Cascaded Resource Allocation among Prioritized Shared Spectrum Blocks

Jeounglak Ha, Jin-Up Kim Smart Radio Research Team

Electronics and Telecommunications Research Institute Daejeon, 305-350, Republic of Korea

{jlha, jukim}@etri.re.kr

Sang-Ha Kim Department of Computer Engineering

Chungnam National University Daejeon, 305-764, Republic of Korea

shkim@cnu.ac.kr

Abstract—Dynamic spectrum access (DSA) has received much attention in recent years as an effort to enhance the spectrum utilization. Primary-secondary usage (PSU) of spectrum is an option of DSA which gathers much study interest. Some modes of secondary usage of spectrum are granted on the condition that the secondary user (SU) releases the spectrum resource immediately whenever the primary user (PU) requests. The SU may hop to an idle spectrum resource, if one is available, or the call is dropped. This spectrum handoff (SH) let the ongoing call continue the session with some degradation of Quality of Service (QoS) for a while. However if the spectrum resource consists of more than one spectrum blocks (SBs) as Public-Private Partnership (PPP) in the upper 700MHz band as Federal Communications Commission (FCC) revised lately, we may reduce the SHs by employing the proposed Cascaded Resource Allocation (CRA) algorithm. In CRA, the SBs are prioritized and the users’ accesses are allocated to the SBs considering the priority of the SBs. In this paper we present a generalized model to efficiently make use of multiple SBs and a basic analytical model for prioritized resource access. The performance of CRA is measured by analyzing Markov modulated Poisson process (MMPP) and simulation results.

Keywords—Dynamic Spectrum Access; Resource prioritization; Public safety; Spectrum handoff; Secondary use

I. INTRODUCTION Numbers of previous studies [1]-[3] pointed out that large

fraction of radio spectrum is underutilized temporally and sporadically and there are numerous activities of regulatory agencies and standardization bodies around the world to facilitate dynamic spectrum reuse [4]. The authors of [5], [6] provide various usage models of Dynamic Spectrum Access (DSA). On the other hand Federal Communications Commission (FCC) adopted Public-Private Partnership (PPP) plan in the upper 700MHz band to increase commercial utilization as well as to ensure reliable and accessible communication of public safety purpose [7][8], which has been working based on licensed exclusive spectrum plan.

The primary user (PU) who owns the license of the spectrum resource may let the secondary user (SU) use some parts of the resource while the PU does not use it [6]. This behavior is referred to as rent or lease, and we use the term rent hereafter. However the SU should vacate the occupation of the rented spectrum resource shortly as the PU requests.

The situation is quite similar in PPP plan even though it has a unique spectrum usage scheme, which will be described in the next section. Under the PPP plan the public safety user and the commercial user can be considered as the PU and the SU of broadband public safety band respectively.

Regardless of which spectrum usage plan is employed the SU, which is interrupted by the PU, may perform spectrum handoff (SH) [3] to maintain its ongoing session. Otherwise the session will be disconnected. Ref.’s [9]-[11] are studies related to SH. Giupponi et al. [9] proposed a Fuzzy-based SH algorithm, in which they applied Fuzzy logic in making decision of SH. Zhang [10] provided performance analysis of SH in terms of link maintenance probability, number of the SHs, switching delay, and non-completion probability. Wang et al. [11] studied the SH’s impact on the performance of the SU. Since the SH may cause degradation in Quality of Service (QoS) there are some previous works try to reduce SHs [12]-[14]. Jo et al. [12] proposed a spectrum matching algorithm to reduce SHs, in which user’s service time is used in matching spectrum holes’ holding time. Tzeng [13] applies multiple threshold reservation policy for handoff calls considering the amount of rented spectrum, where the threshold is controlled according to the total available spectrum size including the rented. Zhu et al. [14] proposed reservation of sub-bands for the SHs so that the spectrum handed off calls can be received on the reserved sub-band when the PU appears.

This paper has two contributions. The first one is that it proposes a generalized model for efficient utilization of multiple spectrum blocks (SBs) [15] under primary-secondary usage (PSU) environment. The spectrum usage plan of PPP is an actual one that has already appeared and secondary usage is expected to run in the future with multiple SBs which may include its own SB as well. Under the PSU scenario some part of the PU’s spectrum may be rented to the SU [7][8][13][14]. The other one is that it proposes a basic analytical model for prioritized Cascaded Resource Allocation (CRA). CRA utilizes the difference of PUs’ utilization [16] of the SBs to reduce SHs.

