A new algorithm for admission control of secondary users in CDMA

Int’l Conf. on Computer & Communication Technology Ň,&&&7¶Ň

A New Algorithm for Admission Control of Secondary Users in CDMA based Cognitive Radio Network Sanjay Dhar Roy

Soumen Mondal

Sumit Kundu

ECE Department, NIT, Durgapur Durgapur, India, Pin-713209 [email protected]

ECE Department, NIT, Durgapur Durgapur, India, Pin-713209 [email protected]

ECE Department, NIT, Durgapur Durgapur, India, Pin-713209 sumit.kundu @ece.nitdgp.ac.in

Abstract— In this paper, a simulation study has been carried out to evaluate performance of a new call admission control algorithm for secondary users in CDMA based Cognitive Radio Networks (CRNs). Performance has been evaluated for secondary users (SUs) in an uplink scenario of CDMA network consisting of cognitive users or secondary users (SUs) and Base Station (BS). A primary receiver (PRx) is also assumed to be present in our network model. We assume a network where primary radio network (PRN) and cognitive radio network (CRN) coexist. The new algorithm for admission control of cognitive radio (CR) users considers two constraints. BS computes instantaneous SIR of a SU and PRx computes interference from all SUs present in the network. Specifically, the proposed CAC algorithm considers uplink Signal to Interference Ratio (SIR) of a SU to be above an SIR threshold level and instantaneous interference at primary receiver to be below a predefined interference threshold. Performance of the algorithm has been evaluated in different scenarios in terms of blocking probability of SU in presence of a PRx. Keywords- Cognitive Radio, admission probability , CAC algorithm, CDMA

I.

control,

blocking

INTRODUCTION

In recent years, the demand for the spectrum resource is increasing rapidly with the emergence of various wireless services providing very high speed communication. Since there is only a finite amount of the spectrum resource, the remaining spectrum is being exhausted and it leads to the spectrum scarcity problem. Cognitive radio [1] is an enabling technology that allows unlicensed users to opportunistically access the spectrum in order to enhance the spectrum efficiency. Cognitive radio can be defined as a radio that can change its transmitter parameters based on interaction with the environment in which it operates. Cognitive radio networks allow presence of two types of users, Primary Users (PUs) and Secondary Users (SUs) or Cognitive Users. A primary user is the right full owner of a channel, while a secondary user periodically scans the channels, identify currently unused channels and access the channel opportunistically in case of spectrum overlay type of spectrum sharing. However, in case of spectrum underlay type of spectrum sharing, SUs may transmit all the time, in presence of PUs, maintaining a predefined interference threshold at primary receiver. We consider a network model that consists of one Base Station (BS), several cognitive users and one PRx.

__________________________________ 978-1-4244-9034-/10/$26.00©2010 IEEE

35

Cognitive Users can be admitted to the BS only if the interference to the PRx is less than the predefined threshold [2]. At the same time cognitive users should adapt their transmission powers in order to satisfy their own Signal to Interference Ratio (SIR). This algorithm may be used to reduce interference to the PRx for the PRN as call is admitted on the basis of the interference threshold. Admission of a SU depends also on the required SIR threshold of SU. Contribution of the paper: We propose a new algorithm for admission control of SUs in a CDMA based CRN. A primary receiver is also assumed. We assume that all users are active i.e., all admitted SUs are using traffic channels. That is, all users are interferer to each other in our CDMA based CRN. We consider centralized admission control of all SUs by the BS. In future, we shall assume decentralized admission control of SUs in similar kind of network. Here, PRx is sharing information with the BS. Whenever, a new user seeks admission into the network, interference at PRx may go above the interference threshold. PRx would send an alarm to the BS when overall interference crosses the tolerable interference limit. A new SU may degrade interference scenario for CRN. So, a new SU would be allowed in the network only if overall interference to PRx remains below the threshold and SIRs for all users including the new one remain acceptable. In our present work, we assume interference at PRx due to SUs only, i.e., we are considering that there is some room for SUs always. II.

SYSTEM MODEL

A. Cell Scenario We consider a set of cognitive users in a hexagonal cell and a BS is located at the centre. Cognitive users are covered by the BS. We also assume that a PRx is near to our cell. In our assumed network model Code Division Multiple Access (CDMA) is considered as multiple access mechanism, where all SUs can access the same spectrum band simultaneously. PRx is considered in the receiving mode and SUs are trying to transmit data in the uplink to the BS. Hence, PRx will receive the interference from cognitive users [3, 4].

