Using TV White Space for Interference Mitigation in LTE Femtocell Networks

Proceedings of ICCTA2011 Using TV White Space for Interference Mitigation in LTE Femtocell Networks

Fei Peng, Yue Gao, Yue Chen, K K Chai, Laurie Cuthbert

Queen Mary University of London School of Electronic Engineering and Computer Science London, United Kingdom [email protected]

Abstract technology to get access to certain licensed bands, like TVWS is being promoted by international The traditional Femtocell shares the same licensed organisations, such as Federal Communications frequency band with Macrocell, thus introducing Commission (FCC) in US, and recently by Ofcom cross-tier interference. Interference mitigation in UK. between the Femtocell and Macrocell is considered to be one of the major challenges in Femtocells Among many potential applications for the TVWS, Network. In this paper, the released TV spectrum one promising application network scenarios is by the digital switchover named TV White Space Long Term Evolution (LTE) Femtocell Network, in (TVWS) is proposed to reduce the interference for which it is referred as the HeNB [4]. It is a home the LTE Femtocell Network. Based on the base station with short range, low cost, and being combination of cognitive sensing and TVWS, a linked with the cellular network via a broadband novel resource allocation scheme is applied to connection; it could offload the traffic in cellular mitigate the interference that Macro users are network. Therefore Femtocell is an effective suffering from nearby Femtocells. A single cell approach to improve the indoor coverage and cell Long Term Evolution (LTE) Femtocell simulator is capacity. The main challenge in Femtocell established to demonstrate the system performance. deployment is the cross-tier interference, since The cases with traditional resource allocation traditional Femtocell exploits the same frequency scheme and the proposed scheme are compared and band as the cellular network, cellular users that are analysed. located in close proximity to Femtocell BSs may suffer heavy interference from the nearby Keywords: Femtocell, TV White Space, Cognitive Femtocells. Sensing, Cross-Tier Interference, Mobile Network Current research of approaches on solving cross- tier interference problem includes: 1) Fractional Frequency reuses and resource partition, in which 1. Introduction the main idea is to divide the entire frequency spectrum into several sub-bands, and each sub-band Cognitive Radio, which was firstly introduced by is differently assigned to sub-area of Macrocell and Joseph Mitola in 1999 [1], is being intensively Femtocells [5]; 2) Power control method focuses on researched as the key technology for getting access reducing transmission power of HeNBs, the to TV White Space (TVWS). The TVWS is a large advantage is that base station and HeNB can use the portion of vacant spectrum in UHF/VHF bands that entire bandwidth with interference coordination [6]; are created during the Digital Switchover (DSO) for 3) Collaborative frequency scheduling. In this the higher spectrum efficiency of Digital TV. In a approach, the HeNBs receive Macro Users given location there are some vacant channels from scheduling information from base station, and guard bands and locally unused TV channels in TV compare them with their sensing result to find band. These channels could be utilised by low spectrum opportunities [7]; and 4) Cognitive power devices on the basis that not causing approach uses distributed sensing and shared path- interference to TV service. In the UK over 50% of loss information among HeNB neighbours [8]. geographical locations are likely to have more than However, the above approaches are still based on 150MHz of interleaved spectrum (TVWS) [2]. The the traditional mobile frequency band, and have great attractiveness of TVWS comes from its limited performance improvement at the expense of advantage in the combination of bandwidth and complexity. The TVWS was applied to a pure coverage [3]. Using Cognitive Radio (CR) Femtocell Network in our previous study [9]. In

1 this paper, we innovatively exploited the TVWS for the LTE Femtocell Network. A novel Cognitive Sensing combined with TVWS resource allocation scheme is proposed to solve the cross-tier interference problem in LTE Femtocell deployment. The improvement of the system performance is house demonstrated by simulation results.

