Random Access Protocols for Massive MIMO

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  • Department of Electronic Systems

    Random Access Protocols

    for Massive MIMO

    Emil Bjrnson

    Erik G. Larsson

    Linkping University

    Sweden

    Elisabeth de Carvalho

    Jesper H. Srensen

    Petar Popovski

    Aalborg University

    Denmark

    2016 Tyrrhenian International Workshop on Digital Communications (TIW16)

    Sept 12-14, 2016

  • Department of Electronic Systems

    CSI acquisition and data transmission in

    Crowd Scenarios

    Megacities

    Hotspots

    Machine type

    communications

    http://edition.cnn.com/2013/05/02/travel/london-city-airport-internet-

    of-things/

  • 3

    Presentation Content

    Massive MIMO: massive number of spatial degrees of

    freedom

    Crowds: exploit very large multiplexing gain

    Uplink Training based on orthogonal pilot sequences

    Length/number of orthogonal pilots limited

    Pilot shortage

    One solution: random access to the pilots and possibly the

    data

  • 4

    CSI Acquisition in Massive MIMO

    Time-division duplexing and channel reciprocity

    CSI is acquired using uplink training

    Exploited for downlink transmission

    Orthogonal pilots

  • 5

    Orthogonal Pilots are a limited resource

    Pilot Sequence length limited by:

    # of pilots limited by channel coherence time

    # of pilots limited by transmit power

    Number of orthogonal pilots = pilot sequence length

    Pilot ShortageNumber of pilot sequences is much smaller than the

    number of pilot sequences

  • 6

    Traffic Burstiness

    Email

    Video streaming

    Social network

    Cloud

    Machine-type communications: crowd of devices transmitting

    sporadically

    time

    unpredictable and intermittent traffic

    Non-streaming internet applications

  • 7

    Random Pilot Access

    Crowd of devices with unpredictable and intermittent traffic

    Makes pilot pre-allocation very inefficient

    Need for:

    Scalable and efficient pilot access and

    data transmission protocols.

    Random Access to

    Pilot sequences

    Proposed solution:

  • 8

    Random Access to Pilots

    Total number of terminals and orthogonal pilots available

    Random pilot selection COLLISIONS

    Collisions = Pilot contamination

    Users select a pilot sequence uniformly at random with

    probability

    Intra-Cell Pilot contamination

  • 9

    Pilot Collision

    Users with same pilot sequence: their channel cannot be

    distinguished:

    Beamforming at BS based on contaminated channel estimation:

    results in inter-user interference

    =

    all colliders

    +

    UL Pilot

    transmission

    11

  • 10

    Two kinds of approaches

    Random Access to Pilots Random Access to Pilots and

    Data

    11

    Pilot Contention resolution

    Terminal sends payload when no

    pilot contention

    11

    Uplink data is embarked with the

    pilots

    Data affected by collision-induced

    interference

    Collision in the pilot domain only Collision in the pilot and data

    domain

  • 11

    Pilot Contamination

    Suppression

    Methods based on spatial separation, diversity in path loss,

    timing offsets

    If successful, BS can decode ID of separable users

    which are admitted for DL/UL data transmission

    UL Pilot transmission

  • 12

    Pilot Contamination

    Detection of collision at the BS

    BS sends a message to colliding devices:

    try again until no collision

    Avoidance

    try again

    New random

    accessUL Pilot transmission

  • 13

    Pilot Contamination

    Detection of collision at the device

    BS sends a precoding pilot

    1) Device detects collision

    2) Device decides whether to retransmit the pilot

    sequence

    Avoidance

    Precoded pilot

    Try again

    Decision:

    Retransmit UL Pilot?

    UL Pilot transmission No collision

    (with high probability)

  • 14

    Pilot Contamination

    training

    = 1 + 2 + w =1+2+

    1+2+

    y1 =1

    +2+

    1+2+1 +

    2 =

    y12

    12

    1+2+2

    y12 =

    12

    1Expected

    Training based Estimate of 1 + 2 from 12:

    Compare 1 to

    Compare 1 to /2

    Channel

    hardening

    Contaminated channel estimation Precoding

    STRONGER user

    Retransmit the pilot sequence

    Collision detection

  • 15

    Collision resolution

  • 16

    Uplink Joint Pilot and Data Transmission

    2(1) 2(3) 2()

    3(3)

    4(1)

    4(3)

    3(2)

    1()

    4(3)

    4()

    2

    Time slot

    1

    2

    3

    4

    One codeword sees an asymptotic number of:

    channel fades (small and large scale)

    pilot contamination events

    For delay-tolerant communications

  • 17

    Uplink Sum Rate Bound MRC at BS

    Large scale

    fading

    Rate user 0 (, 0, large scale fading)

    Number of

    active

    users

    Number of

    contaminators

    to user 0

    EEKa, c

    = Sum Rate (, )

    ~(, )

  • 18

    Uplink Sum Rate Bound- MRC at BS

    Probability of having

    Ka active terminals

    out of K

    Probability of having c

    contaminator to a given user

    conditioned on Ka active users

    Optimization wrt and

    ~Binomial(, ) |~Binomial( 1, 1/)

    Channel energy

  • 19

    Heuristic Solution

    Sum Rate ~

    Depends on the channel

    energy variations

    =

    =3

    2(1) 2(3) 2()

    3(3)

    4(1)

    4(3)

    3(2)

    1()

    4(3)

    4()

    2

    Time slot

    1

    2

    3

    4

  • 20

    Rate~

    Average Sum Rate

    M=400

    M=100

    0.5 b/s/Hz per user

    1 b/s/Hz per user

    Average sum rate as a function of , K=800

  • 21

    Average Number of Active Users

    M=400

    M=100

    as a function of , = 800

  • 22

    Conclusions

    New services and scenarios in 5G: new way to access

    the pilots and transmit the data

    Massive MIMO is a fundamental enabler for crowd MBB

    and mMTC

    Creation of an efficient standard for wireless networks

    based on massive MIMO technology will require a

    complete re-design of the multiple-access layer.