The rest of this paper is organized as follows. In section II we describe system model in which CRA is applied and in section III the performance analysis is given using Markov modulated Poisson process (MMPP) for CRA and another

This work was supported by the IT R&D program of MKE (Ministry ofKnowledge and Economy of Republic of Korea) and IITA (Institute forInformation Technology Advancement) [2008-F-001-02, Research onenvironment-adaptive autonomous technologies for mobile wireless access].

978-1-4244-2519-8/10/$26.00 ©2010 IEEE

MMPP for Random Resource Allocation (RRA) to be compared. Expected number of the SHs and utilization of each SB and gain of using CRA are proposed as well. Discussion of numerical and simulation results is presented in section IV and conclusion is in section V.

II. SYSTEM MODEL

A. Dynamic access scenarios Figure 1 shows example scenarios of prioritized and shared

use of SBs. Figure 1(a) depicts an illustrative case that eight SBs are used by seven Radio Access Technologies (RATs). RAT1, RAT2 and RAT3 are the PUs of some of the SBs, who hold licenses of the SBs. RAT4, RAT5, RAT6 and RAT7 are the SUs of the SBs, with the exception that RAT6 is the PU of SBe. It should be noted that the secondary usage may include SBs from one or more primary RATs and that a RAT may have both primary and secondary usage rights to the SBs at the same time. Figure 1(b) shows the lately revised band plan of PPP [7], which is licensed to a Public Safety Spectrum Trust (PSST), who is a national non-profit organization. PSST is obliged to share the band with the D-block operator. The D-block operator is provided with right to access the broadband public safety band on a secondary basis and obliged to build a national commercial network and to allow PSST to expand and have priority access into the D-block in case of an emergency [8].

Assuming the opportunistic use of the SBs of the SU it is inevitable that the PU occasionally reclaim the whole or part of the SB of which channels are being used by the SU. If it is occupied by the SU, the calls should be handed off to available channels in other SBs of the SU in order to avoid the call being dropped from the PU’s interruption.

B. Resource prioritization We assume that there are M of SBs are available to a RAT, RATS. The SBs are represented by B , B , … , BM , BM and

their sizes are ST, ST, … , SMT , SMT bandwidth units (BUs). Let each Bi is rented from RATi and it is licensed to RATS if i is same with RATS as the case of SBe to RAT6 in Fig. 1(a). Then we may set Bi’s priority, Pr(Bi), as (1). Variable ρ is the

Pr(B ) Pr(B ) , where Pr(B ) 1/ρ RATS∞ RATS (1)

utilization [16] of Bi from its PU. Let the amount of occupied spectrum of Bi be S , then S ( ST S ) of Bi is the unused. Let’s assume that the PU

reclaims (0 γ ST) amount of spectrum of Bi from RATS during a certain short time, τ . We assume during τ the reclaim comes only from one primary RAT.

Figure 2(a), (b) and (c) assume Pr(Bi) follows (1) and the primary RAT that reclaims is I, which is RATI. The ongoing calls of RATS which have been using the spectrum that has to be returned to RATI should be handed off to other SBs according to their priorities. The dashed lines represent cascaded SHs among the SBs of RATS, while the solid lines do acceptance of the new calls or the handed off calls from the neighboring cells or the neighboring RATs. Regardless of whether it is a new call, a SH call or a traditional handoff call caused by the change of the channel quality the spectrum resource with higher priority are allocated preferentially.

When RATI reclaims of BI, RATS may not have to vacate any of the spectrum resource of the ongoing call if γ SI . Otherwise RATS should vacate NI ( γ SI ) amount of spectrum from BI. In order to vacate NI amount of spectrum and to maintain the calls those are using the NI spectrum RATS is to hand off the calls to other SBs. The maximum number of the SHs that Bi can support is subject to the spectrum usage state of RATS. The general form of the amount of the spectrum of Bi that has to be vacated is N , and possible number of the SHs from BI that Bi can support is NH. Equations (2) and (3) show N and NH in generalized form. It should be noted that N is amount of spectrum while NH is number of the SHs. Since Pr(B1) is the highest one, NI is directly transferred to N . Variable WA is the average bandwidth of calls that use the N if I 1max (0, γ SI ) if I 1N max (0, γ SI ) if I 1N N NH · WA if IN NH · WA if I (2)

NH min NWA , SWA if Iif I (3)

(a) I=1 (b) I=2 (c) I=3

(d) Spectrum handoff among spectrum blocks

Figure 2. Cascaded use of spectrum blocks

(a) Spectrum blocks shared by RATs

(b) Spectrum pooling of the 700MHz broadband public

safety and D-block bands

Figure 1. Shared spectrum blocks

spectrum which has to be withdrawn, and ⌊x⌋ means the largest integer less than or equal to x. Figure 2(d) shows the cascading relationship of the amount of the spectrum resource to be vacated and that of the SBs, and the number of calls to be handed off.