Max femto BS The rest of the paper is organised as follows: Part II area introduces the system model being studied and the proposed resource allocation scheme in LTE Max UE area Femtocell Network. Part III presents the simulation model and parameters used for demonstrating the Figure 2 Illustration of a Femtocell Deployment Model proposed scheme, followed by the analysis on the obtained simulation results in Part IV. Finally, B. Downlink Femto-Macro Interference Scenario conclusions are drawn in Part V. The Macro BS transmission power is typically far larger than the Femto BS transmission power, so that the interference experienced by Macro users is 2. Using TV White Space For LTE dominated by interference from Macro BS. Femtocell Network However, when a Macro user is very close to a Femto BS or even within Femto BS coverage, it A. System Model experiences high Femto BS interference, as shown 1) Network Layout in Figure 3. If a Macro user is located in an indoor Our simulator employs a single Macro LTE environment, the signal received from Macro BS is network with several Femtocells located randomly generally very poor due to the wall penetration loss. that represents the actual Femtocell deployment In traditional Femtocell Network, the Femtocell distribution as shown in Figure 1. There are mainly shares the same spectrum with Macrocell, thus indoor Macro users suffer unacceptable low SINR. four types of network entities in this scenario, namely Macro Base Station, Macro user, Femto BS and Femto user.

Femto BS Femto User

Macro BS

Macro User

Figure 3 Example of Cross-Tier Interference between Macrocell and Femtocell

Figure 1 Illustration of a single cell LTE Femtocell Network C. Cognitive Sensing combined with TVWS Resource Allocation Scheme 2) Femtocell Deployment Modelling For the proposed interference scenario, an effective Femtocell deployment model is built according to approach to mitigate the Cross-Tier Interference 3GPP specification on HeNB suburban deployment would be assigning different spectrum bands for modelling [10]. HeNBs are dropped within the Macrocell and Femtocell respectively. For instance, macro cell area with a random uniform distribution Femto BS cannot get access to Resource Blocks Figure 2. For each house the HeNB and HeNB UEs (RBs) allocated to Macro users in order to avoid the are randomly dropped within a specific distance of co-channel interference. The traditional way of doing this is using resource partitioning, which is to the centre point of the houses. All Macro users are allocate separate spectrum band for Macrocell and assumed to be in an indoor environment. A macro Femtocell. However, this approach is not frequency UE may be within a HeNB house. efficient and it mitigates the interference at the expense of spectrum utilisation.

2 A Cognitive Sensing combined with TVWS scheme is proposed to exploit the newly opened 500 unlicensed TVWS spectrum for LTE Femtocells, so 400 that to achieve the interference mitigation, while not occupying the spectrum for Macrocell use, and 300 improving spectrum utilisation. 200

100 In this scheme, TVWS availability is obtained by Macro BS retrieves information from local TVWS 0 Distance (m) Distance database through our previous study [11]. Macro -100 users will sense nearby Femtocells transmission power. Based on the sensing result, by comparing -200 with pre-defined threshold Macro users will -300 categorised nearby Femtocells as interfering and non-interfering Femtocells. This Information is sent -400 to Macro BS from each Macro user. -500 -500 -400 -300 -200 -100 0 100 200 300 400 500 Distance (m) The Cognitive Sensing combined with TVWS resource allocation scheme is summarised as Figure 4 Network Layout of Simulation follows:

START˖ 350

Step1: Cognitive Sensing Stage 300 1. All Macro Users sense nearby Femtocells transmission power 250

2. Compare the received power with power decision 200 threshold. 150 1) If PPri, ! threshold

Distance (m) 100 Femtoi o Interfering Femtocell List 50 2) If PPri, threshold 0 Femtoi o Non-Interfering Femtocell List

-50 3. Macro Users send the Interfering Femtocell List as feedback to Macro BS. -100 0 50 100 150 200 250 300 350 400 450 500 Distance (m) Step2: Resource Allocation Stage 1. Macro BS allocates Resource Blocks (RBs) to Figure 5 Detailed Views of Houses with Femto BSs Macro Users and Femtocell BSs in the Non- Interfering Femtocell List In the simulation, the path losses for each Macro 2. Based on the TV White Space availability and Femto users are calculated according to the information from local TVWS Database, Macro 3GPP specifications on HeNB [10]. BS allocates available TVWS RBs for Femtocells in the Interfering Femtocell List 1. Macro BS to Macro User END a) Macro User is outside a house

