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Cross-Tier Coordination in Spectrum Sharing: A Blockchain Approach

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  • Sony (China)
  • Sony China Research Laboratory

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Cross-tier Coordination in Spectrum Sharing: A
Blockchain Approach
1st Shuo Wang
Research & Development Center
Sony (China) Limited
Beijing, China
shuo.wang@sony.com
2nd Chen Sun
Research & Development Center
Sony (China) Limited
Beijing, China
chen.sun@sony.com
3rd Haojin Li
Research & Development Center
Sony (China) Limited
Beijing, China
haojin.li@sony.com
4th Tao Cui
Research & Development Center
Sony (China) Limited
Beijing, China
tao.cui@sony.com
Abstract—To solve the contradiction between the increasing
spectrum demand of wireless services and the finite and un-
derutilized spectrum resources, the concept of spectrum sharing
was proposed to allow unlicensed secondary users to temporarily
share unused spectrum of licensed primary users. Currently,
approaches based on geolocation databases have been standard-
ized and commercialized, such as TV white space, license shared
access (LSA) and the citizen broadband radio services (CBRS).
These are centralized schemes where all spectrum allocation
decisions are made by a third party or a cloud-based spectrum
manager. Recently, blockchain technology has drawn the atten-
tion of researchers for its characteristics, such as decentralization,
transparency, privacy preservation, and immutability. In this
paper, we propose a blockchain-based spectrum sharing scheme
to enable the direct coordination of the interference budget
between priority users and general access users in the CBRS
network. Evaluation results show that our proposed approach
can increase the total number of users that can access the shared
spectrum.
Index Terms—blockchain, Spectrum sharing, interference bud-
get; CBRS
I. INT ROD UC TI ON
A. Background and Motivation
Due to the increasing number of wireless communication
services and user equipments, the scarcity of spectrum re-
sources poses a great challenge to wireless service providers.
However, under the traditional spectrum licensing framework,
some of the licensed spectrum is not fully utilized. There-
fore, dynamic spectrum sharing technology was proposed
to improve spectrum efficiency by opportunistically sharing
the under-utilized spectrum of licensed primary systems [1].
Some successful examples include TV white space, license
shared access (LSA) and the citizen broadband radio services
(CBRS), where a centralized geolocation database calculates
the available spectrum for secondary devices without causing
harmful interference to the incumbent systems.
In 2018, the Commissioner of Federal Communications
Commission (FCC) made a remark that smarter and more
decentralized dynamic spectrum access techniques based on
blockchain had the potential to reduce the administrative
expense of dynamic access systems and increase spectral
efficiency at a lower cost [2]. Subsequently, various studies
on blockchain-based spectrum sharing were conducted to
investigate the pros and cons of the combination of these two
technologies [3] [4]. Despite the advantages of transparency,
privacy preservation and immutability, blockchain technology
can also enable more flexible spectrum coordination among
wireless devices, both in the same tier and among different
tiers.
B. Related Work
Some researchers have started investigating the application
of blockchain technology in spectrum sharing. In [3], the
authors proposed a blockchain-based media access protocol to
verify spectrum sharing among mobile cognitive radio nodes.
This method enables access to available licensed spectrum
resources without the need for persistent spectrum sensing.
The authors in [4] analyzed the benefits and limitations of
blockchain solutions and their potential applications to four
major types of spectrum sharing. In [5], the authors leveraged
blockchain technology in the CBRS system to improve the
spectrum management efficiency and quality-of-service of the
secondary users. A blockchain-based decentralized spectrum
access system was proposed in [6], which provided secure SAS
services without relying on the trust of each individual SAS
server for the overall system trustworthiness. In our previous
study [7], we proposed blockchain-based spectrum trading in
the same tier focusing on an interference-based consensus
mechanism. The aggregated interference from all secondary
users to incumbent users should be guaranteed below a certain
threshold to protect the incumbent users. In this paper, we
propose a cross-tier coordination approach where higher-tier
users can sell their interference budget to lower-tier users,
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allowing more secondary user to have access to the shared
spectrum.
