1. Transmit processing: a viable scheme for MIMO-OFDM in 802.11n
André Bourdoux
Bart Van Poucke
Liesbet Van der Perre
IMEC, Wireless Research

2. Need for 4G High-Speed WLANs
5 m
1 Mbps
10 Mbps
100 Mbps
1 Gbps
Maximum Data rate
100 kbps
50 m
500 m
Range 1G
WLAN
802.11
1-2 Mbps 3G
WLAN
802.11a/g
6-54 Mbps 4G
WLAN
802.11n
> 100 Mbps 2G
WLAN
802.11b
Mbps
IEEE802.11n calls for 100 Mbps at the MAC SAP in a 20 MHz band
MIMO is seen as a ley technology to meet this requirement
MIMO techniques can result in rate increase and/or range increase
Higher data rates
Larger range
More users

3. MIMO-OFDM boosts performances in frequency selective environments
Higher capacity SDM, SDMA
MIMO offers:
Higher robustness Diversity (MRC, STBC)
Frequency
Magnitude
The MIMO advantages:
- Multiplexing (either SDM or SDMA) allows dramatic increase in capacity. Multiple antennas are needed at both ends of the link. Theoretical capacity improvement is approximately equal to min(NT,NR), hence linear in the number of antennas.
- Diversity (Combining diversity such as Maximum Ratio Combining or transmit diversity such as Space-time Block Coding) allows to improve the link budget by combining signals travelling over independently fading paths. Multiple antennas are needed at one side of the link only. A single stream is transmitted. There is still some capacity gain as the SNR improves, but the gain is only a logarithmic in the number of antennas.

4. A smart MIMO system adapts to scene and actual user needs
SDMA multiplies cell capacity
SDM brings higher throughput in DL/UL
MIMO-TX !
MRC brings robustness
STBC brings robustness
MIMO transmission can be used in an intelligent (adaptive) manner:
different user scenarios exist: single user, multi-user, file transfer, multimedia, upstream, downstream, etc..
Different environment: corporate, home, malls, …
Different user termanals classes and needs: single antennna, multi-antenna, power- or not power-limited, legacy terminals, etc…
MIMO-TX can play a significant role for
SDMA downlink and TX-MRC for legacy terminals
SDM downlink for multi-antenna terminals

5. A wide variety of MIMO schemes are available
“MIMO-TX” schemes, focus of this presentation
SDM s1 s2
TX-SDM
SDMA
TX-SDMA
MRC
TX-MRC
STBC
stbc
Downlink H
SDMA s1 s2
RX-SDMA
SDM
RX-SDM
MRC
RX-MRC
STBC
Uplink
MIMO-TX schemes are not the only MIMO mode. A variety of MIMO modes is possible and should be included in 11n.
The rest of this presentation focuses on specific topics for MIMO-TX
NB: RX-SDMA is not recommended because it requires a complicated pre-synchronization of the transmitters so that the OFDM symbols from different transmitters arrive with good time alignment at the AP antennas. This is especially acute in environments with large delay spreads.

6. MIMO-TX requires “TX-CSI”… H S1
MIMO TX SDM(A)
MIMO with TX pre-processing SN h S1 H
MIMO TX MRC …
MIMO with TX pre-processing
Simpler receiver (no MIMO processing needed, SISO equalizer needed)
Transmitter processing: low complexity (Zero-forcing, MMSE, MRC)
Channel H needed at TX ( “TX-CSI” )
Option 1: by feedback
Option 2: from channel estimation in reverse link, reciprocity needed only works for TDD set-ups
Reciprocity:
Propagation channel: OK if delay kept short
Front-ends: not OK, calibration needed to measure transfer function of the front-ends
MIMO-TX simplifies the receiver processing. The SISO equalizer is still needed to compensate for scaling and rotation of the constellation at the receiver.
For SDM, suitable schemes are zero-forcing or MMSE. This is just a linear processing (matrix multiplication). Non-linear schemes, dual of the successsive interference cancellation for MIMO-RX schemes, also exist but require cooperation of the receiver. Hence, they are not suitable for SDMA with legacy terminals
The rest of the slide is self-explanetory or detailed in the next slides

7. Example of Channel estimation and TX processing
DL 2 s2 AP s1
UT1
UT2
DL 1
UL1 AP
UT1
UL2 AP
UT2
Spatial dimension
Channel estimation, AGC couples H
Time
Spatial pre-filter
Example of Channel estimation procedure for MIMO-TX processing:
The intended receivers are the UTs in this example
Short uplink bursts from the UTs are used to estimate the channel at the AP side. IF the UTs are single antenna UTs (SDMA) then each UT transmits a burst in turn. If the UT is multi-antenna (SDM), a single burst is transmitted. The AGC coefficients used during the channel estimation must be known (4 coefficients for SDMA, 2 coefficients for SDM).
The estimated channel and the front-end calibration values (including AGC) are then used to compute the TX pre-filter, MMSE in this example
Parallel SDM or SDMA transmission can then take place.
H with FE. C.

8. Front-end non-reciprocity can be solved by calibration
The channel includes:
the propagation channel ( H )
front-end circuits (filters, etc..) linear and non linear ( Dxx,yy )
Base Station Receiver Front-end
From modulator
To demodulator
Terminal Transmitter Front-end
Terminal Receiver Front-end
Base Station Transmitter Front-end H
DTX,AP
DTX,MT
DRX,AP
DRX,MT
Propagation Channel
Downlink:
Uplink:
Beause the estimated channel includes the RF, IF and baseband circuits that have an impact on the frequency response, the uplink and downlink channel are not reciprocal, even in a TDD set-up. uplink and downlink channel matrices are just the transpose of each other for perfectly reciprocal channels.
It can be shown that the TX processing is only affected by the non-reciprocity at the side where MIMO-TX is applied, usually at the AP side.
To solve the problem, the front-end frequecy responses must be known at this side.
Hence, front-end calibration is required at this side only.
HDL  (HUL)T

9. Non-perfect reciprocity  MUI
Downlink model (for Inversion) :
Not diagonal anymore  Multi-user interference

10. The Base Station Transceiver alone is responsible for the MUI
Only the base station creates MUI (DTX,BS & DRX,BS)
The terminal effects (DTX,MT & DRX,MT) only create scalar complex multiplication, can be equalized
H includes all common (reciprocal) terms, including antenna coupling
Common LO is mandatory
Valid for all pre-filtering technique, including MIMO-TX, beamforming, SVD

11. OFDM-MIMO Demo Set-up 2 Terminals with 1 antenna each
Access Point with 2 antennas

12. Our advise for n
MIMO-TX and MIMO-RX schemes are both interesting for n
MIMO-TX needs channel knowledge at TX side
Estimation in reverse link has lower latency
Delay between reverse link estimation and MIMO-TX transmission must be minimized
must be supported by MAC Protocol
MIMO-TX has been demonstrated
Real-time (VHDL, 5GHz band)
Wireless, 2x2 antennas
MIMO-OFDM-SDM (108 Mbps) and MIMO-OFDM-MRC (8 dB SNR improvement)