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Interference Avoidance for In-Device Coexistence in 3GPP LTE-Advanced: Challenges and Solutions (visit for more insights)


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Interference Avoidance for In-Device Coexistence in 3GPP LTE-Advanced: Challenges and Solutions.

Interference Avoidance for In-Device Coexistence in 3GPP LTE-Advanced: Challenges and Solutions.

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  • 1. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® TECHNOLOGY UPDATES ON LTE ADVANCED Interference Avoidance for In-Device Coexistence in 3GPP LTE-Advanced: Challenges and Solutions Zhenping Hu, China Mobile Communications Corporation Riikka Susitaival, Ericsson Research Zhuo Chen, Huawei Technologies Co., Ltd I-Kang Fu, MediaTek Inc. Pranav Dayal, Qualcomm Incorporated Sudhir Kumar Baghel, Samsung India Software Operations ABSTRACT concurrent operation of multiple radios; for example, for Long Term Evolution (LTE), radio With the ever growing usage of various wire- transceiver for short range communications (e.g. less technologies and services, more and more Bluetooth, BT) and/or WiFi, as well as a Global handheld devices are equipped with multiple Navigation Satellite Systems (GNSS) receiver; radio transceivers (e.g., LTE, WiFi, Bluetooth, which can potentially interfere with each other. and GNSS). As a result, in-device coexistence Extreme proximity of collocated radios due to interference has become a serious problem due small form factor of handheld devices and the to the extreme proximity of multiple radio scarcity of spectrum makes this problem more transceivers within the same device. It has been severe. If these radio technologies within the shown by the studies of 3GPP that for some spe- same device are working on adjacent frequencies cific frequency bands, concurrent operations of or sub-harmonic frequencies, interference power LTE and ISM/GNSS radios working in adjacent due to out-of-band emission from a transmitter of or sub-harmonic frequencies will result in signifi- one radio may be much higher than the signal cant in-device coexistence interference that can- strength of the desired signal for a receiver of a not be completely eliminated by filter collocated radio. This situation results in so-called technology. This motivates 3GPP to introduce in-device coexistence (IDC) interference, as signaling mechanisms and procedures, which shown in Figs. 1a and 1b. enable the devices to effectively solve the in- A straightforward way to mitigate or reduce device coexistence problems with help from the IDC interference is to rely on radio frequency LTE network. This article first discusses the sce- (RF) techniques, such as sufficient filtering or narios in which coexistence interference may isolation. But the analyses made by the Third happen, and then provides an overview of main Generation Partnership Project (3GPP) from the solutions and related procedures specified in RF point of view showed that current state-of- 3GPP Release-11 standards for LTE. In addi- the-art filter technology cannot provide suffi- tion, some aspects of device-internal coordina- cient interference rejection for certain IDC tion and implementation to facilitate the scenarios, which means that alternative solutions coexistence scenarios are also discussed. other than RF design must be considered. If the IDC problem cannot be resolved by the INTRODUCTION terminal itself, the proper operations of different radio technologies cannot be guaranteed; and Interference in wireless networks has been identi- more severely, the connection between the ter- fied as one of the main hurdles toward achieving minal and the cellular network might be lost. In higher network capacity. However, most of the lit- this case, coordination between the terminal and erature has focused on solving interference prob- the cellular network become crucial to avoid lems between devices using the same air interface unacceptable impact on user experience, net- and sharing the same channel. A different type of work performance, and operator key perfor- interference, which could be even more limiting mance indicator (KPI). This motivated 3GPP to to performance, is that produced by concurrent initiate a study item in LTE Release-10 with the radio operation in the same device. There is focus of investigating suitable mechanisms for increasing demand for new use cases that require interference avoidance from signaling and proce- 60 0163-6804/12/$25.00 © 2012 IEEE IEEE Communications Magazine • November 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  • 2. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Power Tx power of ANT #1 ANT #2 ANT #3 Tx signal of ISM transmitter ISM transceiver Interference from LTE Out of band (OOB) emission by ISM Band filter transmitter BT/WiFi Antenna LTE RF GPS RF RFFE Spurious emission isolation Interference by ISM transmitter LTE GPS BT/WiFi from BT/WiFi Rx signal of Unacceptable interference baseband baseband baseband LTE receiver level to LTE receiver Frequency (a) (b) ISM band 2400~2483.5MHz Band 7 UL Band 38 Band 7 DL WiFi channels 2500~2570MHz2570~2620MHz 2620~2690MHz Ch1 Ch7 Ch13 FDD mode TDD mode FDD mode 2401-2423 2431-2453 2461-2483 Ch2 Ch8 Ch14 2406-2428 2436-2458 2473-2495 Band 40 Ch3 Ch9 2300~2400MHz 2411-2433 2441-2463 TDD mode Ch4 Ch10 2416-2438 2446-2468 Ch5 Ch11 Band 41 2421-2443 2451-2473 2496~2690MHz Ch6 Ch12 TDD mode 2426-2448 2456-2478 Bluetooth channels 79 channels: 2402~2480MHz (c) Figure 1. a) Coexistence interference between different radio transceivers within the same device platform; b) example of coexistence interference from in-device transmitter to receiver; c) 3GPP frequency bands around ISM band. dure point of view. With the participation of ter- situation also happens between the higher por- minal, chipset, and network vendors and opera- tion of the ISM band and LTE TDD Band 41. tors, the coexistence interference scenarios and In the case of LTE frequency-division duplex usage scenarios that need to be considered were (FDD) Band 7, uplink (UL) LTE transmission identified. Over the course of more than one causes interference to WiFi/BT reception, but year, potential solutions were studied and even- the impact on the LTE receiver from WiFi/BT tually some feasible solutions were selected for transmitter might be less significant because the further standardization work. As the continua- corresponding LTE FDD Band 7 downlink (DL) tion of the study item phase, a work item speci- is farther away from ISM band. fying the necessary signaling and procedure to Another coexistence scenario example relates support the feasible solutions has been included to GNSS navigational systems developed by vari- as a part of LTE Release-11. ous countries such as GPS, Galileo, GLONASS, This article is organized as follows. Coexis- Compass, and Indian Regional Navigation Satel- tence scenarios considering spectrum and usage lite System (IRNSS). Commercial civil purpose aspects are described first. Then results from RF GNSS receivers use L1 (1575.42 MHz) frequen- analysis are presented to show the necessity of cy for location-based services. UL of LTE Band signaling and procedure based solutions. Subse- 13 (777–787 MHz) and Band 14 (788–798 MHz) quently, an overview of the whole solution to can disrupt the reception of a GNSS receiver solve IDC problems is provided. Both signaling using L1 frequency. This is because the second and procedural parts are covered. Finally, some harmonic of Band 13 (i.e., 1554–1574 MHz) and conclusions are given. Band 14 (i.e., 1576–1596 MHz) are close to L1 frequency. When the Galileo system is support- ing the new global allocation of 2.5 GHz, it will COEXISTENCE also be affected by LTE FDD Band 7 UL [1, 2]. SCENARIOS AND CHALLENGES Based on the above descriptions, some exam- ples of the problematic coexistence scenarios FREQUENCY BANDS WITH IDC INTERFERENCE from 3GPP frequency bands perspective are The frequency range of simultaneous operation summarized as follows: for WiFi/BT and LTE causing IDC interference • LTE Band 40/41 radio Tx causing interfer- is shown in Fig. 1c. The lower portion of the ence to ISM radio Rx industrial, scientific, and medical (ISM) band is • ISM radio Tx causing interference to LTE adjacent to LTE time-division duplex (TDD) Band 40/41 radio Rx Band 40 without guard band in between. This • LTE Band 7 radio Tx causing interference causes mutual interference like LTE transmis- to ISM radio Rx sion affecting WiFi/BT reception and WiFi/BT • LTE Band 7/13/14 radio Tx causing interfer- transmission affecting LTE reception. The same ence to GNSS radio Rx IEEE Communications Magazine • November 2012 61C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  • 3. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® there can be limited attenuation of each filter LTE BW = 10 MHz over the part of the transition band that overlaps -60 with the band of the other technology. This can lead to a large interference to the victim due to -70 WLAN freq (MHz) one or both high spurious emissions and imper- fect blocking performance. In some cases, such 2412 as LTE in Band 40 and ISM starting at 2400 -80 2437 In channel interference (dBm) 2462 MHz, there is almost no guard band between the frequencies of the two technologies. There are -90 further challenges in RF design due to variations WLAN noise floor in filter response across manufacturing process -100 as well as a wide temperature range over which the terminal must operate. We present some representative results to -110 illustrate the extent of IDC interference. The results shown below are based on analysis assum- -120 ing state-of-the-art film bulk acoustic resonator (FBAR) filters. Nominal filter response is assumed (i.e., no temperature variations). The -130 cumulative effect of the interference due to both spurious emissions and blocking is shown. -140 Figure 2 shows the interference caused by an 5 10 15 20 25 LTE @ 2375 MHz transmit power (dBm) LTE jammer to three WiFi channels (CH1 @ 2412 MHz, CH6 @ 2437 MHz, and CH11 @ 2462 MHz). The jammer is