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Dynamic Frequency Allocation in Fractional Frequency Reused OFDMA Networks

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Dynamic Frequency Allocation in Fractional Frequency Reused OFDMA Networks Syed Hussain Ali, Member, IEEE Victor C. M. Leung, Fellow, IEEE University of British Columbia, – PowerPoint PPT presentation

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Title: Dynamic Frequency Allocation in Fractional Frequency Reused OFDMA Networks


1
Dynamic Frequency Allocation in Fractional
Frequency Reused OFDMA Networks
  • Syed Hussain Ali, Member, IEEE
  • Victor C. M. Leung, Fellow, IEEE
  • University of British Columbia, Vancouver,
    Canada
  • TWC 2009

1
1
2
Outline
  • Introduction
  • System Architecture and Model
  • Problem Formulation
  • Proposed Solution
  • Numerical Results and Discussion
  • Conclusion

2
2
3
Introduction
  • OFDMA allows dynamic assignment of
    channels/subcarriers to different users at
    different time instances
  • Dynamic subcarrier assignments (DSA) to multiple
    users improve the system data rate of an OFDMA
    system
  • This improvement is due to the multiuser
    diversity gain as the channel characteristics for
    different users are independent of one another
  • In systems where adaptive modulation and coding
    (AMC) techniques are employed, a better channel
    response results in a higher data rate

4
Equally Distribute Power
  • In principle, allocating different power levels
    to individual subcarriersshould improve
    performance
  • Previous studies 6 and 7 haveshown that the
    performance improvements are marginal
  • 7 Users with the best channel gain for each
    subcarrier are selected and then transmit power
    is equally distributed among the subcarriers
  • A simpler solution involving DSA with equal power
    per subcarrier is preferred over more complex
    joint DSA and APA solution

6 G. Song and Y. G. Li, Cross-layer
optimization for OFDM wireless networkspart I
theoretical framework," IEEE Trans. Wireless
Commun. 2005. 7 J. Jang and K. B. Lee,
Transmit power adaptation for multiuser OFDM
systems," IEEE J. Select. Areas Commun. 2003
5
Frequency Reuse Factor of 1 is Better
  • 13 reported large performance losses due to the
    frequency reuse schemes and suggested a frequency
    reuse factor of 1
  • 13 A fractional loaded 1-reuse, with admission
    control or blocking, is a better alternative than
    a 3-reuse
  • Our proposed scheme dynamically assigns carriers
    to different regions, allocates them to different
    users and maintains a frequency reuse factor of 1
  • Subcarriers are assigned to a user depends on the
    signal-to-interference-noise ratio (SINR) of the
    subcarrier and the fairness requirements of the
    users

13 Downlink inter-cell interference
co-ordination/avoidance evaluation of frequency
reuse," 3GPP TSG-RAN WG1 Contribution, Tech. Rep.
R1-061374, May 2006. Online. Available
http//www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1_45/D
ocs/R1-061374.zip
6
Static FFR Scheme (1)
  • Fractional frequency reuse (FFR) scheme
    partitions the cell surface into two distinct
    geographical regions
  • Inner cell area
  • users present in this cell area is called the
    super group
  • Outer cell area near the cell edge
  • users located in this cell area is called the
    regular group
  • We identify the original FFR as the static FFR
    scheme

7
Static FFR Scheme (2)
  • Shortcomings of static FFR scheme
  • It divides users in two groups on the basis of
    static distance or SINR thresholds
  • The trunking gain is reduced because only a
    fraction of the total cell population is part of
    a group
  • It partitions the available subcarriers randomly
    into the groups
  • The subcarrier partitioning does not consider
    their radio channel states
  • We feel that FFR could benefit if only users in a
    cell are virtual members of both the groups
  • This way the users will be able to get access to
    the subcarriers of both groups and result in
    increased trunking gain

8
Objective
  • The objective of this research is to improve the
    long-term system data rate of a downlink OFDMA
    multicell network
  • by intelligently distributing and allocating
    subcarriers first among geographical locations of
    cells and later, within a cell, among users
  • We assume that the cell power is equally
    distributed among the subcarriers
  • We propose a dynamic FFR cell architecture
  • Subcarriers are dynamically partitioned among
    geographical locations by radio network
    controller (RNC)
  • The BS schedules those subcarriers to the users
    opportunistically

9
Proposed Dynamic FFR
  • Both user groups (super and regular) cover the
    whole cell surface.
  • All users of a cell are virtual members of both
    groups
  • The subcarriers assigned to the super and the
    sectors within the regular groups are orthogonal

10
System Model (1)
  • We consider K-cell OFDMA cellular network and A
    central radio network controller (RNC) manages
    the K BSs
  • Let ?k denote the set of users and
    be the number of users in a cell k
  • is the total number of users
  • is the set of all users in the
    network
  • Assume that each cell is partitioned into L
    sectors where l identifies a particular sector
  • denotes the set of users and
    is the number of users in a sector l of a cell k
  • represents the total number
    of users in the lth sector of all cells

11
System Model (2)
  • Let N be the number of subcarriers available
  • Csup and Creg be the set of subcarriers assigned
    to the super and regular groups, respectively
  • be the set of subcarriers allocated to a
    sector l
  • be the set of
    interferers for a user in the super group and a
    sector l of the regular group, respectively
  • For a 19-cell grid, there are 18 and 7
    interferers experienced by the super and regular
    group users, respectively

12
Example
  • The set of interferers for a user in sector A of
    cell 1 includes
  • all the adjacent cells in the super group
  • cells numbered 5, 6, 13, 14, 15, 16, 17 in the
    regular group setting

