COMMUNICATIONS NETWORKS

Field of the invention

The present invention has its application within the telecommunication sector, particularly in cellular (wireless) communications networks. More specifically, the present invention proposes a method and device to minimize inter-system and intra system interferences in order to allow coexistence between cellular networks (cellular systems) using Time Division Duplex (TDD) technology and, therefore, in Time Division Duplex (TDD) bands.

Background of the invention

TDD bands (frequency bands assigned to be used by TDD systems) are attractive for Fifth-Generation (5G) systems, as they have the potential to offer large bandwidths for future cellular use. Particularly, the electromagnetic spectrum above 6 GHz (generally referred to as millimetre-waves) is usually offered in contiguous mode for simplicity of the UE frontends in cellular networks, hence enforcing TDD operation. The millimetre-wave (mmWave) frequency region comprises radio frequencies in the range from 30 GHz to 300 GHz although in some practical applications, frequencies above 6 GHz are also regarded as mmWaves. The availability of large bandwidths, together with the potential to allocate cellular services, makes these frequencies very well suited for Fixed Wireless Access (FWA) applications, but mobility services can also be offered.

Time Division Duplex mode is attractive because it can cope with asymmetric traffic demands, as opposed to Frequency Division Duplex (FDD) mode where capacity for uplink (UL) and downlink (DL) is symmetric. In FDD, different frequencies are used in UL and DL. In TDD technology, usually the same carrier frequency is used for both uplink (from the user equipment to the base station) and downlink (from the base station to the user equipment) transmissions; the carrier is subdivided in the time domain into a series of timeslots and the single carrier is assigned to uplink during some timeslots and to downlink during other timeslots. One of the drawbacks of TDD bands is, however, the potential coexistence issues that appear between systems operating in the same band, because interference from BS to BS and from UE to UE can be very strong unless special protection mechanisms are devised. The presence of multiple cross interferences in TDD systems makes it essential to follow strict coexistence rules between systems (e.g. networks or operators). These rules will be either imposed by the regulator or agreed among operators, and usually include the timeslots to be used in each frame for UL and DL transmissions, which is called the UL:DL pattern.

The usual approach for coexistence in TDD bands is to define a strict time synchronization scheme (in order to minimize cross interferences) that must be followed by all systems. This time synchronization scheme involves: unambiguously defining the start instants of all the transmission and reception opportunities (e.g. using Global Navigation Satellite Systems, GNSS, a transport network synchronization protocol as Precision Time Protocol or signals or synchronization protocols like IEEE 1588v2) and specifying the exact UL:DL (Uplink : Downlink) pattern detailing the transmission opportunities for Base Stations (BSs) and User Equipments (UEs) in all the systems which are deployed in a certain zone (e.g. a country).

Following a strict time synchronization has the drawback of restricting the freedom to allocate time resources according to the traffic needs in each system. All the systems must agree on the exact UL:DL pattern to use, or the Regulator may impose a common pattern to be adopted, with the subsequent loss in flexibility. Moreover, compliance with an agreed or imposed pattern forces devices to introduce a timing advance for UL transmissions in order to compensate propagation delays. These mechanisms can be easily adopted when all systems belong to the same baseband technology but are far more difficult to apply when coexistence between different technologies is sought. In addition, imposing strict UL/DL patterns can reduce the ability of the system to cope with varying types of traffic, e.g. from Voice over IP (where very small packets are preferred) to high-quality video traffic (where large packets are more dominant).

To circumvent the above drawbacks, some FWA systems in mm Waves operate in FDD mode, i.e. they use different frequencies in UL and DL (either from the same band or from different bands) with a given duplex gap separation in between. In such cases, no strict time synchronization is required, and systems only need to follow certain out-of-band emission limits to keep interferences to a minimum. However, leaving a duplex gap at these high frequencies significantly reduces spectrum efficiency, and some mmWaves bands (like 26 GHz) will only work in TDD mode for 5G New Radio (NR).

Other procedures rely on strict filtering characteristics and appropriate guard bands between coexisting systems while avoiding time synchronization. This approach is however unfeasible when the incumbent systems cannot be changed to apply the required filtering. This is the case of some mmWave systems, where the presence of so many antenna elements makes it very challenging to apply strict filters at the outputs of the power amplifiers, hence making impossible to fully avoid time synchronization.