PL( dB ) 15.3 37.6log10 , R in m (1)

3. Simulation Model and parameters b) Macro User is inside a house In this section, we study the performance of the PL( dB ) 15.3 37.6log10 R Low , R in m (2) proposed scheme by simulations. The network layout in simulation is illustrated in Figure 4. The 2. Femto BS to Macro User big black triangle represents the Macrocell BS, c) Macro User is inside the same house as green points are macro users, and blue squares are houses with yards. Small black triangles are Femto Femto BS BSs, and red points inside the houses are Femtocell PL( dB ) 38.46 20log10 R +0.7d2,D indoor users as in Figure 5. Simulation parameters are n2 given in the Table 1. (0.46) n 1 18.3n (3)

3 traditional resource allocation scheme that all spectrums are shared among Macro and Femto d) Macro User is outside a house Users. As shown in Figure 6, the vertical axis

PL( dB ) max(15.3 37.6log10 R ,38.46 20log 10 R) indicates the interference suffered by Marco users, n2 (0.46) and horizontal axis is the number of simulation +0.7d 18.3nLn1 rounds. In the simulation scenario, there are 40 2,D indoor ow (4) Femtocells with 2 Femto users in each Femtocell and 60 Macrocell users. The red line indicates the e) Macro User is inside a different house traditional all-shared resource allocation scheme, PL( dB ) max(15.3 37.6log10 R ,38.46 20log 10 R) while the blue line is the Cognitive Sensing n2 (0.46) combined with TVWS scheme. n1 +0.7d2,D indoor 18.3nLLow,1 ow ,2 (5) In Figure 6, it can be seen that when the proposed scheme is applied, the cross-tier interference Where R is the Tx-Rx separation in meter, Low is suffered by Macro users (MUEs) reduced by about the penetration loss of an outdoor wall, which is 50% compared to that in the traditional scheme. 10dB or 20dB. d2D, indoor is the distance inside This is because the potential interfering Femtocells the house. d2D, indoor is the total distance inside in the proposed scheme are allocated with TVWS the two houses. Low,1 and Low,2 are the from Macro BS rather than the same frequency band penetration losses of outdoor walls for the two as Macro Users. Femto BSs have no access to RBs houses. that assigned to Macro users, thus cross-tier interference from Femto BSs to Macro Users is Table 1 Simulation Parameters significantly mitigated. Parameter Assumption

Femtocells per Macrocell 40 -7 x 10 Femto users per Femtocell 2 14 Traditional resource allocation No. of Macro users 100 Cognitive sensing+TVWS resource allocation

Macrocell radius 500 m 12 Femtocell radius 12 m Carrier Frequency 2000 MHz 10 Shadowing standard 8 dB deviation 8 Wall Penetration Loss 10dB or 20dB 6 Carrier bandwidth 5 MHz

Total No. of RBs 25 (mW) Total Interference MUE 4 RB bandwidth 180kHz Total Macro BS TX power 46 dBm 2 HeNB Tx power 20 dBm 5 10 15 20 25 30 35 40 45 50 Minimum distance >= 35 m Simulation Round between UE and cell Min separation UE to 20 cm Figure 6 Interference Comparisons of Two Schemes HeNB UE distribution UEs dropped with uniform Figure 7 shows the interference mitigation density within the performance variation when different power indoors/outdoors macro coverage area, subject to a decision thresholds are being used, with threshold = minimum separation to macro -50 dBm, -55 dBm, -60 dBm, -65 dBm and -70 and HeNBs. dBm, respectively. As shown in Figure 7, lower threshold leads to higher interference mitigation performance. This is because more interfering Femto BSs can be sensed, and then avoided allocating the same band as Macro users. It can also 4. Results and Analysis be seen that Thresholds = -60 dBm, -65 dBm, -70 dBm achieve similar performance. Since lower The comparison of the improvement in the power decision threshold results in higher interference mitigation is carried out first in our requirements on sensing equipment for performing simulations. In order to compare with the newly sensing. From the obtained results, an optimised proposed cognitive sensing combined with the threshold value should be around -60 dBm to -65 TVWS scheme; a traditional resource allocation dBm in this scenario by taking into account both scheme is also implemented. It is assumed in the performance and cost.

