Cross-tier interference is defined as the interference between two different cell types, while co-tier interference is defined as the interference between two similar cell types.
In [87], the authors proposed a 2-step strategy to address the cross-tier and co-tier downlink interference in the HetNet. To tackle this problem the interference alignment (IA) strategy was used to mitigate the interference so that more small cells can transmit data using the same time slot on a given channel. Then a link scheduling algorithm was used to reschedule small cells to another time slot when the interference could not be avoided. Also in [88], the authors proposed a radio resource management strategy to avoid interference within the HetNet and a decision algorithm to determine whether CoMP transmission was required for the user’s data transmission.
In Chapter 2, 3 and 4, CoMP transmission has been considered for interference mitigation in interference limited networks, as a strategy to improve the performance of the users, especially at the cell edge. CoMP transmission was shown to improve the overall system performance (especially for the cell-edge users) by transmitting data signals from neighbouring BSs to the users. Other forms of interference mitigation techniques like adaptive beamforming were highlighted in Chapter 2. This form of interference mitigation is used to cancel or minimise the unwanted interference to other users, by designing precoders and/or receive beamformers. In Chapter 5, the RBA was analysed as a technique for radio resource management based on an interference avoidance strategy within the network, where RBs are assigned to users while avoiding the allocation of RBs with high interference to the users, thereby improving the overall system capacity and user data rate. Two distributed RBA techniques were proposed based on maximising the sum-SINR and maximising the sum-SLINR and they were shown to improve the system performance compared to other known strategies.
In this chapter, both forms of ICIC, i.e. interference mitigation or cancellation and interference avoidance will be applied jointly to further reduce the high interference observed in the HetNet especially during peak times, thereby improving the user’s performance. Using the distributed RBA techniques based on maximising the sum- SLINR as proposed in Chapter 5, the qualification matrix is obtained by estimating the SLINR of each user’s data transmission and then allocating RBs to all users to maximise the sum-SLINR within the macro cell or pico cell. Subsequently, using an interference
mitigation scheme, the unavoidable interference is cancelled to obtain a further reduction in the interference and an improved system performance. The two interference mitigation techniques considered in this chapter are: Beamforming and CoMP transmission.
For the purpose of this work, cross-tier interference will be mitigated between the macro cell sector and pico cell sector and co-tier interference will be mitigated between interfering macro cell sectors. The cross-tier interference considered for each macro cell sector includes: macro cell BS to pico cell user interference and pico cell BS to macro cell user interference both within the same cell sector. It also includes the macro BS of another cell sector to pico cell users in another macro cell sector. The co-tier interference considered is the ICI from a macro cell sector to the macro cell users in a different cell sector.
Table 6.1: Summary of variable notations and definitions
Notation Definition
δ Number of macro cell sectors in each macro cell site. NRB Number of available RBs at each time slot.
¯
K(m,c) Number of users served by the c-th eNB in the m-th macro cell sector, m= [1, 2, · · · , M].
¯
Km Total number of users in the m-th macro cell sector, m = [1, 2, · · · , M].
˜
Kw Total number of users in the w-th macro cell site, w = 1: W.
W, M Number of macro cell sites and macro cell sectors respectively, w = [1, 2, · · · , W] and m = [1, 2, · · · , M], M = δW
C Number of transmitting eNBs in each macro cell sector, c = 1 indicates a macro cell, otherwise a pico cell, c = [2, 3, · · · , C].
Tm Set of interfering macro cell sectors on the m-th macro cell sector.
(m,c) The c-th cell in the m-th macro cell sector.
a(m,c)k,r The bit-wise element that indicates if the r-th RB is assigned to the k-th UE in (m, c).
s(m,c)k,r The k-th user data transmitted on the r-th RB from the eNB in (m,c), E{||s(m,c)k,r ||2} = 1.
v(m,c)k,r The precoder used to transmit the k-th user’s data on the r-th RB from the eNB in (m,c), ||v(m,c)k,r ||2= 1.
¯
v(m,d)k,r The precoder used to transmit the data to the k-th user in (m,d) on the r- th RB from the eNB in (m, 1) (i.e. the m-th macro cell eNB), ||¯v(m,d)k,r ||2
= 1.
u(m,c)k,r The receiver beamformer used at the k-th user on the r-th RB in (m,c) to cancel the received interference, ||u(m,c)k,r ||2= 1.
H(m,c)k,r The flat-fading channel on the r-th RB, from the eNB in (m,c) to the k-th UE.
¯
H(m,c,o)k,r The flat-fading channel on the r-th RB, from the o-th interfering macro cell sector eNB to the k-th UE in (m,c).
g(m,c)k,r The channel gain on the r-th RB, from the eNB in (m,c) to the k-th UE. ¯
g(m,c,o)k,r The channel gain on the r-th RB, from the o-th interfering macro cell sector eNB to the k-th UE in (m,c).
ρ(m,c)k,r The power allocation from eNB in (m,c) to the k-th user on the r-th RB. ¯
ρ(m,d)k,r The power allocated to transmit data from the eNB in (m, 1) (i.e. the m-th macro cell eNB) to the k-th user in (m,d) on the r-th RB.
n(m,c)k,r The noise vector received by the k-th user on the r-th RB in (m,c), elements are complex random Gaussian variable with zero mean and variance (σk,r(m,c))2.
y(m,c)k,r The received signal vector of the k-th user on the r-th RB in (m,c). γk,r(m,c) The SINR of the k-th user on the r-th RB in (m,c).
R(m,c)k,r The rate of the k-th user on the r-th RB in (m,c).
R(m)T , R(m,c)T The sum-rate of the users in the m-th macro cell sector and in (m,c) respectively.
6.4
RBA with Beamforming
MC BS PC BS interference signalFigure 6.2: Interference cancellation from a macro cell (MC) BS to the pico cell (PC) users.
The joint RBA with beamforming uses the proposed distributed RBA strategy for HetNets (proposed in Chapter 5) to assign the RBs. For the r-th RB assigned, transmit and receive beam-formers are designed to further mitigate the interference within each cell type. The HetNet system model in Fig. 5.15 is considered and each variable used subsequently is defined in Table. 6.1.