In some proposals, as in CN101282168 and US20130028151 , the new TDD system aimed to coexist with an incumbent TDD system adapts its transmission parameters in such a way that all UL and DL transmissions of the new system are contained within the opportunity intervals defined by the incumbent system. This scheme can be appropriately considered as following strict time synchronization, whose time pattern is in some proposals signaled to devices. Other procedures consider blank periods to avoid interferences as in US20100135272 or US20130301420 with the subsequent loss in efficiency.

Thus, current existing techniques are suboptimal. Smarter strategies for coexistence in TDD bands are therefore required in order to avoid the high complexity and/or reduced spectrum efficiency that result from applying strict time synchronization rules in multi-system scenarios.

Summary of the invention

The present invention solves the aforementioned problems and overcomes previously explained state-of-the-art limitations by proposing a method and device to facilitate coexistence between a (first) TDD cellular system, comprising at least one Base Station (BS) and at least one user equipment (UE) and one or more other TDD systems (called from now on incumbent TDD systems) operating in the same TDD frequency band, in such a way that no modifications are required to the incumbent systems (i.e. the incumbent systems remain unchanged) while interferences between the users and base stations of the respective systems are minimized, as well as any self-interference occurring between sectors of the same system. Thanks to the proposed mechanism, it is ensured that the resulting interferences between systems are minimized to the extent possible, while keeping the radio characteristics and the downlink, DL, and uplink, UL, time intervals defined by said incumbent system for radio transmission and reception unchanged.

The invention proposes a hybrid synchronization scheme with strictly synchronized DL transmissions but asynchronous operation in UL. The synchronous DL timing is assumed to be imposed by the incumbent system(s) (DL pattern agreed between the systems or imposed by a regulator) hence avoiding some of the interference terms between coexisting systems. However, UL operation can observe potentially different timings hence achieving a flexible UL:DL pattern. This represents an advantage over traditional fully synchronized TDD networks, where UL:DL ratio is fixed and cannot adapt to traffic demands. Interferences between systems can be minimized, as well as self-interference between adjacent sectors, while not requiring any changes to the incumbent systems. That is, thanks to the proposed invention the resulting UL:DL ratio of the UL and DL transmissions duration in said TDD system is flexible and dynamically determined by the scheduler decisions at the base station of the first cellular system, whereas the UL:DL ratio of the UL and DL transmission duration in said incumbent system is fixed.

The radio scheduler at the base station of the first cellular system determines the UL transmission durations and start instants of the UL packets, in such a way that no collisions between the user equipments of the first and of the incumbent cellular systems will occur. DL transmissions, however, always stick to the predefined DL slots. Whenever an UL transmission from a given UE is active and occupies part of the DL resources, the BS will refrain from initiating any DL transmission until no UL signal intended for that BS is present. Prior state-of-the-art techniques usually impose strict time synchronization, including a predefined UL:DL ratio and a Timing Advance procedure. The resulting loss in flexibility can also lead to high spectral inefficiencies as per the fixed size of the packets. Other asynchronous alternatives require guard bands and strict filtering capabilities, and incumbent systems may not be allowed to incorporate such modifications hence making this option unavailable. The proposed invention can overcome the limitations of prior techniques by introducing synchronous DL but asynchronous UL operation, hence allowing flexible UL packet sizes while not requiring any Timing Advance algorithm. UEs can therefore be simpler while achieving better spectral efficiency than in a fixed UL:DL configuration. Interferences towards incumbent base station, and from incumbent user equipment, are also avoided, in this proposal. The required filtering characteristics only apply to the system willing to coexist with other incumbents in the same band. Interference between nodes from the same system can also be minimized in the proposed invention, hence leading to simpler operational deployments where nodes need not synchronize their scheduler decisions but just the DL transmission occasions.

