Coexistence of asynchronous TDD networks


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Coexistence of asynchronous TDD networks

  1. 1. Nokia Siemens Networks Coexistence of asynchronous TDD networksWhite Paper
  2. 2. 2 Contents Overview 2 Overview 3 Coexistence and Interference During June 2010, India conducted As evident from the table given below, 4 BS-BS Interference Broadband Wireless Auctions (BWA) the 2.5 MHz guard band is only 6 UE-UE Interference for two TDD blocks of 20 MHz per available between two operators in 10 circle within the 2.3GHz band. Given out of 22 circles . This, in turn, leads to 7 Interference mitigation in below are details of the auctioned strong interference between both the TDD environment blocks. operators as the 2.5 MHz guard band is not sufficient. Apart from these 9 Summary Reliance infotel acquired BWA circles, interference is expected to spectrum in each of India’s 22 circles impact many border areas. (regions) nationwide, while the remaining BWA licenses were divided region-wise amongst five operators; including Airtel in 4, Aircel in 8, Tikona in 5, Augere in 1 and Qualcomm in 4 circles respectively. Auctioned blocks Circles Inter operator Guardband "Operator1: 2305.0 - 2325.0 Delhi, Mumbai, Maharashtra, Gujarat 2.5 MHz Operator2: 2327.5 - 2347.5" "Operator1: 2302.5 - 2322.5 Kerala, Assam, North East, Karnataka 2.5MHz Operator2: 2325.0 - 2345.0" "Operator1: 2312.5 - 2332.5 Tamil Nadu 2.5MHz Operator2: 2335.0 - 2355.0" "Operator1: 2302.5 - 2322.5 Andhra Pradesh 22.5 MHz Operator2: 2347.5 - 2367.5" "Operator1: 2305.0 - 2325.0 Kolkatta, West Bengal 7.5MHz Operator2: 2332.5 - 2352.5" "Operator1: 2320.0 - 2340.0 Punjab, UP(E), UP(W), Rajasthan 17.5 MHz Operator2: 2357.5 - 2377.5" "Operator1: 2322.5 - 2342.5 Haryana 20MHz Operator2: 2362.5 - 2382.5" "Operator1: 2302.5 - 2322.5 Madhya Pradesh 10MHz Operator2: 2332.5 - 2352.5" "Operator1: 2322.5 - 2342.5 Himachal Pradesh 2.5MHz Operator2: 2345.0 - 2365.0" "Operator1: 2302.5 - 2322.5 Bihar, Orissa 12.5MHz Operator2: 2335.0 - 2355.0" Circles prone to interference Fig 1: BWA spectrum auction results
  3. 3. 3Coexistence and InterferenceCoexistence implies that two operators of both the systems that interfere with small (as it only impacts users close towill function in the same geographical each other. the offending BS), but the effect isarea utilizing adjacent frequency persistent in terms of spatial locationbands. However, the situation needs to The MS -> BS interference is seen as (a “blind zone” may exist around anbe handled carefully because working less critical, because it is more interfering BS). The MS -> MSin this kind of an environment may lead transient (i.e. can appear only when an scenario becomes more random andto mutual interference. Given this active interfering MS is close to a unpredictable.scenario, network synchronization victim BS), but it can still potentiallyoffers the most efficient solution in affect a large number of users being We present in this paper a study withterms of handling co-existence in TDD served by the victim BS. In the BS -> deterministic calculation.environment. Asynchronous TDD MS interference scenario, theoperation in the same geographical population of affected users is relativelyarea will lead to high interference if theguard band is too low (e.g. 2.5 MHzcase in India) Operator 1 Operator 2The most critical scenario in case of TDD TDDasynchronized networks is BS -> BSinterference, as it is relatively static (i.e.persists for a long period of time) and 2.5 MHzaffects a large number of users. Itpotentially has an impact on all users Fig2: Guard band 2.5 MHz (TDD networks)
  4. 4. 4 BS-BS Interference If in case the eNBs of two different The resulting additional attenuation is FRC A1-3. In actual practice this value operators are uncoordinated thus 74dB. appears to be quite high. A more (asynchronous), there is a distinct reasonable value can be reached with possibility of interference across both CEPT Report a receiver noise floor of -114dBm/MHz UE-UE and BS-BS. Transmission CEPT report 019 defines several with 4dB noise figure. power and its filter mask must be conditions for interference. The Block taken into account for conducting the Edge Mask (BEM) requirement for The interference sums to the noise feasibility study 2.6GHz FDD-TDD interference can be floor, inducing degradation of the referred to due to the similarity to the sensitivity. Assuming maximum Interfering BS: TX output exercise of the TDD-TDD coexistence receiving interference level is 6dB power and spurious emissions scenarios. The resulting additional lower than the noise floor with 1dB attenuations are in the range bounded desensitization value, we need 104dB Assuming operator 1’s BS has by the 3GPP co-existence additional attenuation in accordance transmission