The rest of this paper is organized as follows. We will firstly
introduce the CBRS and blockchain technology in Section
II, Secondly, a cross-tier spectrum coordination approach is
proposed in Section III. Then, the simulation results are
presented in Section IV Finally, Section V concludes the paper.
II. CBRS AN D BL OC KC HA IN
A. CBRS
In 2015, the FCC established a three-tiered access and
authorization framework to accommodate shared federal and
non-federal use of the 3550-3700 MHz band (3.5 GHz band).
The rules governing the sharing conditions are found in Part
96 of the FCC’s rules [8]. The framework of spectrum sharing
in the CBRS band is shown in Fig. 1. The first tier contains
the incumbent users, which include authorized federal users,
fixed satellite service (FSS) earth stations and grandfathered
wireless broadband licensees. The second tier consists of
priority access licenses (PALs) that will be licensed on a
county-by-county basis through competitive bidding with up
to seven PALs in any given county. Each PAL consists of a
10 MHz channel within the 3550-3650 MHz band. The lowest
tier is called general authorized access (GAA), which enables
flexible access to the band for the widest possible group of
potential users. Incumbent users are protected from both PALs
and GAA users, while PAL users are protected from GAA
users. GAA users do not expect protection from either of the
above tiers or other GAA users. The fixed stations that operate
on a PAL or GAA basis in the CBRS band are also called
CBRS devices (CBSDs).
The spectrum access system (SAS) is fully responsible for
the rule enforcement of CBRS in a centralized manner, such
as providing permissible channel and power to the CBSD,
enforcing exclusion zones and protection zones, and resolving
conflicting uses of the band while maintaining, as much as
possible, a stable radio frequency environment.
Fig. 1. The three-tier CBRS architecture.
B. Blockchain
Blockchain technology was adopted with other technolo-
gies, such as cryptography, to create modern cryptocurrencies
(e.g. Bitcoin) in 2008 [9]. Blockchains are tamper-proof,
decentralized digital ledgers that operate without a central
authority. All participants of the network can independently
verify the validity of all the transactions which are publicly
recorded on the blockchain. Blockchain users utilize public
and private keys to digitally sign the transactions to ensure
security. A consensus mechanism is utilized to determine
which user publishes the next block, , through which mutually
distrusting users can work together.
Two basic categories of blockchain are permissionless and
permissioned blockchains, based on who can maintain the
blockchain. A blockchain is permissionless if anyone can
participate and publish blocks. In contrast, only particular
users can publish blocks in a permissioned blockchain, also
known as a consortium blockchain. Various consensus models
have been proposed for different use cases, such as proof of
work (PoW), proof of stake (PoS), proof of authority (PoA),
etc [10].
Despite the merits promoted by many publications [11],
blockchain technology also has limitations. For example, per-
missionless blockchains can suffer from 51% attack and may
consume a huge amount of resources. With the growth of
the network, scalability becomes a major challenge for its
application in spectrum sharing, we have proposed a multi-
blockchain scheme to solve this issue in [12].
III. CROS S-T IE R SP EC TRUM COOR DI NATI ON S CH EM E
A. System Model
We consider a three-tier spectrum sharing scenario in the
CBRS band based on the FCC Part 96 rules [8]. The incumbent
user is an FSS earth station located in the center of a protection
area with a 150 km radius. Multiple PAL users and GAA
users are deployed in this area. Each PAL user has its own
PAL protection area (PPA) where no harmful interference
from other PALs or GAA users is allowed. The difference
with the existing CBRS architecture is that a blockchain
network is established among the CBSDs. The spectrum usage
information of each CBSD (e.g., channel, bandwidth, transmit
power, etc.) is recorded on the shared blockchain, which is
immutable and transparent. Therefore, it gives an opportunity
for the CBSDs to coordinate their spectrum usage behavior,
both in the same tier and across tiers.