centered at 2375 Figure 2. Interference caused by an LTE @ 2375 MHz and BW = 10 MHz MHz with a bandwidth of 10 MHz. The y-axis jammer on WiFi channels in the ISM band. shows the absolute interference level caused to the different WiFi channels in dBm and the x- USAGE SCENARIOS WITH IDC INTERFERENCE axis shows the LTE Tx power level in dBm. The figure also shows a typical noise floor for WiFi In order to understand the behaviors of LTE reception. The huge desensitization that LTE and other radio technologies when IDC interfer- causes to WiFi can be seen. Desensitization is a ence happens, it is also necessary to identify the common indicator to reflect the degradation in usage scenarios in which multiple radios may performance caused by interference, which is operate simultaneously. Some of the prioritized determined by the ratio between the coexistence use cases are: interference and the noise floor at sensitivity. • LTE VoIP service and possibly other data The situation can get worse if the LTE channel applications + BT earphone — The voice is moved nearer to the ISM band, the LTE band- traffic transmitted by BT is actually from/to width is larger or if the filter responses shift due LTE. to temperature and process variation. For • LTE multimedia service + BT earphone — instance, when a full power LTE jammer is Multimedia (e.g. HD video) is downloaded moved closer to ISM band by centering it at by LTE and audio is routed to a BT head- 2395 MHz, all WiFi channels could suffer more set. than 50 dB desensitization. • LTE + Wi-Fi portable router — LTE is Next we discuss the interference caused by considered as a backhaul link to access the WiFi to LTE. Figure 3 shows the interference Internet, and the connectivity is shared by caused by a WiFi jammer in CH1 to different other local users using Wi-Fi. The Wi-Fi LTE channels in Band 40. Strong desensitization transceiver acts as an access point (AP). is observed in the entire LTE band. As can be • LTE + Wi-Fi offload — In this scenario, expected, detrimental impact of IDC interfer- LTE user equipment (UE) can also connect ence in Band 40 can be seen. to Wi-Fi to offload traffic from LTE, and To conclude, the coexistence interference level the Wi-Fi transceiver of the UE operates as and its impact on the receiver performance a terminal (not AP) in infrastructure mode. depends on transmit power and receiver blocking • LTE with GNSS Receiver — LTE is run- characteristics of each radio and physical charac- ning a data application resulting in heavy teristics of transceivers (filter responses, antenna transmit activity while still maintaining isolation, etc.). A comprehensive set of results positioning by a GNSS receiver. across a wide range of aggressor and victim chan- nel combinations has been captured in [1]. CHALLENGES FACED BY RF DESIGN The filtering assumptions are critical in quantify- MAIN SOLUTION DIRECTIONS ing the impact of adjacent channel interference. Both transmit filter on the aggressor radio and UE INTERNAL ARCHITECTURE TO receive filter on the victim radio should be con- HANDLE IDC PROBLEMS sidered. The transmit filter reduces the out-of- band spurious emissions falling into the receive A basic assumption in handling IDC problems is band of the victim technology. The receive filter that the different radios on the same terminal reduces the blocking effect due to the transmis- are aware of the activities of each other. The sions in the band of the aggressor technology. types of information exchanged between each When the two radios operate in adjacent bands, radio may be classified into static, semi-static, or 62 IEEE Communications Magazine • November 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  • 4. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® real-time. Examples of static information are the traffic profiles and modes of operation. Exam- 0 LTE freq = 2310 MHz ples of semi-static information could be the LTE freq = 2330 MHz error rate and duty cycle experienced over a cer- LTE freq = 2350 MHz -20 LTE freq = 2370 MHz tain observation interval or the relative timing LTE freq = 2390 MHz between the radios. Real-time information may LTE RSSI correspond to each event happening at the radios In channel interference (dBm) -40 such as the knowledge of transmission/reception of each packet including the timing, frequency, and transmit/receive signal power levels. The -60 level of detail of exchanged information depends on the terminal hardware and software imple- mentation. The time of availability of real-time -80 information also depends on protocol operations of each radio. For instance, the terminal is typi- cally aware of LTE uplink transmission only -100 after receiving a grant from a base station a few milliseconds before the actual transmission. -120 It may also be possible for one of the radios to adjust its activities to accommodate another radio. However, this is constrained by the fact -140 that each radio must operate in accordance with -10 -5 0 5 10 15 20 the standardized protocols meant for communi- WLAN transmit power (dBm) @ 2412 MHz cation with their remote peers. For instance, when LTE is operating in a channel centered at Figure 3. Interference caused by a WiFi @ 2412 MHz and BW = 20 MHz 2375 MHz, there is an IDC problem when WiFi jammer on LTE channels in B40. is in Channel 1 of the ISM band. If WiFi on the terminal is operating as an access point (AP), then in the presence of LTE activities it can division multiplexing [FDM]-based solutions). switch to Channel 11 to reduce the mutual inter- The basic concept of FDM is to move the victim ference. However, this would not be possible if (or aggressor) signal farther away from the in- WiFi is operating as a station and communicat- device aggressor (or victim) signal and increase ing with a remote AP. Another example is relat- the frequency separation for better filtering per- ed to the case of LTE and BT coexistence formance. This kind of solution can be further wherein BT can adjust its timing relative to the divided into two main branches, LTE FDM solu- LTE frame structure only if it is operating in a tion and ISM FDM solution. master mode but not a slave mode. Some legacy An LTE FDM solution is to move the LTE BT devices (e.g. BT earphone) may by default signal farther away from the ISM band (e.g., by configure themselves as masters, thus in-device performing interfrequency handover within LTE BT may only behaves as BT slave under certain according to the existing procedure specified in scenarios. On the LTE side, the standards 3GPP). On the contrary, in the ISM FDM solu- changes in Release-11 are designed with the tion, the ISM radio signal is moved away from help of eNB involvement to facilitate the coexis- the LTE frequency band in the frequency tence with the other radios in situations where domain (e.g., by switching the WiFi channel in the actions taken by BT/WiFi are not sufficient the portable router scenario as mentioned earli- to solve the IDC problem. These are described er). Another example is BT making use of the in more detail in the subsequent sections. adaptive frequency hopping (AFH) feature to There can be different architectures to sup- select a set of frequencies away from the LTE port the coordination between the radios. One channel. But ISM FDM solutions are beyond the kind of architecture is that a central coordinator 3GPP scope and will not be addressed in 3GPP may be used to interface with multiple radios. specifications. This is a flexible architecture with the coordina- The LTE FDM solution is applicable to all tion functionality limited to one block, and each usage scenarios as long as an alternate LTE fre- radio only needs to interface with the central quency channel is available. In some network coordinator. Another kind of architecture is that deployments, however, using the FDM-based there may be a direct interface between each solution to resolve IDC interference problems is pair of radios. An example of this is the wireless not possible or desirable. It is likely that all coexistence interface 1 (WCI-1) and wireless available LTE frequency carriers are impacted coexistence interface 2 (WCI-2) (i.e. 1-wire and by ISM traffic, or the UE is in an area with only 2-wire) between LTE and BT standardized by one LTE frequency deployment, or the UE does BT SIG [3]. However, this can become more not have good channel quality in the alternate complicated when coordination between more frequencies. There is also a possible scenario than two radios is needed. that the operator has alternative frequency carri- ers, but those carriers are congested due to large FDM-BASED SOLUTIONS numbers of UE units connected to those. To Based on RF analysis results, one straightfor- obtain efficient network operation, it can be ward solution to effectively mitigate or reduce beneficial to balance load between carriers and the IDC interference is to let radio transceivers keep the UE in the current frequency even if work in different frequency bands with sufficient LTE is disturbed by ISM signals. For such cases, frequency separation (i.e. so-called frequency- time sharing between LTE and WiFi/BT/GNSS IEEE Communications Magazine • November 2012 63C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  • 5. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® LTE perspective. Unscheduled time can be used Pattern periodicity (128 ms) for ISM communication. To facilitate the DRX solution for coexis- tence of LTE and other radios, the UE should provide assistance information to the eNB. Information can include desired unscheduled period cycle and period duration. For traffic pat- terns where scheduled and unscheduled time alternate rapidly, a subframe reservation bitmap Scheduling period Unscheduled period (60 ms) (68 ms) can also be provided by the UE. Based on assis- tance information, the eNB configures the DRX parameters of the UE. A DRX-based IDC solu- (a) tion is illustrated with two examples, one for coexistence of LTE and WiFi offload, and one for coexistence of LTE VoIP and BT earphone. DRX cycle (128ms) In the LTE and WiFi offload scenario, scheduling and