The total number of all SHs within RATS is NH(γ)∑ NHM and the number of dropped calls is given in (4), when the PU reclaims γ of BI from RATS.

NHD(γ) NM /WA NMH if I MNM/WA NMH if I (4)

III. PERFORMANCE ANALAYSYS For the sake of simplified and tractable analysis we assume M 2, and ST S, ST D BUs. We also assume that the

reclaim of the rented spectrum is only comes from the owner of B2, i.e. I 2. Letting the average bandwidth of calls, WA, be 1 BU/call makes the analysis much clearer.

A. Markov chains Building the MMPP the call arrival rate to RATS is modeled

as Poisson process with the mean of and the call duration follows exponential distribution with the mean of 1/ . We build two Markov chains: one is for CRA and the other is for RRA. Pr(Bi) of CRA follows (1) and that of RRA is 1 for all i’s. States of both Markov chains are defined by (s, d), where s is the number of B1 BUs, and d is of B2 BUs allocated to the users in RATS . In Fig. 3(a) the resource of B1 is selected whenever . In Fig. 3(b) the resource of B1 and B2 are randomly selected with equal chance according to the amount of left resource of each SB. From the Markov chains in Fig. 3(a) and (b), we can determine P( , ),P and P( , ),P . The probability P( , ) is the probability that s and d BUs are being occupied among S of B1 and D of B2 resources. The subscript P and P represent the resource allocation algorithm applied, i.e. P for CRA and P for RRA. Following the basic characteristic of the steady state probability ∑ P( , ),P(S,D)( , ) 1

and ∑ P( , ),P(S,D)( , ) 1. Figure 4 shows the SH and the dropping of calls at each

state of both Markov chains according to which ranges from 1 to D. For examples, assuming S=8, D=7 and state is (7, 3) and applying (2) to (4):

if =2 then there will be no SH and no call dropping,

if =5 then there will be one SH and no call dropping, and

if =6 then there will be one SH and one call dropping.

B. Expected number of SHs and call droppings With any given , S, D, and we can determine the

average number of the SHs and the call droppings, however we omit parameters S, D, and for simple presentations in (5) to (8). The average number of the SHs for each algorithm is given in (5), and the average number of the call droppings is given in (6). The average number of the call droppings for

both algorithms are the same, because CRA may reduce SHs but it may not increase the system capacity.

E NH,P (γ) ∑ NH(γ) · P( , ),P(S,D)( , ),PE NH,P(γ) ∑ NH(γ) · P( , ),P(S,D)( , ),P (5)

(0,0) (1,0) (2,0) ... (S-2,0) (S,0)

(0,1) (1,1) (2,1) (S-2,1) (S,1)

(0,2) (1,2) (2,3) (S-2,2) (S,2)

...

...

(0,D-1) (1,D-1) (2,D-1) (S-2,D-1) (S,D-1)

(0,D) (1,D) (2,D) (S-2,D) (S,D)

...

...

(0,D-2) (1,D-2) (2,D-2) (S-2,D-2) (S,D-2)...

(S-1,0)

(S-1,1)

(S-1,2)

(S-1,D-1)

(S-1,D)

(S-1,D-2)

γ=D

γ=D-1

γ=1

γ=2

SH SH SH SHDrop

SH SH SH SH SHDrop

SH

SH SH SH SHDrop

SH

SH SH SH SHDrop

SH

...SH SH SH SH

DropSH

Figure 4. Spectrum handoffs and call droppings

(a) Markov chain for CRA

(b) Markov chain for RRA

Figure 3. System state transition diagrams

E NHD,P (γ) ∑ NHD(γ) · P( , ),P(S,D)( , ),PE NHD,P(γ) ∑ NHD(γ) · P( , ),P(S,D)( , ),PE NHD,P (γ) E NHD,P(γ) (6)

In reality is not a determined value and it varies according

to the PU’s traffic situation. We are to average all possibilities of , P , which represents the probability that the PU reclaims

BUs. It should be noted that the time that the PU does not reclaim any of B2 should also be taken into account when calculating the average number the SHs and the call droppings. Equations (5) and (6) are then developed to (7) and (8) respectively.

