Evaluation of Percentage Capacity Loss on LTE Network Caused by Intermodulation Distortion in a Coexistence Scenario

Transcription

1 International Journal of Science and Engineering Investigations vol. 6, issue 62, March 2017 ISSN: Evaluation of ercentage Capacity Loss on LTE Network Caused by Interodulation Distortion in a Coexistence Scenario Ekwuee E. U. 1, Nosiri O. C. 2, Chukwuchekwa N. 3, Okpara R. C. 4 1,2,3,4 Federal University of Technology, Owerri, Nigeria ( 1 ekwueeeanuel@yahoo.co, 2 buchinosiri@gail.co, 3 nonwuchekwa2002@yahoo.co, 4 chineduokpara@yahoo.co) Abstract- The paper evaluates the effects of third order Interodulation Distortion (IMD3) on the Long Ter Evolution (LTE) receiver due to coexistence between LTE and GSM networks. Aongst the various existing IMD orders which include first order, second order, third order, fifth order and seventh order. Third order is known to have the greatest distortion effects on a receiver due to its strength and its proxiity to the frequency band of interest. It occurs as a result of the non-linear behavior of coponents or circuit at both the transitter and receiver ends of wireless counication networks. IMD has potential negative effects on a victi receiver which ajorly leads to increase in noise floor level and syste capacity degradation. Deterinistic approach was ipleented in the work assuing worst case scenario. MATLA software siulation was deployed to evaluate the capacity loss at the receiver end relative to a range of distances apart. Results obtained showed severe uplink capacity degradation when VISAFONE LTE network was interfered by INTERCELLULAR LTE downlink and ETISALAT GSM uplink. Various distances ranging fro 500 to 3000 were varied between the ETISALAT GSM network and the VISAFONE LTE network. The results obtained showed that at 500 eters, the percentage capacity degradation was as high as 80. The least percentage capacity loss was obtained as 5.97 at 3000 eters. Keywords- Interodulation Distortion, Coexistence, Long Ter Evolution (LTE), Global Syste for Mobile Telecounication (GSM), Capacity Loss, Uplink and Downlink. I. INTRODUCTION The growth and swift spread of obile counication systes across the globe has necessitated the obile network copanies to strategize advance techniques for quality service delivery. This has brought about the proliferation of base station asks fro different network operators aied at iproving the syste coverage and capacity. The increase has led to various coexistence settings. The coexistence of networks is typically classified as coordinated and uncoordinated setting. The coordinated scenario refers to a case in which the coexisting networks belong to the sae network while the uncoordinated scenario is when the networks belonging to different network providers exist in the sae geographical area [1] The key setback to coexistence of networks is the issue of interference. One of the proinent interferences suffered in such scenario is the Interodulation Distortion (IMD). Others include transitter noise and receiver blocking (receiver desensitization). IMD is a phenoenon caused by coexistence. It is a ulti-tone distortion product that results when two or ore signals are present at the input of a non-linear device [2]. The non-linear device leads to a generation of interodulation products which are sus and differences of ultiples of the fundaental frequencies [2]. These frequencies on their own are harless. However, when two or ore of these IMD products fall within the pass band of a receiver, it interferes with the genuine received signal leading to loss of the signal strength, channel capacity degradation and reduced signal to interference ratio. Of all the interference issues plaguing obile and wireless counication systes, it is observed fro literature that the least attention was paid to IMD. Conversely, as counication systes becoe ore advanced with increase in collocation and coexistence deployent and the need to achieve optial signal to noise ratio, IMD analysis becoes very vital to be neglected. The frequency spectru in theory is an unliited resource. ut then, practically it is liited. This is because different frequencies have their characteristic properties which ay ake the unsuitable for certain applications. When the fourth generation telecounications standard -Long Ter Evolution (LTE) was deployed, there was no defined frequency of operation. Since ost parts of the spectru where already occupied by other wireless systes, the Third Generation artnership roject (3G) recoended that LTE be deployed on any available frequency slot fro 700MHz upwards [3]. This connotes the affiration that the operating frequencies for LTE could vary fro one country or region to the other. In Nigeria, there are four ajor teleco operators naely MTN, AIRTEL, GLOACOM and ETISALAT. These 153

