Multi-hop Based Network Synchronization Scheme for Femtocell Systems

Jangho Yoon, Jaeki Lee and Hwang Soo Lee Division of Electrical Engineering, School of Electrical Engineering and Computer Science

KAIST Daejeon, Republic of Korea

{icespy, ljk}@mcl.kaist.ac.kr, hwanglee@ee.kaist.ac.kr

Abstract—A Femto base station (BS) is connected to operator’s network using a residential DSL or cable broadband connection and provides service in the licensed spectrum. When a BS operates with time division duplex mode (TDD), synchronization of the transmission time at the BS is important to avoid inter-slot interference with adjacent BSs. Because femto BSs are generally located indoor, they may not receive the global positioning system (GPS) signal which is used to synchronize the wireless cellular network. We propose a multi-hop based network synchronization scheme for femtocell systems. In the femtocell system, synchronization can be achieved by aligning its frame start point with the neighbor cell’s preamble signal hop-by-hop. We evaluate the performance of the multi-hop synchronization and compare it to the single-hop synchronization.

Keywords- femtocell; synchronization; multi-hop

I. INTRODUCTION Conventional small base station (BS) for indoor

environment like a pico BS is connected to an operator’s network via operator’s backhaul. In contrast, the femto BS is connected to the network through user’s own IP connectivity such as xDSL line or Ethernet line, so operators can reduce budgets for installation and maintenance. Moreover, since the femto BS may only be used by the limited users who purchase it and use a licensed band, the users can get a high speed data transmission service in their own indoor environment with low price [1]. It is considered as one of the major technologies in 3GPP long term evolution advanced (LTE-Advanced) which is a standardization activity targeting IMT-Advanced system. So, it is required to develop an air interface, which is optimized for indoor environment [2]. Thus, it is necessary to define femto deployment environment and requirements to ultimately replace wireless local area network (WLAN) environment. Interference and synchronization problems are considered as critical issues for this purpose.

For time division duplex (TDD) systems, synchronization between adjacent cells should be maintained to avoid inter-slot interference. Especially, since femto BSs can be installed in unplanned and uncoordinated way by users, femto BSs can be located near to each other and cause severe interference to each other due to imperfect synchronization. Furthermore, various

interference mitigation schemes such as femto BS cooperation, soft frequency reuse schemes etc., need synchronization of femto BSs.

Global positioning system (GPS) is commonly used for BS synchronization. Since femto BSs are located indoor, GPS signal may not be useful. For distributed and indoor environment IEEE 1588 can be used. IEEE 1588 enabled devices are interconnected with each other through IP connectivity and can act as master or slave devices [3]. Each device can measure timing differences between master device and itself by exchanging time stamp packets. Thus each slave device can correct its clock using the estimated timing offset. However, since this method utilizes round trip delay, and packet delay and jitter in intermediate nodes between master and slave devices cannot be estimated, estimated values of timing offset may not be imperfect. For another method a femto BS can directly measure control channel of a macro BS, synchronize itself to the macro BS and then service its users. But this method cannot guarantee synchronization between adjacent femto cells, and femto cells for indoor environment may not receive the correct macro signal. Furthermore, periodic clock adjustments due to clock drifts of the femto BS can cause service interruptions to its users since the femto BS should act as a mobile station during macro signal reception.

For these reasons, estimating correct timing offset of BSs can be assisted by mobile stations. In [4-5], synchronization between macro and femto BSs is maintained by using timing offset measurement results estimated by a mobile station within both coverage areas. In [6-7], synchronization for general BSs in a system can be maintained by using mobile station’s timing offset measurement between adjacent cells. However this method needs several iterations for entire system synchronization.

In this paper we propose synchronization scheme of femto BSs in hot spot areas using timing offset measurement results of mobile stations. For this purpose we propose group generation method for synchronization and group management process. Within a group femto BSs can synchronize to a reference femto BS using multi-hop synchronization path based method, which doesn’t need iterations for full network synchronization.

This work was supported by the IT R&D program of MKE/KiAT under Contract number 2005-S-609-04 in Korea.

978-1-4244-5213-4/09/ $26.00 ©2009 IEEE

The rest of this paper is organized as follows. Section II provides target environment for our synchronization method. Group management scheme including initial group generation and group merging process is described in Section III. Detail process of synchronization within a group using mobile station’s timing offset measurement results is provided in Section IV. Section V provides simulation environment and results. Finally, conclusion and further works are presented in Section VI.

II. SYSTEM MODEL Our proposed method targets synchronization between

femto BSs in an environment in which femto BSs located in unplanned and uncoordinated way. As in Fig. 1, Femto BSs can be located near to each other, some femto BSs can be grouped autonomously. Within each group, synchronization between member femto BSs can be maintained to a reference femto BS which is selected autonomously. Some kind of control server to control group management may exist within operator’s network or not.