unscheduled periods should be rather long in both sides to be able to finalize pending transmissions of data. On the other hand, too long periods would increase latency of packet transmission too much. In addition, since WiFi side acts as a terminal, it should be able to receive beacon signal transmitted once every 102.4 ms. Taken these aspects into account, a LTE on-duration Opportunity for ISM (40 ms) operations (88 ms) suitable pattern periodicity suggested by the UE could be 128 ms, where LTE occupies first 60 ms as illustrated in Fig. 4a. But due to scheduling (b) restrictions and limited availability of DRX parameters, the pattern with long DRX cycle to Figure 4. a) Example of UE suggested TDM pattern through IDC indication; 128 ms and onDurationTimer to 40 ms may be and b) example of DRX configured by eNB to enable TDM IDC solution for configured by the eNB, which is illustrated in LTE+WiFi offload scenario. Fig. 4b. It should be noted that because of pend- ing Hybrid Automatic Repeat Request (HARQ) retransmissions in LTE side, transition from is another kind of solution to mitigate interfer- scheduling time to unscheduled time is not sud- ence (i.e. ensuring that transmission of a radio den. Due to this, LTE transmissions might need signal does not coincide with reception of anoth- to be stopped well in advance before the er radio signal). OnDurationTimer ends. In a LTE and BT voice scenario, scheduling TDM-BASED SOLUTIONS and unscheduled periods for each radio alter- In time-division multiplexing (TDM)-based solu- nate quicker than in the WiFi coexistence sce- tions, the basic idea is to share transmission and nario explained previously. In enhanced reception times between different radios. Time synchronous connection oriented (eSCO) mode division can be done by either adapting LTE in BT, the transmission interval is 3.75 ms having transmission and reception timing to ISM timing 6 Tx/Rx slots. To provide sufficient quality of or vice versa. service (QoS) for voice conversation, the net- To be able to share transmission media work should ensure at least a pair of clean BT between multiple radios in the time domain, Tx/Rx instances in each BT interval of 3.75 ms. traffic characteristics of both radios need to be Rest of the capacity can be allocated to LTE. A known. Traffic patterns vary from one service to detailed analysis of LTE and BT time line for another. Even for BT voice, many different traf- different LTE TDD configurations is available in fic patterns can be expected. Diversity of appli- Annex B of [1] to suggest how to find optimal cations and services introduces a particular subframe reservation bitmap. As an example for challenge for TDM based solutions. the BT slave mode, one possible time division In 3GPP, discontinuous reception (DRX) is between LTE and BT is shown in Fig. 5. In the adopted as a TDM solution for IDC interference example, LTE operates in TDD DL/UL configu- avoidance. The DRX mechanism for RRC con- ration #1 (3DL:2UL). The UE indicates to the nected mode was introduced in the first LTE network the desired subframes reservation Release in order to improve power efficiency of bitmap. In this particular example, the bitmap the terminal [4]. In the DRX mechanism, the would be [0011011001], where 1 refers to a sub- UE is allowed to switch its LTE receiver off and frame used in LTE side and 0 a subframe stop monitoring the physical downlink control reserved for BT communication. For short term channel (PDCCH). DRX operation is based on subframe reservation patterns, it is desirable that sleep cycles and timers defining the time when the pattern is hybrid automatic repeat request the UE is active. Because UL transmissions are (HARQ) compliant. HARQ compliance means controlled by scheduling assignments on the that HARQ feedbacks, HARQ retransmissions, PDCCH, gaps in PDCCH reception imply also and UL/DL assignments can be performed that UL transmissions cannot be scheduled when according to the same fixed timeline as used for the UE is in DRX. Thus, time is divided into any data transmission in LTE so that none of scheduled and unscheduled periods from the the useful HARQ processes are broken. Based 64 IEEE Communications Magazine • November 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  • 6. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® LTE subframes (ms) 0 1 2 3 4 5 6 7 8 9 0 1 In general, the terminal should try LTE RX X X X to resolve the LTE TX X X problem through onDurationTimer internal coordination first and only ask for BT TX X X X help from the BT RX X X X network by sending an IDC indication eSCo interval 3.75 ms eSCo interval 3.75 ms eSCo interval 3.75 ms when the problem really cannot be Figure 5. DRX solution in LTE + BT voice scenario with TDD configuration 1. Time slots marked with X are used for transmission/reception in LTE and BT side, correspondingly. resolved by itself. on the assistance information, the network con- a network controlling TDM solution or even figures a suitable DRX configuration. In this solving the IDC problem without