beam forming, null steering, and sdma

Proprietary of NTHU Communication SOC Lab, Copyright @ 2006

Beam Forming, Null Steering, and SDMA

• Selecting the weights correctly allows transmitter (receiver) to steer the energy toward a receiver (or listen in the “direction” of a transmitter). This is called beam forming

• In selecting the weights, transmitter can also steer energy away from unintended receivers (or not listen in the direction of interfering transmitters). This is called null steering.

• Beam forming can be used to extend range

• Null steering can be used to mitigate interference from other sectors

• Beam forming and null steering can be used to implement SDMA, where multiple SSs within a sector transmit/receive on the same subchannels at the same time.


Beam Forming, Null Steering, and SDMA

Base Station

Subscriber

Beam Forming

Base Station

IntendedSubscriber

Null Steering

InterferingSubscriber

Base Station

Subscriber 1

SDMA

Subscriber 2


Channel Rejection

• Measured by setting transmitting power 3dB larger than the minimum receiver sensitivity

• Adjacent channel rejection– Conforming OFDMA signal

– At least 11 dB power above than desired signal when 16-QAM-3/4


• Non-adjacent rejection– Any channel other than adjacent channel or co-channel



• BER < 10-6


AAS Support

• Indicated by IEs in the DL and UL broadcast maps• AAS zone

– A contiguous block of OFDMA symbols

– Defined preamble structure

– May contain an optional Diversity-Map scan zone (D-Msz) • Used only with FFT size larger than or equal to 512 • Used to transmit AAS-DLFP

• AAS frame structure– Consists of subchannels

– PUSC, FUSC, oFUSC permutation• Two highest numbered subchannels of DL frame may contain D-Msz

– AMC permutation• The first and last numbered subchannels of AAS DL zone may contain D-Ms

z• A 2 bin by 3 symbol tile structure is used


AAS Support

– In a given AMC subchannel, the beam pattern for all pilot and data subcarriers is the same

– In a PUSC permutation, the SS assume the major group is beamformed• Channel may very slowly over the zone


Optional Diversity-Map scan

• AAS-DLFP (Down Link Frame Prefix)– A robust transmission of the required BS parameters

• Enable SS initial ranging• SS paging and access allocation

– QPSK-1/2, 2 repetitions

– Start with an AAS DL preamble

– Specified the permutation of AAS UL Zone

– May, but need not carry the same information

– Supports the ability to transmit a compressed DL-MAP IE

– Not randomized


AAS Network Entry

• AAS-SS synchronizes frame timing and frequency by DL preamble• If decoding of broadcast map fails, search for AAS-DLFP over

several permutations• The SS may receive DCD and UCD pointed from AAS-DLFP• Perform initial ranging using information from DCD and UCD, where

the ranging interval is pointed by AAS-DLFP• Wait the ranging response• Normal operation


AAS Preambles

• AAS preambles– Training information in both UL and DL AAS zone

– Preceding all data allocation and AAS DLFP in AAS zone

– Length is specified in the AAS_DL_IE and AAS-DLFP

– Either time or frequency shifted

• AAS DL preamble– Preamble length of AAS-DLFP is 1 symbol duration

– In PUSC permutation, preamble length is 0 or 2 symbols

• AAS UL preamble– The first Uplink_preamble_config symbols are reserved for UL AAS prea

mbles

– Inserted at the start of an UL data allocation by 3 symbol duration


AAS DL Preamble

• Formed by concatenating the original preamble sequence

• The length of basic preamble is Nused bits

• BPSK modulation• DC carrier shall not be modulated• A subset of the basic preamble is used for burst


AAS UL Preamble

• The basic preamble is the same as AAS DL preamble• A subset of the basic preamble is used for burst• Preamble power level when lower bound < C/N < upper bound

otherwise


Preamble Shift

• Time shift

• Frequency shift

K = [AAS_beam_index (mod 14)]*Nfft/14 for PUSCK = [AAS_beam_index (mod 14)]*Nfft/9 for AMC


STC Using 2 Antennas

• STC may be used on the downlink to provide higher order diversity– 2 transmit antennas on BS

– 1 reception antenna on SS

– Similarly maximal ratio combining (MRC)

• Transmit two different OFDMA symbol in the same time


STC Encoding

• Antenna: A0,A1

• Channel vector: h0, h1

• Transmission complex symbol: s1, s2

– First transmission : A0 for s1, A1 for s2

– Second transmission : A0 for –s2*, A1 for s1

*

• The estimates benefit from second order diversity as in 1Tx-2Rx MRC scheme

• May be used both in PUSC and FUSC configurations


STC2 in PUSC

• The data allocation to cluster is slightly modified– STC encoding is done on each pair of symbols 2n, 2n+1 (n = 0,1,..)


STC4 in PUSC


STC2 in FUSC• Pilot for even symbol

– A0: Variable set #0 and Constant set #0


• Pilot for odd symbol




STC4 in FUSC


Frequency Hopping Diversity Coding

• The downlink preamble shall be transmitted for the duration of one OFDMA symbol from Antenna 0

• Transmission complex symbol: s1, s2

• Antenna: A0,A1

– A0: transmits mapped carriers for subchannel X(s1) onto subchannel X, and mapped carriers for subchannel X+1(s2) onto subchannel X+1

– A1: transmits mapped carriers for subchannel X(-s2*) onto subchannel X,

and mapped carriers for subchannel X+1(s1*) onto subchannel X+1


STC Decoding

• STC using 2 antennas

• FHDC

* *1 0 0 1 1

* *2 1 0 0 1

ˆ

ˆ

s h r h r

s h r h r

*0 ,0 1 ,1 2

*1 1,0 2 1,1 1

x x

x x

r h S h S

r h S h S


Uplink Using STC (1/2)

• A user-supporting transmission using STC configuration in the uplink– 2-transmit diversity data (STTD mode)

– 2-transmit spatial multiplexing data (SM mode)

– Mandatory tile structure shall be used with modification


Uplink Using STC (2/2)

• STTD mode– The tiles shall be allocated to subchannels and the data subcarriers enu

merated

– data subcarriers shall be encoded in pairs

• SM mode (subcarrier)– Two single transmit antenna SS’s can perform collaborative spatial multi

plexing onto the same subcarrier

– A single user having two antennas may do UL spatial multiplexing• Horizontal coding - 2 bursts concurrently• Vertical coding - 1 burst (2 slots) concurrently

• SM mode (subchannel)– one SS should use the uplink tile with pattern-A while the other uses B

– Two dual antenna SS• one SS should use the uplink tile with the pilot pattern A, B• one SS should use the uplink tile with the pilot pattern C, D


STC2 Enhancement

• Using 4 antennas– 2 are used in order to transmit each symbol

– 2 transmit the same signal with a complex multiplication

– Antenna weights may be changed by BS with SS information using feedback channel (CQI channel)

– No change of the estimation process

beam forming, null steering, and sdma

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