13
Channel and Data Rate Models (1)
  • The channel gain for a user i on a subcarrier j
    from the serving BS k is given by
  • where are path
    loss at distance r, shadowing and fading
    coefficient, respectively
  • The corresponding SINR is given as
  • where N0 is the noise power spectral density, ?f
    is the subcarrier spacing, P is the power per
    subcarrier, Q is the set of interferers
  • for the super group
  • for the regular group sector l

14
Channel and Data Rate Models (2)
  • Employing continuous rate adaptation, the SINR is
    mapped to data rate as follows
  • ? is a constant related to the target bit error
    rate (BER) as
  • ?f is the subcarrier spacing
  • Ri,j is the achievable data rate by the ith user
    and jth subcarrier pair
  • RNC algorithm employs average
    whereas the BS uses instantaneous
    values of these achievable rates
  • A subcarrier j which falls within the regular
    group will have achievable data rate for user i
  • identifies the achievable data rate of
    subcarrier j in the super group

15
Problem Formulation (1)
  • The DSA objective is to maximize the system data
    rate while satisfying individual users lower data
    rate requirements
  • Let be the binary decision variable, that
    is,
  • When this variable is 1, it signals that the
    subcarrier j is assigned to the user i and
    belongs to the super group of subcarriers
  • When its complement , it signals that
    the subcarrier j is assigned to the user i and it
    falls in the regular group of subcarriers
  • The super and regular group subcarriers are
    orthogonal
  • i.e.,

16
Problem Formulation (2)
  • The joint DSA solves the following binary integer
    program for every scheduling time slot t
  • where Ci be the lower bounds on data rates for
    user i

A subcarrier can be assigned to only one user in
a cell
If a subcarrier j is assigned to the super group
in one cell then this subcarrier should be reused
in all the cells
Similar reuse constraints for the regular group
subcarriers
17
Proposed Solution
  • We decompose the joint problem into two parts
  • The first part is solved by a central location,
    like RNC which computes the membership of
    subcarriers in the super group or a sector within
    the regular group
  • RNC DSA requires average achievable rates
    information for all user subcarrier pairs in the
    super and regular group settings
  • The second part operates at the BS level and
    allocates subcarriers to the users
  • BS DSA requires instantaneous data rate
    information for all user subcarrier pairs

18
RNC DSA Algorithm
19
BS DSA Algorithm (1)
  • The RNC algorithm forwards the subcarrier
    assignments
    to every BS
  • The BS DSA employs the minimum performance
    guarantee (MPG) opportunistic scheduling rule of
    18
  • We define as the average data rate of
    user i up to time T where x represents the
    decisions made by the BS scheduler
  • where are binary
    integer variables which signal the corresponding
    allocation decisions of the scheduler at a time
    slot t

18 X. Liu, E. K. P. Chong, and N. B. Shroff, A
framework for opportunistic scheduling in
wireless networks," Computer Networks, vol. 41,
no. 4, pp. 451-474, Mar. 2003
20
BS DSA Algorithm (2)
  • is the
    average cell data rate where
  • The MPG problem can be written as
  • For the assignment of the super group
    subcarriers, j ? Csup, the algorithm finds the
    user i that satisfies the following expression
    at every scheduling slot t
  • The true controlling parameters in the above
    solutions are chosen such that for
    all i, E (Ri(x)) Ci for all i, and if E (Ri(x))
    gt Ci then
  • Employing stochastic approximation techniques ,
    the true values of ßi can be estimated in real
    time as follows
  • where is a small positive real number

21
Simulation Parameters
  • We compare the proposed DSA algorithm with
  • Full frequency reuse with full interference
    (FFFI)
  • Conventional sectored
  • Static FFR allocations
  • All the four schemes considered have a frequency
    reuse factor of 1
  • The number of users, the cell dimensions, and the
    BS locations remain the same for the 100
    super-frames
  • User locations vary according to the random walk
    mobility model

22
Difference among Algorithms
  • The algorithms differ in terms of the RNC DSA
  • For FFFI, all the subcarriers are available for
    allocation to all the BSs in the grid with full
    inter-cell interference without any sectoring
  • For the conventional sector allocation, RNC
    randomly selects a subset of the subcarriers for
    a sector.
  • This subset is repeated in the same sector of all
    the cells
  • The static FFR scheme divides users according to
    a distance threshold from the serving BS
  • The users within 70 percent of the cell radius
    are considered members of the super group
  • The remaining users are members of the regular
    group which is divided in 3 sectors
  • The BS part of all the four allocation schemes is
    based on the MPG opportunistic scheduling

23
Comparison of Proposed DSA as a Function of Cell
Radius
24
CDF of Achieved Data Rates and Lower Bounds
25
Cell Edge Throughput (1)
26
Cell Edge Throughput (2)
27
Conclusion
  • We propose a new dynamic fractional frequency
    reused system architecture where a cell surface
    is virtually partitioned into two regions
  • The first region is called super group, and the
    user-subcarrier pairs in this group experience
    interference from all the neighboring cells
  • The second region is called the regular group
    which is physically partitioned into sectors and
    experiences reduced interference
  • Both groups include all the cell users which
    results in increased trunking gain
  • The proposed DSA scheme consists of two
    algorithms
  • The first algorithm runs at the RNC and allocates
    subcarriers to the groups
  • The second algorithm runs at every BS where
    opportunistic scheduling decisions are made and
    subcarriers are assigned to the users
  • For small to medium cell, the proposed scheme
    outperforms the traditional schemes
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