E NH,P ∑ PD · E NH,P (γ)E NH,P ∑ PD · E NH,P(γ) (7)

E NHD,P ∑ PD · E NHD,P (γ)E NHD,P ∑ PD · E NHD,P(γ)E NHD,P E NHD,P (8)

C. Utilization and Gains Utilization of each SB can be defined as (9) and (10)

according to the resource allocation algorithms. The sums of the utilization of both SBs are the same regardless of applied algorithms as (11). The statistics on the utilization of each SB can be used for further decision-making or pricing of the SB renting. Considering the utilization of the SBs operators may decide whether to rent new spectrum resource with a certain price.

UD,P ∑ P( , ),P · D(S,D)( , ),PUD,P ∑ P( , ),P · D(S,D)( , ),P (9)

US,P ∑ P( , ),P · S(S,D)( , ),PUS,P ∑ P( , ),P · S(S,D)( , ),P (10)

US,P UD,P US,P UD,P (11)

The gain of applying CRA can be calculated multiplying the cost of SH, ζSH. The gain of CRA when SH is applied is given in (12).

₡H,P E NH,P E NH,P · ζSH (12)

IV. NUMERICAL ANALYSIS AND SIMULATION RESULTS The settings for analysis and simulation are as following:

M 2, ST S, ST D , 0 S, D 15 and S D 15, WA 1 BU/call,

Nominal utilization, ρN λ/((S D) • µ), is set from 0.2 to 0.6, i.e. 3 to 9 Erlang,

P ~U(0, D), and ζSH 1 currency unit.

The simulation is performed in the time unit of second and 1/μ is 180 seconds. Figure 5 shows the expected number of the SHs and the call droppings during a second. We see that the number of the SHs in CRA is reduced around 1/3 to 1/4 of RRA. The dashed line is simulation result and solid line is analysis result from the MMPP. The expected number of call droppings is also plotted to be compared to the reduction of

(a) E NH,P

(b) E NH,P

Figure 6. Expected number of spectrum handoffs

0

5

10

15

0.2

0.45

0.62

4

6

8

10

12

14

x 10-3

size of D

Expected number of Spectrum handoffs(H,P')

ρN

0

5

10

15

0.2

0.45

0.60

1

2

3

4

x 10-3

size of D

Expected number of Spectrum handoffs(H,P)

ρN

Figure 5. Spectrum handoff and call dropping

0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.650

0.005

0.01

0.015Expected number of SHs and call droppings(D=7, γ=4)

ρN

H,P'

H,PHD,P

SH. Figure 6 shows the expected number of the SHs to ρN and D. Since S+D is fixed in our setting the number of the SHs are decreased from the point where D is around a half of S+D. It is because there are no much resource to take over the SHs from D resource after D is outgrown. As compared in Fig. 6(a) and (b) the number of the SHs is reduced in all combinations of D and ρN by CRA. Figure 7 shows the utilization of the SBs. The slanted plane is the utilization of S or D in RRA algorithm, which is represented in P . In RRA the utilizations of S and D are same. Figure 7(a) shows that the utilization of the SB of D in CRA is less than that of P and Fig. 7(b) shows that SB of S in CRA is higher than that of P . This is because of the prioritized access to S. Figure 8 shows the gain of SHs in (12). The unit of the gain is currency unit per second.

V. CONCLUSION DSA is considered as a smart solution to the scarcity of

radio spectrum and a lot of research activities are being performed for efficient utilization of spectrum resource. One of the well-known approaches of DSA is the PSU which is accompanied by the preemption of the PU to the secondary usage and it may require the SHs of the ongoing call. The SH may lead to degradation of QoS. This paper makes use of statistical difference of traffic characteristics of multiple SBs to reduce the SHs. The background of multiple SBs can be found in some literatures including FCC’s lately revised PPP plan. We analyze and simulate the proposed CRA to show

how it reduces the number of the SHs. We also analyze the utilization of SBs and the gain of applying CRA.

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[16] L. Kleinrock, “Queueing Systems Volume 1: Theory,” Wiley-Interscience, 1975.

Figure 8. Gain of CRA

0

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ρN

(a) UD,P and UD,P

(b) US,P and US,P

Figure 7. Utilization of SBs of S and D

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size of D

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size of D