2 networks are referred to as ajor players by virtue of the arket share they control and their network coverage. Altogether, these copanies own over 98% of the obile telecounication arket in Nigeria [4]. Also, ase Transceiver Stations (TS) belonging to these firs can be seen scattered all over the country. They offer voice and data services on the Global syste of Mobile Telecounication (GSM) 900, GSM 1800 and Universal Mobile Telecounication Syste (UMTS) 2100MHz bands. Recently, the Nigerian Counication Coission (NCC) granted licenses to three copanies which include SMILE, INTERCELLULAR and VISAHONE, to deploy LTE services on the 800MHz band [5]. These LTE networks are yet to have nationwide coverage but have already been deployed in three ajor cities in Nigeria, naely, Lagos, Abuja and ort Harcourt. The deployent of LTE services in areas already doinated by GSM and Wideband Code Division Multiple Access (WCDMA) networks could lead to interference due to close frequencies of operation. Fro the theoretical perspective, it was observed that when GSM network coexists with LTE network due to their close frequency bands, could lead to a third (3 rd ) order IMD. The incongruity could be proinent between the downlink of INTERCELLULAR network and the uplink of ETISALAT GSM network. Hence, this necessitated the study to analyze the interfered frequencies and evaluate their syste capacity loss when interfered. II. RELATED WORKS Fro the articles of [1], carried out an analysis on the effect of transitter end interodulation interference and spurious eissions on a base station receiver in a co-located arrangeent. The interference scenario considered was CDMA2000 ase Station and Mobile receivers degraded by Interodulation generated by the transitters of GSM 900 base station. Deterinistic approach was used to define received signal strength and its degradation as a function of distance. Results obtained showed that co-located base stations suffered greater degradation of received signal strength than standalone base station. The author of [6] carried out a study on the coexistence between LTE and GSM in order to identify potential interference issues that ay be encountered. The author used a statistical ethod based on the Monte Carlo technique. The coexistence scenario considered was one in which LTE was deployed in the 900 MHz band also used by GSM. The interference echanis considered where Unwanted eissions and receiver blocking. In this scenario, receiver blocking was over 5% which is the recoended threshold by 3G. Hence, the author recoended the use of a receiver with a blocking response 8d higher than the 3G iniu requireent. This work ainly considered the interference effect caused by one obile station but failed to investigate the effects of Interodulation Distortion which is a proinent interference challenge. The authors of [7] investigated the ipact of interference fro CDMA 2000 base station transitter and a UMTS base station receiver in a co-location arrangeent. Deterinistic technique was used in the analysis by supposing a worst case scenario for both the interfere and the interfered. However the authors aditted that the real life spurious eissions and blocking specifications of the UMTS receiver are better than the values used. Results showed that an isolation of 65d would be required between the CDMA 2000 and UMTS antennae to avoid blocking. This will only be effective when a filter installed at the UMTS receiver end ust have introduced an attenuation of 60 d. Also a 5 MHz guard band between the CDMA downlink and the UMTS uplink was recoended. III. METHODOLOGY With the proliferation of wireless counication systes which resulted to coexistence and co-location scenario. It is usually very iportant to identify potential interfering systes before a new syste is deployed in any environent. While it is relatively easier to identify co-channel and adjacent channel interferers, identifying interferers which causes Interodulation Distortion is soewhat ore challenging. This is because frequencies when operated in isolation are observed harless but could pose serious threats at a receiver front end when it ixes non-linearly with ore than one frequencies. Fro the study, we chose the interferers as the downlink of INTERCELLULAR LTE and the uplink of ETISALAT GSM networks while the interfered syste is the uplink of VISAFONE LTE. The following steps were deployed to calculate the 3 rd order IM products generated by these two interferers. 