4 -7 Networks,” in Proc. IEEE VTC’11- Spring, Budapest, x 10 3 Hungary, May, 2011. Threshold = -50 dBm Threshold = -55 dBm [7] M. E. Sahin, I. Guvenc, Moo-Ryong Jeong, H. Arslan, Threshold = -60 dBm “Handling CCI and ICI in OFDMA Femtocell Networks 2.5 Threshold = -65 dBm Threshold = -70 dBm through Frequency Scheduling,” IEEE Transactions on Consumer Electronics, vol. 55, no. 4, pp. 1936–1944, 2 November, 2009. [8] L. Zhang, L. Yang, and T. Yang, “Cognitive Interference Management for LTE-A Femtocells with Distributed 1.5 Carrier Selection,” Proc. IEEE 72nd Vehicular Technology Conference Fall (VTC 2010-Fall), pp. 1–5, 6-9 September,

1 2010.

MUE Total Interference (mW) [9] F.Peng, Y. Gao, Y.Chen and L. Cuthbert, “Using TV White Space for Femtocell Aided Future Mobile Network”, 0.5 GreenIT 2010, Singapore, October, 2010. [10] Alcatel-Lucent, picoChip Designs, Vodafone, “Simulation 0 assumptions and parameters for FDD HeNB RF 1 2 3 4 5 6 7 8 9 10 Simulation Round requirements,” 3GPP Standard Contribution (R4-092042), May, 2009. [11] N.Wang, Y.Gao, L.Cuthbert, “Modelling of Spectrum Figure 7 Interference Comparisons for Different Threshold Sensing for Cognitive Radio based on the Geo-location Settings Method”, The Institution of Engineering and Technology Seminar on Cognitive Radio Communication Systems, 2010. 5. Conclusion In this paper, a resource allocation scheme of using TVWS for LTE Femtocell Network is introduced, which has shown the potential to solve the cross-tier interference in traditional Femtocell deployment. Based on the observation that the TVWS channel is becoming unlicensed and widely available in suburban area, the proposed scheme adopts a Cognitive Sensing combined with TVWS resource allocation scheme for interference mitigation in LTE Femtocell Networks. Simulation results have shown that there is about 50% improvement in mitigating the cross-tier interference compared with traditional resource allocation scheme. From the analysis results, an optimised threshold value for interference mitigation should be around -60 dBm to 65 dBm by taking into account both the performance and cost.

References

[1] Mitola, J., III, Maguire, G.Q., Jr., “Cognitive Radio: Making Software Radios More Personal”, IEEE Personal Communications, pp. 13–18, August, 1999. [2] Ofcom, “Statement on Cognitive Access to Interleaved Spectrum”, July, 2009. [3] Nekovee, M., “Quantifying the Availability of TV White Spaces for Cognitive Radio Operation in the UK”, ICC Workshops 2009, pp. 1-5, June, 2009. [4] Vodafone, “LTE-FDD HeNB interference scenarios,” 3GPP Standard Contribution (R4-091976), May, 2009. [5] L. Poongup, L. Taeyoung, J. Jangkeun, and S. Jitae, “Interference Management in LTE Femtocell Systems Using Fractional Frequency Reuse,” Proc. 12th International Conference on Advanced Communication Technology, vol. 2, pp. 1047–1051, Feburary, 2010. [6] M. S. Jin, S. Chae, and D. I. Kim, “Per Cluster Based Opportunistic Power Control for Heterogeneous