involvement of example, the DRX configuration would be as LTE eNB. For example, UE may terminate WiFi follows. The DRX Cycle would be 10 ms, DRX or BT transmission when LTE is receiving criti- drxStartOffset [4] 5 ms, and onDurationTimer 3 cal downlink signal. This would be useful to pre- PDCCH subframes. Downlink subframes 5, 6 vent LTE signaling procedure from being broken and 9 are part of LTE active time because by unexpected IDC interference from the ISM onDurationTimer is running. On the other hand, side. But frequent denial of BT/WiFi transmis- uplink transmissions in subframes 2 and 3 are sion may result in unacceptable performance due to scheduling grants in subframes 6 and 9. degradation or disconnection to BT/WiFi. The To ensure that DRX Active time ends pre- UE needs to judge carefully in order to reach dictably, drx-InactivityTimer and drx-Retrans- the performance trade-off between LTE and missionTimer [4] need to be configured to 0 ms. BT/WiFi connections. In GPS each bit is 20 ms long, and interfer- ence from LTE uplink transmission will be seen as a pulse jammer by the GPS receiver. If LTE SIGNALING AND PROCEDURE FOR uplink allocations can be restricted to a certain percentage of bit length, the GPS receiver can SOLVING THE IDC PROBLEM decode the signal. Therefore, in LTE and GNSS The whole procedure of solving the IDC prob- coexistence scenarios, the LTE onDurationTimer lem is shown in Fig. 6. In the figure, signaling of a few PDCCH subframes and DRX cycle of procedures that are optional and not always nec- 20 ms will help the GNSS receiver to decode the essary are illustrated as dashed. First, a terminal signal. that supports IDC functionality indicates this In cases when interference situations are rare capability to the network, and the network can or sudden, it is impossible or impractical to per- then signal whether the terminal is allowed to form handover or provide unscheduled periods send an IDC indication and for which frequency with the DRX configuration. For example, it can the terminal may report IDC problems. In gen- be that BT connection setup occurs suddenly eral, the terminal should try to resolve the IDC and DRX cannot be configured early enough for problem through internal coordination first and this critical communication. In another scenario, only ask for help from the network by sending some critical signalings occur so rarely and irreg- an IDC indication when the problem really can- ularly that it is not practical to hand over the not be resolved by itself. However, specifying the UE to alternative carrier or configure the UE detailed trigger condition of the IDC indication with a specific DRX setting. Due to these rea- is a complicated problem. From the ISM traffic sons, the UE might deny LTE UL transmissions perspective, it would be good to send IDC indi- autonomously to protect important signals of cation to the network as early as possible. But other radios. However, the denial rate should be from the LTE perspective, unnecessary indica- very low so that the impact on LTE remains tion when the IDC interference problem is not small. For large denial rates, not only are sched- really serious will result in the consumption of uled UL resources lost, but link adaptation of network resources. For example, a network may UL data transmission and DL control channel not be able to balance the load among different might also be disturbed since the network may carriers if it keeps receiving IDC indications compensate missed transmissions by applying a from many UE units and perform an FDM solu- more robust modulation and coding scheme. tion to hand them over to some specific carriers. In addition to the TDM-based solution per- It is complex to specify some unified measure- formed under the control of an LTE network, ments or criteria for IDC triggering for multi- UE may also perform the TDM solutions radio LTE devices. There are many different use through internal coordination. UE internal cases to consider with different interference sce- TDM solutions would be helpful to reduce the narios. Due to these reasons, 3GPP eventually IDC interference to a desired level together with decided that in Release-11 the IDC indication IEEE Communications Magazine • November 2012 65C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  • 7. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® trigger is left up to terminal implementation. patterns which are following HARQ timing However, a guideline is that the IDC indication process are included in the IDC indication should be triggered based on ongoing IDC inter- for the scenario of LTE VoIP+BT ear- ference (or severe interference that will definite- phone. ly happen within a few hundred milliseconds) for Some additional assistance information (e.g., the serving or non-serving frequencies. Any interference direction, interfering technology) unnecessary indications should be avoided to type is still under 3GPP discussion, and may be prevent degradation of the LTE performance introduced later if deemed necessary. experienced by the UE. Upon