To protect the incumbent user from harmful interference,
the aggregated interference to the incumbent user from all the
PALs and GAA users within 150 km should not exceed the
interference threshold, which is a median root mean square
(RMS) value of -129 dBm/MHz for the FSS earth station. For
a specific channel, the aggregated interference to a protection
point is calculated by:
IAgg =
N
X
n=1
In(1)
Where IAgg indicates the aggregated interference to the in-
cumbent, Inrepresents the interference caused by the CBSDn
and is defined as:
In=Pt
n+Gt+GrPL(2)
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Fig. 2. System scenario.
where Pt
nis the transmit power of CBSDn,Gtand Gr
represent the transmitting antenna gain and receiving antenna
gain respectively, PLdenotes the path loss from CBSDnto
the incumbent according to the irregular terrain model (ITM)
defined in the CBRS specification [13].
B. Procedures of cross-tier coordination for spectrum sharing
While in the current CBRS specifications, the SAS is
responsible for all the interference calculation and spectrum
management tasks, our proposed blockchain based solution
allows GAA users directly coordinating with PAL users, which
alleviates the burden on SAS and improves the flexibility of
spectrum sharing. Our method includes the following main
procedures, as shown in Fig. 3.
1) Coordination among GAA users: This procedure con-
sists of steps 1 to 5 in Fig. 3. In our solution, When GAA-
1 sends a spectrum request to the SAS, the SAS not only
returns the available spectrum as in traditional method, but
also returns a coordinating available spectrum. The available
spectrum refers to idle channels and maximum allowed power
that can be provided to the GAA requester by the SAS directly.
The coordinating available spectrum is the channels in use by
multiple CBSDs and can be provided after coordinating with
other users. The first coordination process happens among
GAA users willing to provide interference headroom to the
new GAA spectrum requester by reducing transmit power.
Two kinds of interference headroom are considered. One is the
aggregated interference to the incumbent user from co-channel
PAL and GAA users. The other is the aggregated interference
to the PAL from all the co-channel GAA users.
After receiving the coordinating available spectrum, the
GAA-1 sends a coordination request to other GAA users on
the blockchain and then execute smart contract 1 to determine
a GAA coordination scheme. When selecting the list of coor-
dinating GAA user, a trust value determined by the credibility
of its provided interference headroom according to history
records will be considered. The GAA user with higher trust
value will be selected in priority to improve the probability
of successful coordination. The coordination scheme contains
the list of coordinating GAAs, the interference headroom
provided, the location and transmit power of each GAA user.
The determined coordination scheme will then be broadcast to
the PAL users on the blockchain.
2) Coordination among GAA and PAL users: In this coor-
dination process, the PALs should consider the interference
impact from two perspective. Firstly, the aggregated inter-
ference of all GAA users to the PPA should not exceed
the PPA interference threshold. The PALs can also decide
whether to adjust their interference threshold if they can
tolerate more interference according to their operating status.
Secondly, to ensure the aggregated interference from all GAAs
and PALs to incumbent is below a threshold, the PALs can
sell their interference budget to GAA users by reducing their
transmit power, which will allow more GAA users having
access to the spectrum. Then the PAL coordination scheme
will be sent to GAA-1 which requested the spectrum. To
encourage the participation of PAL users in coordination, a
reimbursement will be paid by the GAA user that benefits
from this coordination.
3) Grant process: When the spectrum requester GAA-
1 receives response from other PALs and GAAs of their
coordination schemes, it will decide whether to accept the
coordination scheme and send a spectrum grant request con-
taining detailed information on how to coordinate spectrum
usage to the SAS in step 8. Then, the SAS will check the
feasibility of this coordination request based on its calculation
and send a response to GAA-1 in step 9.
4) Updating spectrum usage: If a success response is
received in step 9, smart contract 2 will be executed on the
blockchain to adjust the transmit power of each CBSD. A
coordination factor ηis defined to denote the contribution of
interference headroom of CBSDi:
ηi =Pi
red/(Pi
cur Pmin)(3)
where Pi
red is the reduced power of CBSDiin this coor-
dination process, Pi
cur the transmit power of CBSDibefore
coordination and Pmin is the minimum required power of
CBSDito ensure quality of service.