III. GROUP MANAGEMENT

A. Group Generation Group generation for synchronization is performed when

femto BS is turned on. Since femto BS on initialization cannot know interference environment and target femto BS for synchronization. So the femto BS should follow the initialization process for synchronization group generation and/or joining to another synchronization group. On initialization the femto BS itself can be a reference femto BS for synchronization and forms an independent group.

When a femto BS forms a group, it sets the group ID with its own information such as preamble ID or IP address. And the femto BS broadcast group information periodically. The broadcast information includes the group ID and hop count for the multi-hop synchronization. Initially, the multi-hop count should be 0 because the femto BS is not synchronized with other femto BSs.

When it experiences interference from other femto BSs, it can join to another synchronization group under control of a coordinator which is some kind of control server. Fig. 2 shows

the detail procedure of initialization of a femto BS and generation of a synchronization group.

B. Group Merging Process In this subsection we describe a group merging process.

When a synchronization group experiences interference from the femto BSs which are members of other synchronization group, it can be efficient that the groups are merged and they have one reference femto BS for synchronization. So according to the effect of interference between each group, they can be merged. Detail procedure for group merging is illustrated in Fig. 3.

If a group experiences interference from other group, a mobile station (MS) of the group notifies alert of interference to serving femto BS of the MS via FEM_INT_ALT message. When the serving femto BS receives the message, it requests detail information about interference to the MS using FEM_INT_REQ message. Then, the MS responds with information about source of interference such as preamble index of other femto BS which is a source of interference and signal strength of interference in FEM_INT_RSP message. The serving femto BS relays the response to a reference femto BS when the interference source is not a member of the group. Then the reference femto BS requests group merging to the other group which includes the interference source, via GRP_MRG_REQ message. When the reference femto BS receives a successful update message, GRP_UPD_MSG from the other group which includes the interference source, it updates information related with its own group and sends it to the members of the group using GRP_UPD_MSG and UPD_ACK_MSG messages. Finally, multi-hop path selection needed for femto BS synchronization and actual femto BS synchronization with newly selected reference femto BS are performed according to the updated group information as a result of group merging process.

Figure 1. Femto BSs distributed in unplanned and uncoordinated way

Turn on of femto BS

Coarse sync. acquisition from macro signal

Configuration of Femto BS( IP addr. and cell ID acquisition )

Interference from neighboring femto BSs above a threshold

level ?

Operated as an independent group

Forward to the generated group information to a coordinator

Coordinator : Request the correction of sync. and group

merge with interfering neighbor group

Group merge process

No

Yes

Figure 2. Initialization process during turn on of a femto BS

When a femto BS undergoes interference from two or more femto BSs, it should select one femto BS to synchronize. The selection can be decided by the group ID, hop count, and the interference power. When the interference sources have the same group ID, the femto BS can be synchronized with the lowest hop-count femto BS to prevent the synchronization loops. Fig. 4 shows the reference femto BS selection algorithm when multiple femto BSs exist. If the interference sources do not have the same ID, the femto BS can decide the reference with other parameter such as interference power. Detail synchronization process within a group is described in the following section.

IV. SYNCHRONIZATION WITHIN A GROUP

A. Synchronization between Adjacent Cells The synchronization between two femto BSs can be done

by the following method. Femto BSs send requests for measuring the synchronization error as in Fig. 5. The MS

which received the request measures the frame offset between serving femto BS and other femto BS and send a feedback which include the information about synchronization error to the serving femto BS. The femto BS which received feedback control the frame offset to synchronize with other femto BS or send a message to other BS using a backhaul to compensate the error.

B. Multi-hop based Synchronization within a Group Within each group, network synchronization can be

achieved by the process as shown in Fig. 6 utilizing synchronization offsets estimated by MSs between femto BSs as described in the previous subsection. First, a reference femto BS can be autonomously elected as a target femto BS for synchronization within the group when there are more than two femto BSs in the group. The metric such as average turn-on duration of each femto BS can be considered for the election of the reference among femto BSs. It means that the femto BS which has long average turn-on duration may be expected to be stable compared to the others. Next, the path for synchronization from each femto BS to the reference is established. Interference level between adjacent femto BSs can be considered as a main metric for the path selection. For the path selection algorithm using the metric, shortest path selection algorithms which are normally used in internet routing can be utilized. This shortest path selection algorithm can be performed by a distributed or centralized way. According to this path selection principle, a femto BS should synchronize to the femto BS on the path with high interference level toward the reference. Then, the femto BS can correct its

Figure 3. Merging procedure between femto groups

Figure 5. Request and response for synchronization offset measurement

Figure 4. Synchronization BS selection in the same group

clock by using the measured timing offset with adjacent BS by MS. The network synchronization within a group described here can be performed according to some predefined time period or when network topology or interference environment in a group is changed.