1. Specify the Victi receiver s pass band: the pass band for the VISAFONE LTE enode receiver is MHz. 2. Specify the operating frequency range of the two interfering systes: intercellular downlink has an operating frequency range of MHz while the ETISALAT Uplink has a frequency range of MHz. 3. Let the interferers be labelled f a and f b respectively, where f a is {842, 843,, 852MHz} and f b is {890, 891,, 895 MHz} 4. All cobination pairs of the individual f a and f b frequencies are evaluated using odels to derive the third order IM products generated 5. For any IM product derived in the preceding step, a quick check is carried out to verify if the frequency falls within the victi receiver pass band. IM products which fall outside this range are of no interest as they do not pose any threat to the syste. IM products which fall within this range are harful and will interfere with the VISAFONE LTE receiver. Figure 1 illustrates the step-by-step approach towards evaluating the interfering third order interodulation products while Table 1 represents the obtained interfering interodulation frequencies. International Journal of Science and Engineering Investigations, Volue 6, Issue 62, March aper ID:

3 TALE I. INTERFERING IM FREQUENCIES No Start Specify Victi Receiver ass band F pass=> MHz Specify frequency range of interferers LTE: f a => 842, 843,, 852 MHz GSM: f b => 892,893,, 895 MHz Take pair of interferers, f a, f b 2f a ± f b = IM 1 f a ± 2f b = IM 2 Is IM 1 or IM 2 within the Victi Rx pass band? Yes IMD product detected Have all cobinations of f a and f b been evaluated? Yes No Intercellular (MHz) Etisalat (MHz) IM roducts End Figure 1. Flowchart for evaluating interfering third order IM products The values obtained fro the evaluation of the third order IM products generated by the downlink of INTERCELLULAR LTE frequencies and uplink of ETISALAT GSM frequencies are presented in Table 1. Only the IM products capable of causing interference are shown on the table. Other IM products that fall outside the pass band of the VISAFONE enode receiver are not included because they exert no treat on the syste capacity. Table 1 showed that the downlink frequencies of INTERCELLULAR Network ranging fro MHz will generate distortive IM products with the entire uplink frequencies of ETISALAT which ay interfere with the operation of any nearby VISAFONE LTE enode receiver. A. Evaluation of Uplink Capacity Loss The loss of capacity can serve as an indicator as to the ipact of an interference echanis on a victi network. In LTE, any significant capacity loss can have an adverse effect on the services offered to users on the network. LTE network was designed to carry high data rate deanding services such as ultiedia streaing, video conferencing, real-tie internet gaing etc. These activities involve transfer of lots of inforation bits hence any shrink in network capacity will frustrate users especially when network load is high. LTE deands high Signal to Noise Ratio (SNR) and to achieve this, it is necessary to reduce to the barest iniu the effects of all sources of interference. Signal bandwidth in LTE is about 90% of the channel bandwidth. Hence a 10 MHz channel will have a signal bandwidth of 9 MHz [8]. Using Shannon forula for finding axiu channel capacity in bits per second [9]. C log (1 SNR ) bits / sec (1) 2 International Journal of Science and Engineering Investigations, Volue 6, Issue 62, March aper ID:

4 where is the signal bandwidth, SNR is the Signal to Noise ratio. SNR is a ratio of the ower of the received signal to the noise inherent in the syste. This is expressed as [9]. SNR N (2) where is the strength of the received signal strength of the LTE enode, N is the Noise ower. Noise ower (N) is generated in the receiver circuitry and is expressed as N kt0 (3) 23 where k is the oltzann Constant given as 1.38x 10 J / k T is the receiver operating teperature in Kelvin. The widely 0 accepted value is 290K at an abient teperature of approxiately 17 o C. is the receiver noise bandwidth Lets represent in equation 2 as which is given as rx rx txlug (4) where tx is the strength of the transitted signal Lu is the path loss G is the gain of the receiver antenna tx is assued to be 22d which is the axiu transit