To monitor the behavior of GAA users, if PAL users detect
harmful interference, they will report to the SAS. Then the
SAS will determine which GAA user is transgressing the
sharing rules and broadcast the non-compliant GAA users on
the blockchain. Smart contract 3 will be executed to update
the trust value of GAA as follows:
Tn
i=
α·Tn1
iif not coordinating
Tn1
i+Tiif not transgressing
β·Tn1
iif transgressing
(4)
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Fig. 3. Procedures of proposed solution.
where nis the times of a GAA user participating coordination,
αis a declining factor which takes effect when GAA user
does not coordinate, βis a declining factor when transgressing
behavior of GAA user is detected and β > α.Tiis the
increased trust value of GAA iif it successfully participating
in the coordination and without transgressing behavior.
5) Consensus process: The consensus process is key to
ensure the validity and synchronization of blockchain across
decentralized nodes. In step 15, the accounting node responsi-
ble for generating new blocks will be determined firstly based
on the coordination factor and trust value of each GAA user.
Then the accounting node generates the new block containing
latest transactions and updated coordination factor and trust
values. After the new block is generated, it is broadcast to
other nodes in the blockchain network. Each node will verify
the block and append it to its local blockchain.
IV. SIM UL ATION RESULT S
In this section, we will evaluate the performance of our pro-
posed solution through Monte-Carlo simulation using MAT-
LAB.
A. Simulation Setup
In the simulation setup as shown in Fig. 4, an incumbent
user is located at the central coordinate of the simulation area.
A total of 20 PALs and 150 GAAs are randomly distributed
TABLE I
SIM ULATI ON PAR AM ETE RS
Transmit power of PAL 47dBm/10MHz
Transmit power of GAA 23dBm/10MHz
Channel frequency range 3550-3560 MHz
Interference threshold of incumbents -129 dBm/MHz
Propagation model ITM model
in a circular area with a radius of 80km with incumbent as
the center. The simulation parameters are shown in Table I .
B. Performance evaluation
We use the rules defined in Wireless Innovation Forum’s
specifications as the baseline scheme denoted as ”WInnF
Scheme” in Fig. 5. Then we compare it with two other
schemes. The first one is that GAA can only request inter-
ference budget from its nearest PAL user, which is denoted as
”Single PAL w/o coordination” in the figure. The other one
is our proposed scheme where multiple PALs can coordinate
through the blockchain to provide interference budget to GAA
users. AS shown in Fig. 5, with the number of PAL CBSDs
increasing, the number of licensed GAA user also increase for
the schemes with cross-tier coordination (green and red line)
while the number of licensed GAA user remains the same
for the baseline scheme. This is because GAA users cannot
utilize the interference budget of PALs. Furthermore, more
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Fig. 4. Simulation setup.
licensed GAA users can be achieved when multiple PALs can
coordinate their interference headroom. In Fig. 6, it can be
observed that the scheme with multi-PAL coordination can
have higher utilization ratio of the interference headroom of
PAL users. This phenomenon becomes more obvious with
the increase of the number of PALs, resulting a significant
performance gap between the two schemes.
0 5 10 15 20
Number of PAL
90
95
100
105
110
115
120
125
Number of Licensed GAA Users
Multiple PALs Coordination
Single PAL w/o Coordination
WInnF Scheme
Fig. 5. Simulation setup.
V. CO NC LU SI ON
In this paper, we designed an cross-tier coordination scheme
based on blockchain for dynamic spectrum sharing. Our pro-
posed solution enables the lower tier GAA users to utilize
the interference headroom from upper tier PAL users to the
incumbent through a secured blockchain network. The simu-
lation results demonstrate that the proposed method improves
the total number of GAA users that can have access to
the shared spectrum and can fully utilize the unoccupied
0 5 10 15 20
Number of PAL
30
40
50
60
70
80
90
100
Utilization of Interference Headroom/%
Multiple PALs Coordination
Single PAL w/o Coordination
Fig. 6. Simulation setup.
interference headroom to the incumbent. The application of
blockchain technology in spectrum management presents a
promising framework for a more flexible, transparent and
secure utilization of the valuable electromagnetic spectrum
resources.
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Blockchain Technology overview
  • D Yaga
D. Yaga et al., "Blockchain Technology overview, " NIST Internal Report 8202, Available: https://doi.org/10.6028/NIST.IR.8202.