power value 33dBm/ requirement (lower bound) and co- with 3GPP ACLR guidelines. MHz i.e. 40W over 20MHz, the ACLR location scenarios requirement (higher (Adjacent channel leakage ratio) and bound). Mutual Coupling Loss between the the unwanted emission requirements BS’s limits the emissions to the victim BS Case Study of BS-BS Site planning, in this case the antenna band. (interferer and victim) isolation is one of the key factors in the interference mitigation between The 3GPP ACLR requirement [3GPP RX noise and allowed degradation the systems. TS 36.104] provides the limit for the due to interference and blocking TX power leakage over the adjacent The 3GPP Reference Sensitivity is With well coordinated vertical channel of the same BW of the victim defined as -101.5dBm with the separation, it is possible to achieve 70 BS as the transmission. With the said Reference measurement channel dB mutual coupling loss (MCL) TX power, i.e. 33dBm/MHz, and the required 45dB ACLR value, we achieve an average value of -12dBm/ MHz and then compare this value with Antenna Isolation Measurements the following scenarios: Setup Antenna d [mm] / Minisolation [dB] d [mm] / Max isolation [dB] 3GPP co-existence III A Same mast / 52 750 / 71 3GPP TS 36.104 defines the DL B Same mast / 49 1300 / 69 C Same mast / 52 1150 / 76 spurious level at the TX antenna port D, Co-polar Same mast / 38 1250 / 62 as -52dBm/MHz limit with 10MHz D, Cross-polar Same mast / 53 1250 / 62 offset below the lowest and above the IV A 250 / 37 6000 / 57 highest frequency of the operating B 250 / 27 6000 / 52 band. In this study, we take -52dBm/ C 250 / 34 6000 / 48 D, Co-polar 250 / 33 4250 / 53 MHz as the emission limit at 2.5MHz D, Cross-polar 250 / 36 6000 / 57 offset. The resulting additional attenuation is thus 40dB. 3GPP co-location 3GPP TS 36.104 defines the DL spurious level at the TX antenna port as -86dBm/MHz limit with 10MHz IV (Horizontal) III (1800) offset below the lowest and above the highest frequency of the operating Fig3: Antenna Isolation Measurement band. In this study, we take -86dBm/ MHz as the emission limit at 2.5MHz offset
  5. 5. 5between two operators. However, in the required interference levels of An attenuation of 54 ~74 dB atsome locations and configurations -116dBm/MHz, as well as the MCL 2.5MHz offset is clearly a very50dB or even 30dB MCL has to be values of 30dB and 50dB: challenging value, leading to high-costallowed. 30dB can be considered as a • with 50dB MCL, 54 dB additional filter structures.typical value for operators who have filtering is neededcompletely independent planning • with 30dB MCL, 74 dB additional Similar results based on Rx blockingwithout careful separation between filtering is needed scenario can be obtained as well.antennas, and 3GPP refers to thisvalue in the most severe co-locationspecification. Some measured antennaisolations are shown in the figurebelow. 50dB is very close to CEPT Channel fOOR bandwidth fOORcoexistence MCL assumption in its Spurious domain Spurious domainreport 019.In the below calculations, 50dB and30dB isolations are considered.Estimate of needed filtering betweennon-synchronized TDD BS’sThe needed filtering due to the TX E-UTRA Bandoperating band unwanted emissions 33 dBm/MHz (=46dBm/40W / 20MHz)can be estimated using the emissionlevel of -12 dBm/MHz at 2.5MHz offset, Interfering system Filter mask 45 dB* - 12 dBm/MHz Guard band 30dB MCL Performance ~104 dB 50dB MCL Requirement Additional Filtering - 116 dBm/MHz (4dB Noise Figure; 1dB desensitization) Fig4: Case Study: BS-BS Coexistence
  6. 6. 6 UE-UE Interference Case study for victim UE the one of operator 2 is set as 30 dB performance is concerned. This is Assume that UE of TDD operator 2 assuming two UEs are at the vicinity. partly addressed in CEPT report. transmits with full power (23dBm) over 5PRBs (physical resource blocks), The power level of unwanted signal is References from CEPT and the UE is at the cell edge subject set as -105dBm/MHz by considering The case mentioned above reflects a to high interference from the noise floor, 9dB noise figure and 3dB worst-case scenario, assuming that neighbour cells that does not allow the desensitization. To meet the target we both the interfering and interfered UE usage of more PRBs. (otherwise the still need 62dB RF isolation from the are close to each other. In the real min. level of MCS (modulation and UE. world, likelihood of UE-to-UE coding scheme) cannot be agreed). interference should be considered. A From implementation viewpoint, there study in CEPT Report 131 shows that To get the emission power level at is no physical solution. The emission -22.5dBm/MHz TX power from the 2.5MHz offset from the carrier edge, spectrum is purely resulting from the interfering UE is enough to obtain bear in mind that 3GPP compliant