V. SIMULATION

A. Environment First, to evaluate the efficiency of the femto BS

synchronization aided by MS, the environment in Fig. 7 is used. The distances from one femto BS to another are 30 m, 45 m and 60 m. MSs are randomly distributed within the coverage area of two femto BSs. Simulation parameters are listed in Table I.

Next, we evaluate femto BS synchronization performances of a proposed multi-hop based scheme using the environment in Fig. 8. Fig. 8 shows an example in which the distance between a femto BS and a reference BS is 100 m. There exist 1 or 2 femto BSs between the femto BSs in Fig. 8. Synchronization probabilities are measured according to the distance between a femto BS and a reference and numbers of femto BSs (or hop count) between them like in Fig. 8.

B. Results and Discussion Fig. 9, 10 and 11 show that the synchronization

performance between two femto BSs according to the location of MS which measures synchronization error and sends feedback. The simulation results show that the probabilities of the synchronization success. The synchronization success probability depends on the location of the MS. The MS has poor performance if the MS locates far from the femto BS. However, the performance of synchronization is reliable when the distance between femto BS and MS is below 30 m. Considering the coverage of the femto cell, the proposed

algorithm can be effective because the femto BS covers below 30 m.

Fig. 12 shows the performance of the multi-hop based network synchronization. The probability of the successful synchronization in the multi-hop environment can be expressed by the following equation,

,1

( ( 1, ) )n

s n si

P P d i i=

= −∏ (1)

where ,s nP is the probability of the successful synchronization at the n th hop count, ( )sP d is the probability of the synchronization between two node through the distance, and

( 1, )d i i− is the distance between the node i and node i-1. When the reference femto BS and the synchronizing BS have long distance, the probability of synchronization success rate is low because of the path loss effect. However, the multi-hop based synchronization overcomes the path loss effect. But if the distance between the synchronizing femto BS and the reference is short enough, the hop count need not to be increased. In other words, it means that the synchronization performance can be improved through multi-hop based scheme when the distance is long or the channel environment between the synchronizing femto BS and the reference is not so good.

Figure 6. Synchronization process within a group

Figure 7. Two femto BSs with randomly distributed MSs

Figure 8. An example for multi-hop based synchronization environment

TABLE I. SIMULATION PARAMETERS

Description Value Multipath fading Pedestrian A 3km/h Carrier frequency 2.3GHz

Height of MS 1m Height of femto BS 1.5m

Path loss model HATA model Output power of BS 17dBm

VI. CONCLUSION AND FURTHER WORKS We proposed autonomous group generation and merging

scheme of femto BSs and a synchronization algorithm within a group when many femto BSs are located in uncoordinated and unplanned way. Simulation results showed that the network synchronization performance can be improved by the proposed multi-hop based scheme compared to the conventional single-hop based scheme that a BS directly synchronizes to a reference BS. The proposed multi-hop based scheme is useful when the channel environment toward a reference femto BS is not so good.

For further works analysis should be performed according to various channel environments, hop count and measurement error. And algorithms for path selection and reference femto election need to be developed more specifically.

REFERENCES [1] Femto forum, http:// www.femtoforum.org. [2] 3GPP TSG RAN, IMT-Advanced Workshop, Apr. 2008. Available:

ftp://ftp.3gpp.org/workshop/2008-04-07_RAN_IMT_Advanced/. [3] IEEE 1588, Standrd for a precision clock synchronization protocol for

networked measurement and contorol systems. [4] X. Yang, J. Zhu, M. Venkatachalm, S. Talwar and Y. Choi, “Support for

femto-cell”, IEEE 802.16m Session 58, Oct. 2008. Available:http:// http://wirelessman.org/tgm/contrib/C80216m-08_1348.pdf.

[5] S. Tzavidas, G. Han and H. Xu,”Time synchronization for Femtocells,” IEEE 802.16m Session 58, Oct. 2008. Available: http://wirelessman.org/tgm/contrib/C80216m-08_1368r1.doc.

[6] M. Jordan, M. Senst, Y. Cui, G. Ascheid and H. Meyr, “Downlink based intercell time synchronization using maximum likelihood estimation,” in Proc. 16th IST Mobile and Wireless Communication Summit, Jul. 2007.

[7] E. Costa, P. Slanina, V. Bochnicka, E. Schulz and G. Spano Greco, “Downlink based intra- and inter-cell time and frequency synchronization in OFDM cellular systems,” in Proc. 6th World Wireless Congress, May 2005.

Figure 9. Synchronization performance for distance of 30m

Figure 10. Synchronization performance for distance of 45m

Figure 11. Synchronization performance for distance of 60m

Figure 12. Synchronization performance according to hop count and distance