Int’l Conf. on Computer & Communication Technology Ň,&&&7¶Ň where d c is the distance of SU from the PRx. dc

(d * sin(T )  d PRx * sin(T PRx )) 2  (d * cos(T )  d PRx * cos(T PRx )) 2

(4)

T : angle of cognitive radio user from BS with respect to reference line. T PRx : Angle of PRx from BS with respect to reference line.

D. SIR Calculation The SIR of a SU at the BS will be the ratio of power received at the BS for any particular SU and summation of power received from all other users except the particular user. Hence, SIR of i-th SU may be expressed as:

Fig 1: The cellular layout for simulation

SIR (i )

Pr (i )

(5)

n

¦ Pr ( j )

B. Radio Propagation model:

j 1 j zi

Cellular communication is operating at UHF/VHF frequency bands; radio propagation is influenced by three factors: path loss, log normal shadowing and multipath fading [5]. In our case, radio propagation model considers log normal distribution of shadowing with path loss only. A Mobile Station (MS) at distance d from a base station (BS) provides received field strength at BS as follows:

Note that, we did not consider processing gain for SIR expression. Residual capacity can be defined as additional number of users that a base station can serve [5]. Residual capacity is evaluated following Algorithm I of [5] as stated below. Algorithm I





Pr

Pt *10

]

10

*(

R min R

wKHUHȟLQGHFLEHOKDVDQRUPDOGLVWULEXWLRQZLWK]HURPHDQ and standard deviation (ı), 5~12 dB and d o is the close in reference distance. Value of path loss exponent (Į) is 4. Here, Pt is the transmit power of a MS and Pr is the power received at the base station (BS).

C.

Power Control: In CDMA systems, received power at BS is kept at a constant value to avoid near far problem. However, MS situating at far away from BS may need to transmit at very high power, which may drain the battery quickly and may lead to health hazard. For that, transmit power of MS must be limited to a maximum allowable transmit power level. If the required transmit power level is greater than the maximum allowable transmitted power, the transmitted power is fixed at a maximum allowable transmitted power.

if Pt ! Pmax

then Pt

Pmax

(2)

« 1 1 »  « »!0 ¬ SIR th SIR i ¼ otherwise

« 1 1 »  R i « » if ¬ SIR th SIR i ¼ 0

do D )   d

(6)

E. Interference Power: Power received from any SU at PRx is interference to PRx. Total interference will be the summation of all interferences created by all SUs.

I total

n

¦ Prc(i )

(7)

i 1

where n : Number of SUs. Prc : Power received at the PRx.

F. Performance measurement The performance of the network is measured by the blocking probability and outage probability. Blocking Probability is defined as:

PRx will receive interference from each cognitive user. We assume that PRx is situated at a distance d PRx from the BS. The received power at PRx for each user will be

Pblk

Pr ob^R

0`

(8)

Outage Probability is defined as: Prc

Pt *10

]c

10

d * ( o )D dc

(3) Pout

36

Pr ob^SIR d SIRth `

(9)

Int’l Conf. on Computer & Communication Technology Ň,&&&7¶Ň

Call block due to low SIR. block counter=block counter+1 end if 13. End of the loop 14. End of the loop 15.Blocking probability=block counter/(iteration*new call)

SIR Vs cognitive user 0.12 with powercontrol,in this model in 7cell without power control,zarki model in 7cell with powercontrol considering only one cell

0.1

0.08

SIR

III.

SIMULATION MODEL

0.06

0.04

0.02

0 10

20

30

40

50 60 70 No. of cognive users

80

90

100

Fig 2: Signal to Interference Ratio Vs Cognitive users.

Now, we present our proposed algorithm for call admission control (CAC).

G. Call Admission Control algorithm:

The simulation has been developed in MATLAB. Several cognitive radio (CR) users or SUs are present in the hexagonal cell. One PRx is present outside the hexagonal cell at a fixed position. Distance and angle of cognitive radio users with respect to BS are generated randomly as per uniform distribution within the cell. We calculate the distances between PRx and cognitive radio users using equation (4). The CR users are power controlled by the BS for SUs. According to the requirement for simulation, (i) the numbers of SUs, (ii) value of interference threshold and (iii) value of SIR threshold are chosen. Algorithm that describes our simulation model is shown in Section 2. Since CDMA is interference limited, noise term in the SINR expression has been neglected in our simulation.

1. Input: n, int TH , SIRth , Pr , Pmax , Radius, iteration 2. Initialize iteration 3. Loop for iteration 4. Generate: d ( i ), d PRx , ] i , ] ic 5. Calculate:

d c( i ), Pt ( i )

6. if Pt ( i ) ! Pmax

Pt ( i )

Pmax

end if 7. Calculate: Prc 8. Calculate: Interference

SIR( i ) 10. Calculate: R( i ) 11. Calculate: R 9. Calculate:

12. Loop for arrival calls if R ! 0 then if

Interference ! int TH Call block due to more interference at PU block counter=block counter+1

else Call admitted end if else

III.