reception of the IDC indication, the In principle, the network should trust the ter- eNB will take appropriate actions based on the minal’s reporting and try to help the terminal by received assistance information and the network FDM or TDM solution as early as feasible when situation. If the indicated unusable frequencies the terminal is experiencing serious interference. are all non-serving frequencies, the eNB just In order to let the eNB have sufficient informa- needs to ensure that handover to them is avoid- tion about how to efficiently resolve the IDC ed; or, if handover is performed, the TDM solu- problem within the terminal, all necessary/avail- tion should be applied. If the indicated unusable able assistance information for both FDM and frequencies include the serving frequency of the TDM solutions will be sent together in the IDC terminal, the eNB needs to apply either an indication to the eNB. The following are some FDM-based solution or a TDM-based solution assistance information specified by 3GPP. to eliminate the IDC issue experienced by the • Unusable frequencies — List of LTE carri- terminal. For example, when an LTE FDM solu- ers suffering from ongoing interference. tion is considered feasible, in order to determine • TDM patterns or parameters — Depending to which frequencies a terminal can be handed on the usage scenario (e.g., LTE VoIP+BT over, the eNB may configure further measure- earphone) for which the IDC interference ments on usable frequencies. After receiving occurs, different type of patterns or param- additional measurement results reported by the eters are reported to enable appropriate terminal, the eNB can finally decide whether the DRX configuration for TDM solutions on FDM or TDM solution will be used. the serving LTE carrier. For example, The delay from the terminal receiving the desired LTE subframe reservation bitmap measurement configuration for usable frequen- cies to transmitting the measurement report to the eNB could be up to several seconds. During such a long period, if the ISM transmission is Terminal eNB not allowed, the QoS of ISM will deteriorate quite a lot. On the other hand, if ISM side still Indication of support of IDC solutions continues to transmit and disturb LTE severely, it is likely that the LTE connection cannot be Configure whether and for which frequencies maintained before the terminal successfully the terminal is allowed to send IDC indication sends the measurement report. Therefore, in order to ensure the proper operation of both LTE and ISM during the period of measure- IDC interference cannot be ment, eNB may temporarily configure a TDM solved by terminal itself solution to the terminal. In addition, to ensure connectivity with the eNB to perform necessary IDC indication with assistance information procedures to resolve IDC issues, the terminal may deny ISM transmissions. In case the inter-frequency handover is not eNB decides the need of possible or a TDM based solution is preferred, further measurements the network may enable an appropriate DRX operation by configuring the terminal with nec- Measurement configuration for essary DRX parameters, which could be deduced usable frequencies from the assistance information. Then the termi- nal can start to perform TDM solution in syn- eNB may temporarily chronization with the eNB. configure a TDM solution In case there is a significant change on the IDC interference condition, e.g., IDC interfer- Configuration for temporary TDM solution ence disappears due to one disturbing/disturbed radio being switched off, the terminal will report this to the network and the network may react RRM measurement results accordingly. In case of inter-eNB handover, the target eNB should be aware of the IDC problem eNB decides the within the terminal to avoid ping-pong handover appropriate IDC solution back to the problematic frequency. Therefore, the IDC assistance information is transferred from the source eNB to the target eNB during Signaling related to FDM or TDM solutions inter-eNB handover. Even though triggering of IDC indication is left to terminal implementation, the network can still control the amount of IDC indication sent Figure 6. Signaling and procedure to resolve the IDC issue. by the terminal. In principle, this is done with 66 IEEE Communications Magazine • November 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  • 8. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® two mechanisms. First, in the case IDC solutions 2007 and has been attending 3GPP standardization of LTE since 2008. He is the rapporteur of the study item and are not supported by the eNB (e.g. in the net- work item on interference avoidance for in-device coexis- Different solutions work not upgraded to LTE Release-11), IDC tence in 3GPP. indications are not allowed to be sent. The net- and configurations work also configures with the dedicated signaling RIIKKA SUSITAIVAL _________________ received ( have been specified her M.Sc. and Dr. Sc. degrees in electrical and telecommu- for which frequencies the terminal may indicate nications engineering from Helsinki University of Technolo- in 3GPP to fulfill the IDC problems. In addition, with a prohibit timer, gy in 2002 and 2007, respectively. Her major in both it is possible to limit signaling load due to too degrees was teletraffic theory. She joined Ericsson Research requirements of vari- frequent reports from the terminal side. in 2007 and has worked with research and standardization in the area of wireless access networks. Her particular ous deployments, interests include link layer protocols of LTE networks, usage scenarios, and CONCLUSIONS including concept development and performance analysis with system simulations. network conditions. Driven by an upsurge in smartphone usage, The combination of ZHUO CHEN ( received his electron- _____________ simultaneous operation of multiple radio tech- ics engineering degree from Sichuan University, China, in nologies within the same device has become an 2001 and his Ph.D. degree in electromagnetic field and an LTE network con- increasing trend for many applications. In some microwave technology from Beijing University of Posts and trolled solution and frequencies and usage scenarios, the coexistence Telecommunications, China, in 2006. Since 2007 he has interference between different radio technolo- been with Huawei Technologies Co., Ltd., China, where he terminal internal is currently working as a researcher as well as meeting del- gies cannot be eliminated by a single generic RF egate of 3GPP. His research interests are in the fields of coordination would design. It has been shown by 3GPP studies that wireless communication, 3GPP cellular systems. be a good resolution solutions from the signaling and procedure I-KANG FU ( received his M.S. degree ____________ of the IDC problems. points of view are feasible to minimize the from the Department of Electrical Engineering at National impact of IDC interference and bring acceptable Chung Cheng University in 2002, and B.S. and Ph. D performance for both LTE and WiFi/BT/GNSS degrees from the Department of Communication Engineer- radios. Different solutions and configurations ing of National Chiao-Tung University, Hsinchu, Taiwan, in 2000 and 2007, respectively. He is currently the manager have been specified in 3GPP to fulfill the of MediaTek Beijing standard department and responsible requirements of various deployments, usage sce- for 3GPP LTE-Advanced/Beyond and CCSA standardization narios and network conditions. The combination tasks. Prior to 3GPP, he was heavily involved in the research of an LTE network controlled solution and ter- and standard development for IEEE 802.16m, WiMAX 2.0, and IEEE 802.16j since 2005, with main focus on multicar- minal internal coordination would be a good res- rier, interference management, and multihop relay tech- olution of the IDC problems. nologies. He also served as Chair of WFEG to help with IMT-Advanced evaluation for ITU-R WP5D in 2010. REFERENCES PRANAV DAYAL ( received a B.Tech. ______________ [1] 3GPP, “TR 36.816: Evolved Universal Terrestrial Radio degree in electrical engineering from the Indian Institute of Access (E-UTRA); Study on Signaling and Procedure for Technology, Madras, India, in 2000, and M.S. and Ph.D. Interference Avoidance for in-Device Coexistence,” degrees in electrical and computer engineering from the University of Colorado, Boulder, in 2003 and 2005, respec- [2] ICG United Nations Office of Outer Space Affairs, “Cur- tively. In 2005, he joined Qualcomm, Incorporated in San rent and Planned Global and Regional Navigation Satel- Diego, California, where he has been involved in R&D of lite Systems and Satellite-based Augmentations 4G wireless systems. In recent years, he has worked on Systems,” 2010, multiradio coexistence with LTE including standardization [3] BT SIG, “Bluetooth Core Specification Addendum 3,” in 3GPP RAN2. July 2012 [4] 3GPP, “TS 36.321: Evolved Universal Terrestrial Radio SUDHIR KUMAR BAGHEL ( is a senior _____________ Access (E-UTRA); Medium Access Control (MAC) proto- chief engineer at Samsung Electronics. He received his col specification,” M.Tech in computer science and data processing from the Indian Institute of Technology Kharagpur in 2002. Since BIOGRAPHIES then he has been working with Samsung Electronics in its India R&D center in Bangalore on radio access research and ZHENPING HU ( received his _________________ development. He was involved in design and development Ph.D. degree in information and communications engineer- of LTE modems for UE prior to start working on 3GPP stan- ing from Huazhong University of Science and Technology dardization in 2010. His research interests are especially in in 2005. He was with Siemens Limited China from 2005 to machine type communication, handling signaling overhead 2007, where he worked on research of 3GPP LTE system. in radio networks due to smartphones, and energy efficien- He joined China Mobile Communications Corporation in cy of radio networks. IEEE Communications Magazine • November 2012 67C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®