Soft Fractional Frequency Reuse

Xu Fangmin Tao Xiaofeng Xu Xiaodong

（Wireless Technology Innovation Institute （WTI），Beijing University of Posts and Telecommunications， Beijing 100876，China）

Abstra c t:A soft fractional frequency reuse scheme is p roposed tofu lfill full frequency reuse for the 4G mob ile comm unications system.This schem e can be imp lem ented in three types,and using what type of the scheme can be decided during p ractica l app lications,depend ing on the link gain of the physical layer.The im p lem entation of soft fractional frequency reuse in one of the three types is stud ied.The study results show that the frequency reuse factor of this scheme may reach 1/3-1.The simp lified form of this scheme has been successfu lly app lied in the 4G experim ental network in Shanghai,and p rovides a way tofulfill full frequency reuse in systems.

The frequency reuse factorof the FirstGeneration（1G）system is far less than 1.Forexamp le,for 7-cell frequency reuse,the frequency reuse factor is 1/7.The frequency reuse factorof the Second Generation（2G）system has been imp roved greatly.Forexam p le,in the G lobalSystem forMobile communication（GSM）,which is FDMA p lus Time Division Multip le Access（TDMA）system,the frequency reuse factor can reach 1/4-1/3；in the 2G Code Division Multip le Access（CDMA）system[1]and the 3G digitalmobile communications system,such asWideband Code Division Multip le Access（WCDMA）system,the frequency reuse factor can reach 1,which enhances the efficiency of the spectrum and reduces network dep loymentcosts.Therefore,as for the future Enhanced 3G（E3G）and the Fourth Generation（4G）mobile communication systems,it is hoped that 1 orhigher frequency reuse factorwould be used.

The threemainstream standards of 3G systems are allbased on CDMA technology.After the in-dep th study of E3G technologies such as Orthogonal Frequency Division Multip lexing（OFDM）and Multip le InputMultip le Output（MIMO）,MIMO is regarded as the basic technology for future E3G and 4G system s,w ith its advantage ofhigh communication capacity.Orthogonal Frequency Division Multip le Access（OFDMA）technology,w ith the industrial recognition for its advantages such as higherspectralefficiency,is likely to become the basic multip le access technology for E3G and 4G system s.The 3rd Generation Partnership Projec t（3GPP）Long Term Evolution p lan（LTE）[2]has confirmed to use OFDMA[3]as the multip le access technology fordownlink in the E3G system.Moreover,a large numberof research institutions has c laimed that they w illuse OFDMA to rep lace CDMA as the basicmultip le access technology.ButOFDMA technology,based on FDMA,also b rings a new challenge tofrequency reuse.If inter-cellinterference,intra-frequency network and the im p rovementof frequency efficiency in OFDMA system can notbe solved well,the OFDMA-based E3G and 4G system s are bound toface app lication difficulties.Thus,the frequency reuse scheme based on FDMA becomes one of the research p riorities in E3G and 4G systems.

The studies of frequency reuse scheme for E3G have started in 3GPP LTE.This paperp roposes a soft fractional frequency reuse scheme.The scheme p rovides amethod to solve the p rob lem of frequency reuse inmobile communication system s.It's simp lified form has been used in Shanghai's 4G experimentalnetwork,successfully.This paper is structured as follows；in section 1,system modelofsoft frac tional frequency reuse is introduced；section 2 introduces three forms of the soft fractional frequency reuse scheme,and analyzes the features of the second form；section 3 p resents theirapp lication in 4G experimentalnetwork.Conc lusion is given at the end of the artic le.

1 System Model

The users in every cellof the w ireless cellularnetwork are d ivided into two g roups.One is the edge usergroup,and the other is the centraluserg roup.The whole availab le frequency is divided into two non-overlapp ing parts:G and F.F is furtherd ivided into 3 sub-channels:F1,F2and F3,allofwhich are availab le for edge users in ad jacentcells 1,2,and 3 respectively.Each celledge is divided into 12 edge fields,and the 12 edge fields are then divided into two sorts,namely Sort1 Edge Field and Sort2 Edge Field.Sort1 Edge Field refers to the junction area of the three bordering cells（see the edge fieldsmarked"1","2"and"3"in Figure 1）.Sort2 Edge Field refers to the remaining edge areas,excep tSort1（see the edge fields marked"4","5","6","7","8"and"9"in Figure 1）.For the bad communication environmentat the celledge,the nine edge fields in the dashed circ le become themain study ob jectduring frequency p lanning.

◄Figure 1. System model.

▼Table 1. Mapping relationship of frequency sets and edge fields

2 Soft Fractional Frequency Reuse Scheme

Tab le 1 shows a frequency p lanning solution based on soft fractional frequency reuse in the edge fields 1-9（named C1-C9）in Figure 1.

Take Form IIin Tab le 1 as an examp le.The soft frac tional frequency reuse scheme is introduced in detailas follows and illustrated in Figure 2.

