Study or Work Item Proposal
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Title <Provide the name of the SWIP in the below box. Don’t use bold letters.>
Full Duplex Relaying for Small Cell Self-Backhauling
Study Item/Work Item
Submitted For
TSDSI-SG1-SI10-V1.0.0-20150807
Information/Discussion/Decision
Supporters:
Supporter Name
Radha Krishna Ganti
Email ID
[email protected]
Description <Provide the Description in the below box.>
Recent successful demonstrations of radios for in-band full-duplex (FD) wireless systems have opened a
gamut of interesting network case studies. Though radios for in-band FD systems have been developed, the
impact of such a capability on a cellular network as a whole is an open research area. One interesting use
case of this capability is proposed for in-band FD self-backhauling heterogeneous networks. Self-backhauling
is term used for cells, generally from a smaller tier, that can backhaul themselves over the wireless channel
with cells of, generally, a macro tier. The advantage is twofold—efficient reuse of spectrum as well as the
ease of backhauling dense small-cells without having to lay down fiber every hundred meters. We propose
the case of a two-tier cellular network with in-band FD-enabled small cells, wirelessly backhauled by
conventional macro cells. The in-band FD capability allows small cells to self-backhaul themselves with the
macro cells using the same pair of frequencies that they use for providing access to users. Coverage and
throughput trends in the proposed network architecture need to be investigated and novel methods be
proposed for such analyses.
Proposed is a two-tier heterogeneous network (HetNet) comprising of small cell tier with pico base-stations
(PBS) and another tier of macro base-stations (MBS). The PBSs are FD-enabled. The full-duplexing scheme is
used to ease the backhauling of dense small cells. Consider for instance an FDD network with small cells
serving DL and UL access to users on carrier frequencies f1 and f2 respectively. The PBS uses the same set of
f1 and f2 carrier frequencies to wirelessly backhaul itself with the MBS, since it has a FD radio. This offers a
twofold increase in spectral usage where otherwise the backhaul link needs to be orthogonalized in resources
to the access link. Moreover, the problem of laying down fiber to backhaul each small cell is also resolved.
Figure 0-1: DL Interference in FD setup. Users attached to either tier see interference from both tiers, though the entire
depicts such a self-backhauling setup.
I.
Analysis
The proposed network should be analyzed for coverage and average rate parameters. Though one would
expect the average rate to double up in such a network, it might not exactly be the case. This is because the
number of interfering nodes to the user increases. In the network of
Figure 0-1: DL Interference in FD
setup. Users attached to either tier see interference from both tiers, though the entire a user associated to any tier
would see interference from both the tiers. This increased interference would tend to equalize the gain
obtained from doubling up the spectrum.
The interplay between these two conflicting phenomena needs proper modeling and analysis.
Figure 0-1: DL Interference in FD setup. Users attached to either tier see interference from both tiers, though the entire
spectrum is used the backhaul as well as the access link. Total spectrum = 2W Hz. Each link represents a bandwidth of W Hz.
For instance, the DL backhaul link is centered on f1 Hz and has a bandwidth of W Hz.
We report initial results on evaluating such methods using tools from stochastic geometry proposed to be
studied under [1]. Consider a two-tier cellular network modeled using two independent PPPs Φm and Φs of
constant intensity measure λm and λs respectively, over the Euclidean space Ɍ2. The BSs transmit at constant
powers Pm and Ps, with biases Bm and Bs respectively. Association follows the maximum received biased power
rule. That is to say if the closest MBS and PBS to the typical user are at distances rm and 𝑟𝑠 respectively, then
the user associates with the PBS if:
𝑃𝑠 𝐵𝑠 |rs |−α > 𝑃𝑚 𝐵𝑚 |rm |−α
and to the MBS otherwise. Small scale fading between any pair of nodes is assumed to follow Rayleigh
distribution and hence the fading power is exponentially distributed with unit mean. Simple path loss model
is assumed with path loss exponent α, such that power received at a distance x from a node transmitting at
unity power is |x|-α, α > 2. This implies that the SINR of the typical user, for instance associated with a PBS,
located at the origin and having a nearest BS at a distance r is given as:
SINR =
𝑃𝑠 ℎ𝑟 −𝛼
𝑁+𝐼𝑚 +𝐼𝑠
(1)
Where N is the noise power at the typical user receiver and 𝐼𝑚 , 𝐼𝑠 denote the interference due to MBS and
the PBS tiers. For analysis in such an interference limited scenario we drop the noise term in the SINR
expression and use SIR instead.