RESULTS AND DISCUSSIONS

In our assumed model, a BS is located at the centre of a hexagonal cell with a radius of 1000m. The distance between a SU and BS is randomly chosen from 100m to 1000m and angle of a SU is randomly generated between >ɉ@. A PRx is located at a fixed position outside the cell at a distance of 1500m as shown in Fig. 1. Two different allowable maximum transmission powers are assumed to be 0.5mW and 0.005mW. We assume propagation model with path loss exponent, Į = 4 and VWDQGDUGGHYLDWLRQı = 8 dB. In Fig. 2, uplink SIR of a CR user or SU is shown as a function of number of CR users. In our model, we have evaluated average SIR for each user considering power control mentioned in equation (2). We find that SIR obtained as per our model is better than SIR obtained as per the model of [5] in the 7 cell pattern for both with and without power control as mentioned in equation (2). Signal strength based power control has been assumed in this paper i.e., BS needs same received power from all SUs to avoid near and far problem. From the received power at BS, the value of required transmitted power of SU can be calculated on the basis of its link gain. If required transmit power of a SU is greater than maximum allowable transmit power, it assigns a fixed value of maximum allowable transmit power. Since maximum allowed transmit power is limited, the interference caused is also less in our case compared to conventional power control. Thus, the SIR is better in our model for each user than the SIR of 7 cell pattern model having conventional power control method.

37

Int’l Conf. on Computer & Communication Technology Ň,&&&7¶Ň

SIR Vs cognitive user

Blocking Probability Vs cognitive user

0.12

0.4

0.35

0.1

Blocking Probability

SIR

0.06

0.04 SIR = -18.9279 dB X: 80 Y: 0.0128

0.02

0 10

20

30

40

50 60 70 No. fo cognive users

80

-5

90

sigma= 8dB, Rxer Sensitivity= -80dB 4.5 4 3.5 3 2.5 2 1.5 Interference = -50 dB

1 0.5 0 10

20

30

40

50 60 70 No. fo cognitive users

80

90

0.2

0.15

0.05

100

interference Vs cognitive user

x 10

0.25

0.1

0 10

Fig 3: Uplink SIR of a cognitive user as a function of number of cognitive users.

Interference

Max.trans power=0.005,int th=-50dB,SIRth=-18.95dB Max.trans power=0.005,int th=-50dB,SIRth=-19dB

0.3

0.08

5

Max.trans power=0.5,int th=-50dB,SIRth=-18.95dB Max.trans power=0.5,int th=-50dB,SIRth=-19dB

100

Fig 4: Interference to PRx vs. number of cognitive users.

20

30

40 No. of cognitive users

50

60

70

Fig 5: Blocking probability versus cognitive

In Fig. 5, blocking probability of cognitive user is shown as a function of number of cognitive users. The blocking probability of CR users in our model is increasing when number of CR users is increasing. The blocking probability is evaluated in four different cases in our model. Four typical cases with various values of maximum allowed transmit power, interference threshold and SIR threshold are studied such as (i) the value of maximum allowable transmitted power (P max ) is fixed at 0.5mW and interference threshold is fixed at -50dB and SIR threshold is fixed at 18.95dB, (ii) P max is fixed at 0.5mW and interference threshold is fixed at -50dB and SIR threshold is fixed at 19dB, (iii) P max is fixed at 0.005mW and interference threshold is fixed at -50dB and SIR threshold is fixed at 18.95dB, (iv) P max is fixed at 0.005mW and interference threshold is fixed at -50dB and SIR threshold is fixed at 19dB. For case (i), maximum allowable transmitted power is more compared to other cases, so interference created by all CR users will be greater than the other cases and SIR would

Number of cognitive users allowed in the network satisfies either of the conditions of our algorithm as mentioned in Section 2, is shown in Fig. 3 and Fig. 4. Uplink SIR of the new user must be above a specified SIR threshold. Secondly, interference caused to PRx by all cognitive users including the new cognitive user must be below a predefined interference threshold. Uplink SIR of a cognitive user reduces if number of users increases (Fig. 3). Interference at PRx would be increased as number of cognitive users increases. If we consider an interference threshold of -50dB without taking SIR threshold into consideration, number of cognitive users allowed into the network would be merely twenty (20). Similarly, less than eighty (80) cognitive users may be allowed if SIR threshold is around – 18.93 dB without taking interference constraints into consideration with same values of parameters. However, a SU is allowed into the network if it satisfies both the conditions simultaneously.