UE physical resource allocation and PA reasonable throughput performance. has 30dB ACLR for adjacent channel linearity, and the TX filter does nothing That is; however, still ~10dB more interference reductions [3GPP TS to filter the adjacent channels. stringent than what the UE really can 36.101]. Furthermore, according to Similar results based on Rx blocking do. 10dB extra filter implementation is Table of 3GPP TS 36.101, scenario can be obtained as well. technically feasible however; it will be UE out-of-band emission at 2.5MHz very challenging to have a device eco offset must not exceed -13dBm/MHz. Assume both system are configured system from the economic aspects. We take the minimum value as the to be frame configuration 1 (3DL:2UL) target from the ACLR requirement and according to [3GPP TS 36.211] , there Filtering isn’t the ideal solution as it is out-of-band emission requirement, i.e. are some chances of avoiding direct not possible across all mobiles. -13dBm/MHz as depicted in the figure collision between UL and DL, and that 5. will relax the additional attenuation requirement as far as throughput The antenna coupling loss between the UE belonging to operator 1 and 23 dBm over 5 PRB @ OP1 = 23 dBm/MHz Ref: 36.101 Min (power –ACLR, SEM) = -13dBm/MHz ACLR 2.5 MHz -22.5dBm/MHz CEPT report 131 TDD OP 1 Ant. Isolation - 30 dB TDD OP2 Additional 62 dB Isolation still needed* Noise Floor/ Interference Level @ Receiver with 3dB desensitisation: -105dBm/MHz Ref: 36.101 Fig5: Case Study: UE-UE Coexistence
  7. 7. 7Interference mitigation in TDDenvironmentDeploying two TDD cellular systems in UL parts of the transmitted radiothe same coverage area on adjacent frames. TDD switch point need to becarriers requires synchronization of the coordinated with neighbors to avoidnetworks and alignment of the DL and interference in border areas as well. Transmit-to-receive and receive-to-transmit transition gaps (TTG and RTG) Operator 1 on carrier f1 DL UL DL UL Operator 2 on carrier f2 DL UL DL UL Time Recommended System 1 DL UL DL UL on carrier f1 MS-to-MS interference BS-to-BS interference System 2 DL UL DL UL on carrier f2 Time Not recommendedFig6: Design Principle of TDD Coexistence So that between operators guard band is not required. 10 MHz 10 MHz 20 MHz LTE TDD LTE TDD Fig6: Synchronized TDD network without Guard band
  8. 8. 8 • Use additional guard band (min 7 to uncoordinated and asymmetric with 12 MHz) from the allocated send/receive frame structures. spectrum in addition to 2.5 MHz 3GPP has 7 TDD configurations with guard. But this may not be a viable 5ms and 10ms switch points. Nokia solution to use precious spectrum Siemens Networks strongly for guard band recommends TDD configurations with • Alternate option is to deploy 5ms switch point as it provides the expensive external filters. This is not lowest latency. recommended as it will increase operator’s capex. UL/DL Switch-point Subframe number However Interference has a negative configuration periodicity 0 1 2 3 4 5 6 7 8 9 impact on capacity. Once the capacity runs out, there will be a real incentive 0 5 ms D S U U U D S U U U for the operators to synchronize, 1 5 ms D S U U D D S U U D because it will increase the capacity for everyone. This is particularly valid for 2 5 ms D S U D D D S U D D cases in TDD networks which are 3 10 ms D S U U U D D D D D 4 10 ms D S U U D D D D D D 5 10 ms D S U D D D D D D D 6 10 ms D S U U U D S U U D Fig7: TDD UL/DL configurations
  9. 9. 9Summary• In India 10 out of 22 circles are Given the options at hand, the easiest functioning on the 2.5 MHz inter and most viable solution is to have operator guard band that is prone to synchronization of two TDD operators interference networks through a joint agreement per• Providing additional guard or telecom circle to decide the “UL:DL deploying expensive filters on BS is ratio”. not a profitable solution.In addition, filtering isn’t possible on all mobiles
  10. 10. Nokia Siemens NetworksP.O. Box 1FI-02022 NOKIA SIEMENS NETWORKSFinlandVisiting address:Karaportti 3, ESPOO, FinlandSwitchboard +358 71 400 4000 (Finland)Switchboard +49 89 5159 01 (Germany)Copyright © 2011 Nokia Siemens Networks.All rights reserved.Nokia is a registered trademark of Nokia Corporation,Siemens is a registered trademark of Siemens AG.The wave logo is a trademark of Nokia SiemensNetworks Oy. Other company and product namesmentioned in this document may be trademarks oftheir respective owners, and they are mentioned foridentification purposes only.This publication is issued to provide information onlyand is not to form part of any order or contract.The products and services described herein aresubject to availability and change without notice. Every effort is made to ensure that our communications materials have as impact on the environment as possible