer
       With independent synthesizers, a new tracking
        algorithm may be needed.
   Other impairments
       1/f noise, I/Q mismatch, spurs, etc.
       Estimated and corrected for each chain
Backup Slides

               0.18um
                digital CMOS
               7.2x7.2 mm2
                die size
               15x15mm2
                BGA with 261
               Ref: ISSCC’05
                    Backup Slides
MIPS R4Kc, 16kB I and D caches     180 MHz

16b SDRAM interface                100 MHz

9b ADCs (4x)                       < 0.65 LSB INL&DNL, -48dB
                                   SNDR, 27mW
9b DACs (4x)                       <0.25 LSB INL&DNL, -51dB SNDR,
Total power, PCI mode, CPU off     690 mW

Total power, MPEG-TS mode, CPU     1.8W
Supports 802.11 a, b, g, 20 and 40 1 to 108 Mb/s raw data rates
MHz channel BW
                 Backup Slides

                         SDRAM Controller
                                                                          SDRAM and
                           and Memory        MPEG-TS                        Flash
Tx  2.4/5 GHz

                                             Local Bus
Rx Transceiver
                            MIPS Processor
                                                 I2C                        Video
                             WLAN MAC                                      Decoder

                               MRC/BF          UART             RS232
Tx  2.4/5 GHz

Rx Transceiver                                              LED Control
                             OFDM Mod/       Peripheral
                               Demod         Interface                    Host System

                                                             IR Remote
                             Radio Control   IR Interface       Control

                 WLAN SOC

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