Blocking Probability Vs cognitive user 0.32 Max.trans power=0.005,int th=-50dB,user=50 Max.trans power=0.5,int th=-50dB,user=50

0.3

Max.trans power=0.005,int th=-40dB,user=50 Max.trans power=0.5,int th=-40dB,user=50

Blocking Probability

0.28 0.26 0.24 0.22 0.2 0.18 0.16

-19

-18.8

-18.6

-18.4

-18.2 -18 -17.8 SIR threshold in dB

-17.6

-17.4

-17.2

Fig 6: Blocking probability versus SIR threshold in dB

38

-17

Int’l Conf. on Computer & Communication Technology Ň,&&&7¶Ň

-4

5

Transmitted power Vs distance of user from BS

4 3.5 3 2.5 2 1.5 1 0.5 0 500

600

700

800 900 1000 1100 1200 Distane of user from BS(m)

1300

1400

1500

Fig 7: Analytical plot of transmit power vs. distance of user

the rate of increase in blocking probability with SIR threshold is slow. This paper shows impact of number of cognitive users, SIR threshold, and interference threshold on the admission of a CR user in an underlay spectrum sharing model where a primary receiver is assumed to be present in the vicinity of a cognitive radio network. In future, more comprehensive analysis for our network model in terms of throughput, delay and outage probability would be carried out. REFERENCES [1]

[2]

[3]

[4]

[5]

V.

x 10

4.5

Transmitted power(mW)

be low. Hence, blocking probability is more than other cases. As maximum allowable transmitted power is fixed at 0.005mW, interference threshold is fixed at -50dB and SIR threshold is fixed at -19dB, maximum allowable transmitted power is the lowest among all cases. Hence the interference to PRx will be the lowest compared to the other cases and average SIR of SU will be lowest amongst all SIRs. Thus, blocking probability for this case is lowest amongst all four cases. In other two cases, i.e., case (i) and case (ii), i.e., when the maximum transmitted power is 0.5mW, blocking probability is more. This is due to the fact that effect of transmit power is more than the effect of SIR threshold on probability of blocking. In Fig. 6, blocking probability of a cognitive user is shown as a function of SIR threshold in dB. Blocking probability also depends on the SIR threshold. Blocking probability increases very slowly in nature with increase in SIR threshold. When SIR threshold is high, instantaneous SIR of a SU would be lower than the SIR threshold for many cases. This further increases blocking probability. Note that a SU would be allowed in the network when its SIR is above an appropriate SIR threshold. For a transmitted power of 0.5mW and interference threshold of -50dB, blocking probability is large. As the maximum allowable transmitted power is more, interference to PRx created by the SUs is more. Blocking probability decreases as interference threshold is increased from -50dB to -40dB. Total interference created by all users should not be more than interference threshold. Many users are blocked in case of low interference threshold compared to the case of high interference threshold. For that reason, first two curves show higher blocking probability in this figure. As maximum allowable transmitted power is reduced to 0.005mW and interference threshold is -40dB, transmit power is low. Since interference to the PRx will be low and interference threshold is high, the system can allow more number of users. This results in low blocking probability in this case. In Fig. 7, transmit power of a CR user is shown as a function of distance of it from BS. Here, we are considering only path loss. Shadowing and multipath fading are not considered. The transmitted power of a CR user situated at long distance from the BS should be higher than the transmitted power of user near to the BS. This is needed to avoid near far effect.

CONCLUSIONS

In this paper, we have proposed a new admission control algorithm for cognitive radio users. The algorithm is based on simultaneous satisfaction of SIR of SUs as well as interference conditions to primary radio network. The algorithm improves the performance of the system as it also takes care of interference to the primary radio network. Our proposed scheme provides better SIR for a SU than the conventional methods. We have also evaluated blocking probability of SUs with respect to number of cognitive users and SIR threshold. Blocking probability increases with increase in number of CR users. Blocking probability also increases with increasing value of SIR threshold. However, 39

J. Mitola and G. Maguire, “Cognitive radio: making software radios more personal” IEEE Personal Communications., Vol. 6, No 4, pp. 13–18, Aug. 1999. Ki Tae and Seong Keun Oh, “Cognitive Ad-hoc Networks under a Cellular Networks with an Interference Temperature Limit,” ICACT, pp 879-882, February 2008. Karama Hamdi, Wei Zhang, and Khaled Ben Letaief, “Uplink Scheduling with QoS Provisioning for Cognitive Radio Systems” WCNC 2007, pp-2594-2598. Tev¿N