According to a first aspect, the present invention proposes a method for minimizing interferences between a first (cellular) Time-Division Duplex, TDD, communications system and at least a second (e.g. existing) TDD communications system (usually operating in the same frequency band as the first system) wherein each TDD system comprises at least one base station, BS1 (first system) and BS2 (second system) respectively, in charge of scheduling downlink, DL, and uplink, UL, transmissions and each TDD system comprises at least one user equipment served by the correspondent base station, wherein the DL and UL transmissions scheduling in the second TDD system is made according to a pre-established fixed UL:DL pattern which indicates which time intervals (e.g. time slots) of each frame are reserved (may be used) for DL transmission and which for UL reception in the base station side (and for DL reception and UL transmission in the UE side), that is, the second system follows a full time synchronization scheme. Wherein the method comprises the following steps performed by BS1 : a) (dynamically) scheduling (assigning) one or more time intervals (e.g. time slots) for transmission of DL packets only if said time interval is contained within the time intervals reserved (established) for DL transmission in said second TDD system in the pre-established fixed UL:DL pattern (that is, the time slots that may be used for DL transmission in the first system are the same time slots which may be used for DL transmission in the second system) and said time interval does not overlap with any UL time interval scheduled by the base station BS1 for UL reception; b) (dynamically) scheduling (assigning) one or more time intervals (e.g. time slots) for transmission of UL packets by a user equipment of the first TDD system, wherein said one or more time intervals are at least partially outside (i.e. they are not completely contained by) the time intervals reserved for UL reception (in the pre-established fixed UL:DL pattern) in said second TDD system.

In an embodiment step b) comprises: BS1 scheduling one or more time intervals for transmission of UL packets which at least partially overlap one or more time intervals reserved for DL transmission in the pre-established fixed UL:DL pattern of said second TDD system (that is, the UL time interval of UE1 at least partially coincides with the time interval reserved for reception of DL transmissions on the second system or, in other words, the UE in the first system transmits during part or all of the time intervals established for DL reception in the second system).

Usually, BS1 schedules a time interval for transmission of UL packets by a user equipment only if said time interval does not overlap with any DL time interval scheduled by BS1 for DL transmission to any other user equipment served by BS1 (that is, only if not other DL transmission is active in the BS1 cell).

Analogously, in an embodiment, BS1 schedules a time interval for transmission of UL packets by an user equipment only if said time interval does not overlap with any UL time interval scheduled by BS1 for UL transmission of any other user equipment served by BS1. In an alternative embodiment, several UEs may share UL or DL time interval if Multiple Users MIMO is used.

The second TDD system may follow a (full) time synchronization scheme (unambiguously fixed defining the start instants of all the possible UL and DL time intervals), by using a Global Navigation Satellite Systems, GNSS, a protocol for time synchronization, a transport network synchronization protocol, such as Precision Time Protocol, PTP, or any other suitable means for time synchronization.

The first and second systems use the same communication technology or a different communication technology. The user equipments may be mobile telephones, tablets, smartphones, laptops, computers or any other type of user equipments served by a base station of a TDD communications system.

Guard bands are preferably reserved at both edges of the frequency carriers (frequency used to transmit UL and DL transmissions to each base station) assigned to the base stations and user equipments of the first TDD system.

BS1 preferably schedules the UL time intervals based at least on the transmission traffic requirements of the user equipments served by it and/or on the transmission traffic requirements of the BS1.

The base stations (as BS1) preferably have specific reception filters centered at its carrier frequency (in order to ensure that the unwanted signal level at BS1 receiver created by said second TDD system is below thermal noise).

In order to further minimize the interference between BS2 and BS1 (to minimize the radio frequency coupling), at least one of the following actions may also be taken: maximize the separation of the carrier frequencies used by BS2 and BS1 , maximize the physical distance between BS1 and BS2, or avoid direct visibility between both base stations by adjusting their relative tilts and azimuth orientations.

The first TDD system may further comprise at least a second Base Station BS’2, where said second base station also performs steps a) and b) for scheduling the transmission of UL and DL packets. In order to further minimize the interference between BS1 and BS’2, at least one of the following actions may also be taken: assign different carrier frequencies to BS1 and BS’2, avoid direct visibility between both base stations by adjusting their relative tilts and azimuth orientations, or reserve suitable guard bands at both edges of the carrier frequencies of BS1 and BS’2. The base stations BS1 and BS’2 of said first TDD system may estimate the round- trip-time between them and the user equipments connected to the respective base station, and said estimated round-trip-time is taken into account when scheduling UL and/or DL transmissions, with the intention to avoid any time overlap between the received UL signals and the transmitted DL signals when radio resources are to be scheduled by the base stations.

In order to further minimize the interferences created by a User Equipment UE1 connected to (served by) BS1 , to a User Equipment UE2 connected to BS2, and/or to a User Equipment UE’2 connected to BS’2, UE1 fulfils the necessary out-of-band and spurious emission limits (to ensure that the unwanted signal levels at UE2 and UE’2 receivers are below thermal noise).

The carrier frequency used for communications between BS1 and UE1 may be the same carrier frequency used for communications between BS2 and UE2 or different.

The base stations of the first and second TDD systems (BS1 and BS2) may be located in different positions or co-located in the same position and, in this second case, the co-located base stations (their antennas) are pointing towards non-overlapping angular regions.

According to a second aspect, the present invention proposes a base station, BS1 , of a first Time-Division Duplex, TDD, communications system, for minimizing interferences with at least a second TDD system comprising at least one Base Station, BS2, wherein the DL and UL transmissions scheduling in the second TDD system is made following a pre-established fixed UL:DL pattern which indicates which time intervals of each frame are reserved for (may be used for) DL transmission and which for UL reception in the base station side (and for DL reception and UL transmission in the UE side), wherein BS1 comprises a base station scheduler (a processor) in charge of scheduling DL and UL transmissions, the base station scheduler being configured to: a) schedule one or more time intervals for transmission of DL packets from BS1 only if said time intervals are contained within the time intervals reserved for DL transmission in the pre-established fixed UL:DL pattern of said second TDD system and said time interval does not overlap with any UL time interval scheduled by BS1 for UL reception; b) schedule one or more time intervals for transmission of UL packets by a user equipment, wherein said one or more time intervals are at least partially outside the time intervals reserved for UL transmissions in the pre-established fixed UL:DL pattern of said second TDD system.

According to a third aspect, the present invention proposes a cellular TDD system comprising at least one base station BS1 as described above.

In a last aspect of the present invention, a computer program is disclosed, comprising computer program code means adapted to perform the steps of the described methods, when said program is run on processing means of a network entity of an OFDMA network, said processing means being for example a computer, a digital signal processor, a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), a micro-processor, a micro-controller, or any other form of programmable hardware. In other words, a computer program comprising instructions, causing a computer executing the program to perform all steps of the described method, when the program is run on a computer. A digital data storage medium is also provided for storing a computer program comprising instructions, causing a computer executing the program to perform all steps of the disclosed methods when the program is run on a computer.

Consequently, according to the invention, a method, base station, system and storage medium according to the independent claims are provided. Favourable embodiments are defined in the dependent claims.

These and other aspects and advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

Description of the drawings For the purpose of aiding the understanding of the characteristics of the invention, according to a preferred practical embodiment thereof and in order to complement this description, the following figures are attached as an integral part thereof, having an illustrative and non-limiting character:

Figure 1 depicts a diagram showing the interference terms between two TDD systems coexisting in the same frequency band, where the proposed solution may be applied according to an embodiment of the invention.

Figure 2 shows a schematic diagram of a full time synchronization scheme between base stations BS1 and BS2 according to prior art solutions.

Figure 3 shows a schematic diagram of a Timing Advance mechanism according to prior art solutions.

Figure 4 shows a schematic diagram of a full asynchronous operation scheme between base stations BS1 and BS2 according to prior art solutions.

Figure 5 shows a schematic diagram of an example of the proposed hybrid synchronization scheme between base station BS1 and user equipments UE1 and UE1A according to an embodiment of the invention.

Figure 6 shows a schematic representation of the frequency guard bands that should be reserved at the edges of the TDD system carriers to minimize interferences to/from the incumbent systems, according to an embodiment of the invention.

Figure 7 shows a schematic representation of the filter to minimize interferences from BS2 to BS1 , according to an embodiment of the invention.

Figure 8 depicts a diagram showing the interference terms that appear when both base stations and both user equipments adopt the proposed hybrid synchronization scheme, according to an embodiment of the invention.

Detailed description of the invention The present invention proposes a method and device to minimize interferences between wireless cellular systems operating in TDD mode (using TDD techniques), where at least one of them is an incumbent system whose radiofrequency (RF) and software (SW) characteristics must not be modified. The proposed synchronization scheme (optionally together with additional filtering stages, guard bands and site engineering actions), can minimize interferences while at the same time achieving flexibility in UL:DL traffic ratio.

Figure 1 shows an embodiment for application of the proposed invention. A first (new) TDD system comprising at least one base station (BS1) and at least one user equipment (UE1) has to coexist in the same band with at least one second TDD system comprising at least BS2 and UE2, in such a way that BS2 and UE2 remain unchanged while minimizing interferences. That is, Figure 1 illustrates a cellular TDD scenario containing at least two Base Stations, named BS1 and BS2, and at least two User Equipment (UEs), named UE1 and UE2. Both BS1 and UE1 operates in a given carrier frequency that may be different or not to the one corresponding to BS2 and UE2, but in the same band. If BS2 and UE2 operate at a different carrier frequency, they are considered in what follows as the incumbent system, with which BS1 and UE1 must coexist. BS1 and UE1 can use the same baseband technology as BS2 and UE2, or a different one.

Figure 1 shows all possible interference terms in the uplink (UL) and downlink (DL) of this scenario, denoted as 11 , I2, ... , I6. Some of these interference terms are avoided when strict time synchronization is observed. When BS1 , BS2, UE1 and UE2 operate at the same carrier frequency, 11 , I2, ... , I6 are usually denoted as self-interference terms. The interference terms are the following:

- Cross BS-to-UE and UE-to-BS interference terms 11 and I2. These interference terms are always present regardless of whether the systems are time-synchronized or not. If the carrier frequencies of both systems are different, the impact of 11 and I2 will be minimal if proper out of band and spurious emission requirements are fulfilled at both BS and UE. In such a case, its only effect will be a reduction in the effective signal to noise ratio (SNR) caused by the spectral leakage at the edges of the signal carriers. If both systems use the same frequency, then these interferences might be significant.

- UE-to-UE (I3, I4) and BS-to-BS (I5, I6) interference terms. These terms can be very significant if UEs or BSs are close to each other. In particular, I3 and I4 are the hardest to mitigate because UEs are typically uncoordinated and its relative positions cannot be controlled beforehand. These interference terms are especially high when the UL transmissions and DL transmissions are partially or totally overlapped, for example, when the uplink transmission of UE2 coincides with the downlink reception of UE1 (I3), when the uplink transmission of UE1 coincides with the downlink reception of UE2 (I4), when the downlink transmission of BS1 coincides with the uplink reception of BS2 (I5) or when the downlink transmission of BS2 coincides with the uplink reception of BS1 (I6).

In order to mitigate the above sources of interferences, different coexistence schemes are devised between BS1 and BS2. Two schemes are commonly adopted, full time synchronization and fully asynchronous operation:

- Full time synchronization

In this case, both systems are strictly time-synchronized as shown in Figure 2. The DL and UL slots are the same and synchronized (they start and finish at the same time instants). Interference terms 13, 14, 15 and 16 are not present in this case.

Full time synchronization implies that systems must ensure the following:

1. A common time reference that must be shared among the synchronized BS nodes, unambiguously defining the starts of frames, subframes, and slots at the BS side. It can be provided by e.g. Global Navigation Satellite Systems (GNSS) or Precision Time Protocol (PTP), as long as it ensures a given precision. As an example, a possible frame structure for 5G NR (New Radio) Technologies in mm Waves may have a subframe duration of 1 ms divided in 8 slots of 125 microseconds each one. With full time synchronization, the beginning of slots and subframes are exactly aligned at all the BSs operating in the same band, by means of a common time reference that must be shared by all the nodes. 2. A common UL:DL pattern that specifies the expected (reserved) occasions for DL transmission and UL reception at the BS side (and therefore, for DL reception and UL transmission in the UE side), as well as a guard period for DL-to-UL transition. A possible UL:DL pattern that could be adopted by mm Wave 5G NR systems, referred to as DDDSU, with three DL slots (D), then one special (S) slot mainly intended for DL-UL switching and one UL slot (U). In this example each slot will have a duration of 125 microseconds. The special slot can also contain UL (U) symbols, DL (D) symbols and flexible (F) symbols (for example as defined in ETSI TS 138 213: "5G; NR; Physical layer procedures for control 3GPP TS 38.213 version 15.3.0 Release 15)", 2018. For example, the S slot can include n symbols (e.g. 14) which can be only D symbols, U only symbols or only F symbols or it can include a combination of them (that is some D symbols and some F symbols, some U symbols and some F symbols...). The exact structure of the S slot must also be agreed by all systems operating in the same mmWave band.

3. A Timing Advance (TA) value that all UEs must apply according to a common algorithm. TA adjustments are aimed to ensure that UL signals reach the BS at the same time instants, regardless of the UE positions in the cell. TA relies on a closed- loop process, where the BS estimates the time offset that each UE has to apply to the start of all subframes containing UL slots and properly notifies it to the UEs. TA is calculated by means of the following relation: TA = 25 + t _processing where 5 is the propagation time between BS and UE, and t _processing is the estimated processing time at the UE. Figure 3 illustrates the principle of TA algorithm, between two UEs and a BS. The UEs will receive the downlink subframe with a delay corresponding to the propagation time from the BS to the UE (51 for UE1 and 52 for UE2).Then, the UEs will apply the time offset estimated and reported by the BS and send their uplink subframes to the BS, which will be received at the same instants. In figure 3, it is considered that t _processing is very small compared to the propagation times so it is not taken into account. All UEs coexisting in the same TDD band must obey the same TA algorithm to ensure that their transmissions do not collide with the occasions intended for DL.

It is apparent that full time synchronization has different implications for BSs and UEs: - BSs must strictly follow the transmission and reception instants defined by an a- priori UL:DL pattern, whether agreed or imposed by Regulation (e.g. technology standards) with clearly defined start instants. The time source can be obtained by means of a hardware signal provided by e.g. GNSS or PTP.

- UEs acquire synchronization from the received DL signals (e.g. from a special synchronization beacon devised for this) and adjust the start occasions of UL transmissions according to a given TA algorithm. Even if both systems coexisting in the band implement different baseband technologies, with full time synchronization the UEs will have to follow the same TA algorithm to ensure a given agreed UL:DL pattern.

- Fully asynchronous operation

In this case systems operate in asynchronous mode (an example shown in Figure 4). There is no time alignment of transmissions between base stations, and different UL:DL patterns are generally observed by the coexisting systems (by the coexisting base stations). As it can be seen in figure 4, the slots starts and finish at different times in each BS and one of the BS can be transmitting in the downlink and the other in the Uplink. This leads to strong BS-to-BS and UE-to-UE interference terms, which can be alleviated by means of additional filters and guard bands. As it can be seen in figure 4, when BS1 is transmitting in the downlink and BS2 in the uplink there is a strong interference from BS1 to BS2 (and vice versa).

Fully asynchronous operation can be possible in cellular systems (especially in systems operating in the sub-6 GHz frequency range) provided that:

1. BSs and UEs of coexisting systems fulfil strict out of band and spurious emission limits defined by the baseband standards.

2. Operator-specific receive filters are included at the BS side to protect each system from the BS-to-BS interference created by the other systems.

3. Appropriate guard bands are reserved between the carrier frequencies of all coexisting systems to further reduce in-band interferences. The part of frequency spectrum used by said system should be separated by appropriated guard bands. The actual filtering requirements, emission limits and guard bands must be calculated according to the baseband receiver characteristics. Fully asynchronous operation requires special filtering techniques at the transmission and reception stages of all coexisting systems; if any of them does not implement such filters, coexistence will not be possible because the interference would be too high. As an example, the presence of hundreds or even thousands of antenna elements in some mmWave systems generally makes it too challenging, or too costly, to include strict filters after the power amplifiers (PAs) and low-noise amplifiers (LNAs); in such cases, fully asynchronous operation is generally not possible unless extremely large guard bands are reserved to protect the two systems from each other. This can also happen if a new system has to coexist with an incumbent system in the same band and it is not possible to introduce extra filtering on the incumbent equipment.

In view of the above drawbacks of the existing schemes to mitigate interferences, the present patent application proposes an alternative hybrid synchronization mechanism that accomplishes coexistence between a first (new) system and at least one second (incumbent) system without requiring any modifications to the second system equipment.

Proposed hybrid synchronization scheme

In a scenario where the proposed solution can be applied, it is assumed that a first TDD system, comprising at least one BS and one or multiple UEs, must coexist with one or more second TDD systems (called incumbent systems) operating in the same band. The incumbent systems must remain unchanged while ensuring that the resulting interferences between systems are minimized to the extent possible.

In this scenario, the TDD system can be part of any telecommunications network, especially a cellular telecommunication network, for example a 2G, 3G, 4G, 5G mobile telecommunications network or any other type of telecommunications network using TDD technology for communications. The technologies used by the TDD system and the incumbent system are the same or different (in an embodiment, for example, the first system can be a 3GPP or non-3GPP system operating in a mmWave band, and said incumbent TDD system is a 3GPP system operating in the same band). The user equipment may be a mobile telephone, a tablet, a smartphone, a laptop, a computer, a PC... (and generally any electronic equipment or device that can be connected to the TDD system).

The proposal considers strictly synchronized DL transmissions but asynchronous operation in UL, as shown in Figure 5. Figure 5 shows a TDD system, with at least one Base Station BS1 and at least two user equipments UE1 and UE1A. In said figure, Tp1 and Tp1A are the propagation times between BS1 and UE1 and UE1A respectively. In figure 5, the slots where there is a real DL or UL transmission are highlighted. The synchronous DL instants and pattern is imposed by the incumbent systems (previously agreed between the existing TDD systems in a certain area), in order to avoid some of the interference terms in Figure 1. However, UL operation can observe potentially different timings hence achieving a flexible UL:DL pattern.

The BS scheduler determines the UL transmission durations and start instants of the UL packets, in such a way that no collisions between UEs occur. Usually, the UEs transmit to the BS their traffic requirements and, based at least on said information the BS scheduler assigns to each UE, UL transmission durations and start instants of their UL packets (which may stick or not to the predefined UL slots, that is the UL packets can be transmitted in the predefined UL slots or in predefined DL or S time slots, for example). DL transmissions, however, always stick to the predefined DL slots. That is, to the DL time slots predefined by the UL:DL pattern agreed between all the existing TDD systems in a certain geographical area.

Whenever an UL transmission from a given UE is active and occupies part of the DL resources, the BS will refrain from initiating any DL transmission until no UL signal intended for said BS is present in the system. That is, as the BS has assigned UL transmission durations and start instants to each UE (some of them occupying part of the pre-assigned DL resources), the BS knows when there is no UL transmission assigned and will only initiate a DL transmission when there is no UL transmission assigned in said time slot. No Timing Advance mechanism is necessary, as UL transmissions do not need to be confined within the limits of the UL occasions defined by the incumbent systems. That is, if the system does not use the hybrid synchronization scheme (e.g. incumbent system) it should perform a Timing Advance algorithm to ensure that all UL signals under control of BS2 are received at the same instants, while no Timing Advance algorithm is needed at said TDD system applying the proposed hybrid synchronization scheme, to achieve coexistence and minimize interferences.

In order for the BS to keep control of the active UL transmissions from UEs, estimation of the round-trip-time may be needed to avoid any overlap between the active UL transmissions and any DL occasion planned by the BS scheduler. That is, each base station (BS1) estimates the round-trip-time between it and the user equipment connected to that cell, with the intention to avoid any time overlap between received UL signals and transmitted DL signals when radio resources are to be scheduled by the base station. Such estimation can be performed dynamically by the BS (for moving UEs), or statically by the BS or the management system (for static UEs, like in FWA scenarios), without precluding any other possibility.

The actual UL:DL ratio in the system will therefore be dynamic and dependent on the BS scheduler decisions, which are ultimately determined by the traffic demand and the spectral efficiency of the system. This represents an advantage over traditional fully synchronized TDD networks, where UL:DL ratio is fixed and cannot adapt to the traffic demand.

The presence of asynchronous UL transmissions generally introduces interferences between the elements of the system. However with the proposed hybrid synchronization scheme said interference is minimized. For example, if the proposed hybrid synchronization scheme is applied at BS1 and UE1 (the first TDD system), the interference from BS2 to BS1 (I5 in figure 1) and from UE2 to UE1 (I3 in figure 1) will be avoided (because the uplink transmission of UE2 will never coincide with the downlink transmission of UE1 and BS1 will strictly follow same DL time instants as used in BS2), being BS2 and UE2 a base station and user equipment of a second TDD system (an incumbent system coexisting in the same band, where this hybrid synchronization scheme may not be applied).

The base stations BS1 and BS2 may be in different positions or even they can be co located in the same position (in this latter case, the base stations may be pointing towards non-overlapping angular regions, hence serving different cellular sectors). In a preferred embodiment, the following rules are observed in order to minimize interferences between the system with hybrid synchronization and the incumbent system (where this hybrid synchronization scheme may not be applied):

1. Whenever BS1 needs to start any DL transmission, it sticks to the predefined DL time intervals, i.e. those defined by the incumbent system (that is, the time interval between the start and end of all DL packets transmitted by the base station of said TDD system, BS1 , must be contained within the time intervals established for DL transmission in said incumbent system) and avoids any of the UL time intervals. This will avoid interference I5 from BS1 to BS2 and interference I3 from UE2 to UE1 during the DL transmissions of said TDD system.

2. UE1 , upon receiving appropriate scheduling indications from BS1 , can transmit during the UL slots defined by the incumbent system, but can also“invade” part of the slots reserved for DL in the incumbent system, (if no other DL transmission is active in the cell), if the BS scheduler has allowed it to do it. That is, UE1 can transmit during the time intervals established for UL transmission in said incumbent system, and can also transmit during part, or all, of the time intervals established for DL transmission in said incumbent system, provided that no other DL transmission is active as per the appropriate scheduling indications from BS1. This enables a flexible UL:DL pattern whose actual ratio is controlled by the BS scheduler as a response to the traffic demand (however, the UL:DL ratio in the incumbent system is fixed if not applying the hybrid synchronization scheme). In such a case: a) Interference I4 from UE1 to UE2 (which appears specially when UE1 is transmitting, uplink, and UE2 is receiving a downlink transmission) may be minimized by ensuring that (see figure 6): guard bands are reserved preferably at both edges of the carrier corresponding to UE1 , and UE1 fulfils the out of band and spurious emission limits required to ensure that, considering the reserved guard bands, the resulting in-band interference at UE2 remains below thermal noise. That is, the guard bands introduce an appropriate roll-off for the transmit filter response of UE1 in order to ensure that the unwanted signals received by UE2 are below thermal noise. b) Interference I6 from BS2 to BS1 (which appears specially when BS2 is transmitting, downlink, and BS1 is receiving an uplink transmission) may be minimized by additionally ensuring that: BS1 has specific receive filters centred at the carrier frequency to ensure that the unwanted signal levels at BS1 receiver are below thermal noise (see figure 7), and optionally also by site engineering actions are taken to minimize the RF coupling between BS2 and BS1 , e.g. maximize the frequency separation between BS1 and BS2 signal carriers, maximize the physical distance between BSs, and avoid direct visibility between BSs by optimizing their tilts and azimuth orientations, among others. These actions will help keep the resulting in-band interference at BS1 below thermal noise.

Interference can also appear between base stations of the same system in the proposed synchronization scheme. Figure 8 shows an embodiment where the proposed hybrid synchronization is applied at BS’1 , BS’2, UE’1 and UE’2, all of which belong to the same TDD cellular system. In addition to observing the rules already described (for minimizing the interference between the TDD system applying the proposed synchronization scheme and an incumbent system not applying said proposed synchronization scheme), the following rules may be applied:

1. The cross BS-to-UE interference terms G1 and G2 can be minimized if base stations are assigned different carrier frequencies (as it happens in the previous scenario, where the BS1 and BS2 belong to different TDD systems). Proper out-of-band and spurious emission limits can be defined to ensure that the resulting unwanted signal levels at both BS and UE receivers are below thermal noise.

2. Interferences G5, G6 between different base stations can be minimized by additionally optimizing the corresponding tilts and azimuth orientations to avoid direct visibility between BSs. Such spatial isolation can ensure that the resulting unwanted signal levels at the receiver side are below thermal noise, by exploiting the signal loss occurring outside the direction of maximum radiation in the transmit and receive antenna patterns.

3. Interferences G3, G4 between UEs can also be minimized by additionally ensuring that: guard bands are reserved at both edges of the signal carriers, and UEs fulfil the necessary out-of-band and spurious emission limits to ensure that, considering the reserved guard bands, the unwanted signal levels at the receiver side remain below thermal noise. The proposed embodiments can be implemented by means of software elements, hardware elements, firmware elements, or any suitable combination of them. Note that in this text, the term“comprises” and its derivations (such as“comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc. The matters defined in this detailed description are provided to assist in a comprehensive understanding of the invention. Accordingly, those of ordinary skill in the art will recognize that variation changes and modifications of the embodiments described herein can be made without departing from the scope of the invention. Also, description of well-known functions and elements are omitted for clarity and conciseness. Of course, the embodiments of the invention can be implemented in a variety of architectural platforms, operating and server systems, devices, systems, or applications. Any particular architectural layout or implementation presented herein is provided for purposes of illustration and comprehension only and is not intended to limit aspects of the invention.