The sub-band sets of G and F are availab le frequencies forusers at the centers of the cells.

The frequency p lanning forusers at the edges of the cells is:leu5=f8）,where uj（j=1,..,5）rep resents non-overlapp ing frequency sub-band set.The frequencies in each setcan be continuous or discontinuous.Frequency sub-band sets u1,u2and u3are respectively allocated to the edges of Cells 1,2,and 3,p rovid ing services for the edge users.The allocation of sub-band set u4and u5is:

（1）Reassign a frequency sub-band set u4for the field marked“4”in Figure 1；u4serves users in this region.

（2）Reassign a frequency sub-band set u5for the field marked“8”in Figure 1；u5serves users in this region.

（3）Reassign a frequency sub-band set u5for the field marked“5”in Figure 1；u5serves users in this region.

（4）Reassign a frequency sub-band set u4for the field marked“7”in Figure 1；u4serves users in this region.

Characteristics ofsoft fractional frequency reuse scheme Form‖:

（1）Making betterperformance for cell edge users,and im p roving the frequency reuse ratio.

（2）Although the p re-p rog ramm ing of frequency is d ifferent formally,itw illnot resultin unfair resource assignment.For examp le,Fields 4,8,5,and 7 can choose frequencies（or sub-carriers）from frequency sub-band sets u4,u5,u4and u5,respec tively.If frequencies in these sub-band sets are notsufficient for them,Fields 4,8,5,and 7 can then choose frequencies from frequency sub-band sets u1,u2,u3and u3,respectively,accord ing to some rules.However,Fields 9 and 6 w illonly choose frequencies from frequency sub-band sets u1and u2respectively.But they can coord inate availab le frequenciesw ith Fields 1 and 2,respectively.The numbers ofavailab le frequency sub-bands are same in edges of three cells.

（3）When the interference of celledge users is serious,there are two solutions tom itigate the interference:deducing the numberof frequencies in sub-band sets u4and u5；or increasing the area of the edge field app rop riately.When there is hard ly orno serious interference for cell edge users,the numberof frequencies in set u4and u5may be ad justed p roperly,or the area of the edge fieldmay be decreased to im p rove the frequency reuse.

3 App lication in Shanghai FuTURE 4G TDD Experiment Network

The simp lified form ofsoft frac tional frequency reuse scheme has been app lied in the 4G TDD experimental network in Shanghaisuccessfully.As shown in Figure 3,there are two Access Points（AP）,namely AP1and AP2,in the experimentalnetwork.The d istributed antenna arrays at the lecture theatre and the Golden Sun connect to AP1；the d istributed antenna arrays at the small hoteland the lib rary connect to AP2.Each site has fourantennas.In Figure 3,the region circ led by the b lue line rep resents Cell1,and thatcirc led by the g reen line rep resents Cell2.

▲Figure 2. Soft fractional frequency reuse scheme Form Ⅱ.

▲Figure 3. Shanghai 4G experimental network with two users and two base stations.

The soft fractional frequency reuse scheme with two cells is app lied in Shanghai4G experimentalnetwork.Its results are as follows:

（1）When there is only user,MT1,in Cell1,MT1w illuse the whole spectral resources in Cell1,and the rate can reach 100Mb/s.

（2）When there are two users,MT1and MT2,in Cell1,each useruses halfof the frequency resource,enjoying the highest rate of50Mb/s.

（3）When MT1and MT2move inside Cell1,each useruses halfof the frequency resource,and can also get the highest rate of50Mb/s.

（4）When MT2in Cell1moves to the handover region,butmaintains the connectionw ith Cell1,MT1and MT2respec tively use halfof the frequency resource ofCell1,enjoying the highest rate of50Mb/s.

（5）When MT2moves to the handover region,and connectsw ith Cell2,MT1uses halfof the frequency resource of Cell1,and MT2uses the half resource of Cell2.Each userenjoys the highest rate of50Mb/s,and the frequency reuse fac tor is 0.5.

（6）When MT1in Cell1 and MT2in Cell 2move to app rop riate positions,（for exam p le,MT1is near the lec ture theatre,while MT2moves to the lib rary）,the two users enjoy the total frequency resources in both Cells 1 and 2,and can get the highest rate of100Mb/s.The frequency reuse factor is 1.

4 Conclusions

This paper p roposes a soft fractional frequency reuse scheme in order tomeet the requirements of im p roving spectral efficiency and intra-frequency network in 4Gmobile communication system s.The scheme has three form s for imp lementation,and a p roper form may be selected according to the link gain of the physical layer in p ractical app lications.The soft frac tional frequency reuse scheme has been simp ly app lied in ShanghaiFuTURE 4G TDD experimentalnetwork,w ith a successful result.The stud ies show that the system frequency reuse factor can be up to 1/3-1 when adop ting the soft fractional frequency reuse scheme,which p rovides a good solution to im p roving intra-frequency network and frequency utilization.