Probability of coverage for a typical user is modeled as:
𝑃[𝐶𝑜𝑣𝑒𝑟𝑎𝑔𝑒] = 𝑃[𝜀𝑚 ]. 𝑃[𝑆𝐼𝑅𝑢𝑚 > 𝑇𝑚 ] + 𝑃[𝜀𝑠 ]. 𝑃[𝑆𝐼𝑅𝑢𝑠 > 𝑇𝑠 , 𝑆𝐼𝑅𝑠𝑚 > 𝑇𝑏 ]
(2)
Where 𝜀𝑚 and 𝜀𝑠 denote events of macro and pico association. Signal-to-interference ratios 𝑆𝐼𝑅𝑢𝑚 and 𝑆𝐼𝑅𝑢𝑠
denote user-MBS and user-PBS ratios, 𝑇𝑚 , 𝑇𝑠 , 𝑇𝑏 denote the coverage SIR thresholds for user-MBS, user-PBS
and PBS-MBS links.
On similar lines the average conditional rate is derived as:
𝐸[𝑅|𝐶𝑜𝑣𝑒𝑟𝑎𝑔𝑒] = 𝐸[𝑅𝑢𝑚 |𝑀𝐵𝑆 𝐶𝑜𝑣𝑒𝑟𝑎𝑔𝑒]. 𝑃[𝑆𝐼𝑅𝑢𝑚 > 𝑇𝑚 ] +
𝐸[min(𝑅𝑢𝑠 , 𝑅𝑠𝑚 |𝑃𝐵𝑆 𝐶𝑜𝑣𝑒𝑟𝑎𝑔𝑒]. 𝑃[𝑆𝐼𝑅𝑢𝑠 > 𝑇𝑠 , 𝑆𝐼𝑅𝑠𝑚 > 𝑇𝑏 ]
(3)
Equations (2) and (3) also bring a new element of analysis which is the joint analysis of access-backhaul links
that attempts to quantify coverage and average rate right from the core network down till the mobile user.
The analysis in itself is novel as it attempts to derive the composite performance of a cellular network and
therefore needs careful study.
II.
Initial Results
We show some early results on a stochastic geometry based analysis of a two-tier self-backhauling
heterogeneous network.
Figure 0-2: Coverage Probability. Tm = Tb = −10 dB, λs = 4λm, Bs = Bm + 12 dB
Due to the increased interferers in the FD self-backhauling network the coverage for such a network is lower
than that in an HD self-backhauling network.
Figure 0-3: Coverage Probability. Tm = Tb = Ts = −10 dB, λs = 4λm, Bm = 0 dB
Figure 0-4: Average Rate. Tm = Tb = Ts = −10 dB, λs = 4λm, Bm = 0 dB
The rates offered with FD self-backhauling tend towards the double of rates in an HD self-backhauling
network.
III.
Future Directions
The results obtained in this section indicate two major impediments to achieving the full potential of FD selfbackhauling networks that are inter-tier interference from the backhaul to access links and bandwidth
division at the MBS to accommodate backhauling resources for multiple PBSs. These limitations could be
countered by:
1. Using large antenna MIMO in backhauling. The large antenna array could provide form narrow beams
directed precisely towards the P-BSs, thereby reducing interference in the backhaul as well as access
links.
2. Furthermore, such a backhaul link could be thought of as a MIMO broadcast channel with the MBS
serving multiple PBSs using its many antennas, thereby getting rid of the 1/n bandwidth division
factor.
3. Another approach could be thought of, which could potentially do away with the bandwidth
partitioning factor at the MBSs. This could be achieved if the MBSs are also FD-enabled and use the
opposite pair of frequencies in the access and backhaul networks. This is diagrammatically represented
in Figure 0-5: Self-Backhauling Architecture - Future.
Figure 0-5: Self-Backhauling Architecture - Future
Summary <Provide the Summary inathe below box.>
We proposed self-backhauling HetNet architecture for FD as well as traditional HD enabled base-stations. A
tractable and quick-to-compute analytical model for network wide coverage and average rates needs to be
investigated into. Our initial results show that the proposed FD self-backhauling network suffers from
limitations posed by the inter-tier interference and the bandwidth division happening at the backhauling
MBSs.
Though FD capability helps improve the average rates (conditioned on user being covered) by a factor less
than 2, the coverage in such a network is worse than half of its HD counterpart. This may limit the use of FD
radios as-is in next generation self-backhauling networks. To leverage the full potential of FD capability, it
needs to be complemented with other 5G technologies such as large-scale MIMO and millimeter wave
networks that could improve the inter-tier and intra-tier interference as well as the bandwidth sharing
occurring in the backhaul links. This is a practically viable approach since it is relatively easier to mount several
antennas on a MBS due to its larger form factor. It is also worth noting that, since the PBSs could be operator
deployed, they could be positioned so as to take full advantage of the MIMO link with the M-BS. Moreover,
since the MBS to PBS links are stationary with large channel coherence times, channel training may not be a
problem.
Impact <Provide the Impact of SWIP in TSDSI in the below box.>
High
References <Provide the References in the below box.>
[1] Stochastic models for macro cellular base station locations used for system evaluation, SI11, TSDSISI11-SWIP6-V1.0.0-20150814
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