Transcript Session Title

Wireless for Miniaturized
Consumer Electronics
Part II: MIMO or SISO? Wireless
Design Considerations and Trade-offs
15-Jan-2013
Fanny Mlinarsky
1
Outline
•
•
•
•
Multipath in the wireless channel
What’s MIMO?
MIMO vs. SISO performance
Cost/performance/power consumption tradeoffs of different design approaches
2
Wireless Channel
Multipath clusters
Per path angular spread
Composite angular spread
Multipath and Doppler fading
in the channel
3
Cyclic Prefix ↔ Guard Interval
Guard interval > delay spread in the channel
Useful data
TS
copy
• 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.
4
• Frequency and time variable wireless
channel
• Multipath creates a sum of multiple
versions of the TX signal at the RX
… …
Channel Quality
Wireless Channel
Frequency
Frequency-variable channel
appears flat over the narrow
band of an OFDM subcarrier.
OFDM = orthogonal frequency division multiplexing
5
Antenna Diversity
RX
RSSI
TX
Comparator
Antenna selection
RSSI
preamble
data
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 = receive signal strength indicator
6
NxM
MIMO systems are typically described as NxM, where N is the
number of transmitters and M is the number of receivers.
TX
TX
2x2
MIMO
radio
channel
RX
RX
TX
TX
RX
RX
2x2 radio
2x2 radio
7
MIMO vs. SISO Throughput
Draft 802.11n vs. Legacy - Home
Draft 802.11n vs. Legacy - Office
180
160
140
120
100
80
60
40
20
0
Mbps
Mbps
Draft-802.11n
Legacy 802.11g
Draft-802.11n
6 ft
110
40
Measured by octoScope
150
180
200
180
160
140
120
100
80
60
40
20
0
25
feet
35
45
50
feet
Vendor 1
Vendor 2
MIMO = multiple input multiple output
SISO = single input single output
8
Legacy 802.11g
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
9
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
10
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.
Typical 2stream MIMO
channel
Typical SISO channel
Under average
channel conditions
MIMO gain may be
only ~ 20%.
Credit: Moray Rumney
Agilent Technologies Inc.
SNR = signal to noise ratio
α = TX antenna correlation
β = RX antenna correlation
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)
12
Modern Radio DSP
Radio 1
IP
MAC
Baseband
Radio 2
Multiple streams?
MRC?
TX diversity?
DSP
IP
MAC
DSP
RF
DSP = digital signal processing
MRC = maximal ratio combining
MAC = medium access control
IP = internet protocol
Baseband
RF
Decisions on signaling are made
by Baseband DSP based on
channel conditions, such as SNR
and correlation.
13
Beamforming
Beamforming increases
network capacity by
enabling spatial
multiplexing.
Focused RF beam forms
by combining radiation
from multiple phaselocked 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
14
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
15
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.
16
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.
Channel models
Wireless channel
emulation involves
emulating multipath
reflections and
Doppler fading due
to mobile reflectors
or moving radios.
Radio 1
Radio 2
IP
IP
MAC
MAC
Baseband
Baseband
RF
Wireless channel emulator
17
RF
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/
18
802.11n Channel Models - Summary
Model [1]
A*
B
C
D
E
F
Distance to 1st
wall (avg)
test model
Residential
small office
typical office
large office
large space
(indoor or outdoor)
5m
5m
10 m
20 m
30 m
# taps
Delay
spread
(rms)
Max delay # clusters
1
9
14
18
18
18
0 ns
15 ns
30 ns
50 ns
100 ns
150 ns
0 ns
80 ns
200 ns
390 ns
730 ns
1050 ns
* Model A is a flat fading model; no delay spread and no multipath
19
2
2
3
4
6
Cluster Example – Model D Profile
3 overlapping clusters of Model D
20
MIMO vs. SISO Design Tradeoffs
MIMO
SISO
Requires more power for multiple radios, but
transmits information faster, thus balancing out
power consumption
Lower power consumption, but may require
more retransmissions that consume power
Longer range and faster throughput, but at the
cost of power consuming DSP
Smaller size
Affordable for infrastructure devices, such as
APs and base stations
Less expensive
May yield little gain in high SNR, low
correlation conditions, but sophisticated DSP
can optimize range and throughput
SISO client still benefits from MIMO
infrastructure (APs and base stations) in terms
of range and throughput
A compromise is to have a single TX and 2 MIMO RX in a low power client device.
21
Next Session
• Part III: Bluetooth
• Wednesday, January 16th 2013
• 12 pm EST
Visit octoScope publications for more material
22