power of an LTE MS [10]. The path loss is estiated using the Hata Model for urban area as shown in equations (5) and (6) [11]. Lu log f 13.82log h CH [ log h ] (5) C 3.2(log11.75h ) (6) H Where h is the base station antenna height in eters, h is the height of the obile station in eters, f is the frequency in MHz, d is the distance between the Transitter and Receiver and C is the antenna height correction factor. H The interferers are the LTE enode transitter of another operator (INTERCELLULAR) and the Mobile Station transitters of a GSM operator (ETISALAT). It is assued that the interferers are transitting at axiu power. Let the LTE enode transit at axiu power LTEMAX and the GSM MS transit at axiu power GSMMAX, the loss on the path fro the interfering MS to the victi S is calculated fro equations (5) and (6). Taking h = 1.5eters and h = 30 eters, the path loss fro GSM MS to LTE enode is given as L UM log f log d (7) Siilarly taking h = 30 eters and h = 30 eters, the path loss fro the interfering LTE enode transitter to the victi is given as L U log f log d (8) Interfering signals IRX 1 and IRX 2 reaching the victi enode station fro both interferers has to take into account the path loss. Hence, L IRX 1 LTEMAX U (9) L IRX 2 GSMMAX UM (10) The signal strength degradation ( ) is the difference between the signal strength in a non interfering environent and an interfering environent. This is calculated thus 1 LTEMAX pirx 1 (11) 2 p (12) GSMMAX IRX 2 Where 1 is the LTE signal degradation, 2 is the GSM signal degradation. The Interference power receiver is given by I I ( KT NF) log(10 1) d at the victi LTE enode (13) Where (kt+nf) is the receiver noise floor of each interferer. It is calculated using values derived fro equation 13. Thus, I fro LTE enode is given by 1 10 I LTE ( KT NF) 10 log(10 1) d Also fro GSM MS is given by 2 10 I GSM ( KT NF) 10 log(10 1) d (14) (15) Since the nuber of interfering GSM MS is greater than one at any given instance, the total I fro a population of GSM MS (axiu of 60 MS) is given by I TGSM 60 I x 1 TGSM d (16) The power of the 3 rd order IM products, I3, generated by I LTE and I TGSM interfering signals are calculated thus; I3 2I I 2II3 (17) LTE TGSM International Journal of Science and Engineering Investigations, Volue 6, Issue 62, March aper ID:

5 Where II3 is the 3 rd order Interodulation intercept point of the LTE receiver. Then, the Signal to Interference plus Noise ratio is calculated using SINR (18) I N Where is the received signal strength, I is the interference and N is the Noise Replacing I with I3 fro equation 18 and rx fro equation 4, equation 19 then becoes rx SINR I3 N (19) Using Shannon s Capacity forula, the LTE Uplink capacity in the presence of interference is given by C int prx log 2 (1 ) bits / sec (20) I3 N Where C int is the victi LTE Uplink capacity due to interference. Capacity loss is given by c 1 c int C L (21) And the percentage loss is coputed using cint % C L (1 ) X100 c (22) The flowchart of figure 2, suarises the procedure for calculating the uplink percentage capacity loss. Start Specify MS, S Tx frequency Specify MS to Victi Receiver distance Calculate athloss fro MS to Victi, L UM and athloss fro S to Victi, L U Calculate signal reaching Victi Rx front end 1RX1 and 1RX2 Calculate signal degradation ( ) Calculate Interfering power at Receiver Su Interfering power for a population of GSM MS Calculate IM signal strength I3 Calculate Uplink Capacity affected by interference C int Calculate Capacity Loss C L and ercentage Capacity Loss %C L IV. RESULTS The interference effects suffered by the LTE uplink was evaluated in ters of uplink capacity loss. The interfering networks are the INTERCELLULAR LTE Downlink and the ETISALAT Uplink. The scenario deployed a concept that the distance between the two base stations (INTERCELLULAR and VISAFONE LTE) are fixed while the distance and nuber of siultaneously transitting obile stations are varied. The distance between the base stations was fixed at 1000 eters with the obile stations. The victi VISAFONE LTE receiver was increented by 500 eters. Table 2 shows the percentage uplink capacity loss as obtained fro equation 22. The GSM interferer power reduces with distance and increases as the nuber of MS increases. Figures 3 to 7 illustrated plots of the nuber of base stations versus the percentage capacity loss for distances ranging fro 500 to TALE II. End Figure 2. Steps to calculate the capacity loss ERCENTAGE VISAFONE LTE ULINK ERCENTAGE CAACITY LOSS % Capacity Loss No of MS International Journal of Science and Engineering Investigations, Volue 6, Issue 62, March aper ID:

6 Figure 3. Capacity Loss at a distance of 500 eters between the obile and base stations denoted as D MS = 500 eters Figure 6. Capacity loss at D MS = 2000 eters Figure 4. Capacity loss at D MS = 1000 eters Figure 7. Capacity loss at D MS = 3000 eters Figure 5. Capacity loss at D MS = 1500 eters Figure 3 shows the uplink capacity degradation when the obile stations are positioned 500 eters fro the enode receiver. As can be seen fro figure 3, capacity degradation was very severe. A loss of 82.6% was observed when the 60 obile stations were transitted siultaneously. Fro figure 4, the capacity loss stood at 30.2% for 20 obile stations while 60 transitted obile stations resulted in a 37.67% capacity loss. At 1500 eters the severity of the loss in uplink capacity was further reduced for 60 obile stations to 23.51% as shown in figure 5. For 2000 eters and 3000 eters respectively, the capacity loss dropped below 20% as illustrated in figures 6 and 7. Although a 20% capacity loss is a relatively low percentage, it is still significant because in peak periods, subscribers will be short-serviced due to reduced capacity. The results obtained have shown severe degradation of the network capacity. This deonstrates that the IMD generated due to this interference scenario is not suitable for the operation of the VISAFONE enode receiver owing to the fact that Third Generation artnership roject recoended a axiu tolerable loss of 5% capacity [10]. It therefore requires ipleenting itigation techniques such as filtering to reduce the power effect. International Journal of Science and Engineering Investigations, Volue 6, Issue 62, March aper ID:

7 V. CONCLUSION This work evaluated the effects of Interodulation Distortion on the uplink capacity of an LTE network due to coexistence with other networks. Using coputational ethod, the IM frequencies capable of causing interference where derived. The signals fro an INTERCELLULAR LTE downlink and ETISALAT uplink were observed to generate distortive Interodulation frequencies when incident on the front end of VISAFONE LTE base station receiver. An evaluation of the effects of IMD on a receiver was carried out using deterinistic ethod. Worst case scenario in which the interferers were transitting at axiu power was assued. The interference scenario in this work consists of one LTE enode interferer at a fixed distance and a population of GSM obile station interferers at varying distances fro the victi LTE receiver. The degradation suffered by the receiver due to IM interference capacity was evaluated in ters of uplink capacity loss. Loss of capacity was observed to be as high as 80% in soe cases and the least capacity loss at 3000 was 5.97%. REFERENCES [1] Okorogu Victor, Onoh G. N., Onyishi D. U., Utebor N. N., Mitigation of Interodulation Effects due to coexistence of GSM 900 and CDMA x Systes International Journal of Scientific & Engineering Research, Volue 4, Issue 6, June ISSN , [2] aschos G., Kotsopoulos K.A., Zogas D. A., Karagiannidis G. K. (2003), "The ipact of interodulation interference in superiposed 2G and 3G wireless networks and optiization issues of the provided QoS", roc. CCCT03, [3] Maatin Sint ureau Telecounications and ost LTE Options, pp 1-49, [4] Industry Statistics, Market share by technology (2017). Retrieved fro [5] Frequency Allocation Tables National Frequency Manageent Council of the Federal Republic of Nigeria, [6] Cătălina-Marina Crina, Study on Coexistence between Long Ter Evolution and Global Syste for Mobile Counication, TRANSACTIONS on ELECTRONICS and COMMUNICATIONS Volue 59(73), Issue 1, [7] Haeed A., Oudah A., Interference in wireless networks: Causes, analyses and practical itigating techniques, Modern Applied Science, Vol. 8, No. 5, ISSN DOI, [8] 3G TS V GSM/EDGE Radio Access Network; Radio transission and reception (Release 9) 3rd Generation artnership roject (3G), 2009 [9] Shannon C. E., A Matheatical Theory of Counication, The ell Syste Technical Journal, Vol. 27, pp , , July, October, [10] 3G TS V9.1.0, LTE; Evolved Universal Terrestrial Radio Access (E-UTRA) User Equipent (UE) radio transission and reception (Release 10) 3rd Generation artnership roject (3G),2011. [11] Hata, M., Epirical forula for propagation loss in land obile radio services," IEEE Trans. Vehicular Technology, Vol. 29, , 1980 International Journal of Science and Engineering Investigations, Volue 6, Issue 62, March aper ID: