Technical Field

The present invention relates to method in a transmitter for transmitting signals in the downlink of a cellular wireless communication system. Furthermore, the invention also relates to a transmitter device, a computer program, and a computer program product thereof.

Background of the Invention

During the initial access of a mobile terminal (e.g., UE in 3GPP terminology), a connection is established to the cellular network by detecting and synchronizing to a cell. In the prior art 3 GPP LTE system, the UE establishes a connection to a cell by performing cell search and synchronization using a two-step procedure by first detecting the primary synchronization signal (PSS) and thereafter, detecting a secondary synchronization signal (SSS), which are transmitted by the eNodeB.

A candidate cell identity (ID) may be obtained after the PSS and SSS have been detected. There are 504 cell IDs and when a candidate cell ID has been obtained, the UE may perform a third step to verify the cell ID by detecting the cell-specific reference signal (CRS).

The CRS is transmitted in every subframe and resource block. Up to 4 antenna ports can be accommodated (labeled p=0-3) with the CRS and a cell could be configured with antenna ports p=0 or p=0, 1 or p=0, 1, 2, 3. The CRS has a frequency shift (which determines the set of time- frequency resources it is using) and a modulation sequence which are determined from the cell ID. The number of antenna ports with CRS and the bandwidth of the cell are not known to the UE in the cell search process and it may therefore for the cell ID verification, only try to detect the CRS in the 6 central resource blocks of the carrier (i.e., the minimum bandwidth for the system) and assume antenna port 0. The CRS is used for several things, e.g., it is a reference signal for data demodulation such that the receiver can use channel estimates from the CRS when demodulating data and control channels. It is also used for RRM measurements such that a receiver can measure received signal power on the CRS from other cells in order to facilitate mobility procedures. The CRS can also be used for cell identification such that the UE can verify a cell ID being obtained from the cell search. Moreover, after a successful cell search, the UE may start performing fine tuning of the synchronization to the cell, the so called time- and frequency tracking. This is needed for reducing the time- and frequency errors that otherwise may be detrimental for data demodulation. The CRS is typically used as a reference signal for the time- and frequency tracking. Once reliable synchronization been established, the broadcast channel of the cell can be decoded and the UE may start to camp on the cell.

In prior art, it was suggested to transmit a signal using the same resource elements and modulation sequence as the CRS on antenna port 0, but it would only be transmitted in a subset of the subframes. Such a reduced signal density may result in worse cell search performance since the detection probability of the cell ID verification decreases due to less signal energy being present. This may be an issue as it will take longer time to detect cells, in particular for deployment cases where the UE is supposed to detect several cells, e.g., in dense small cell deployments. Similarly, a reduced signal density will degrade the time- frequency synchronization performance.

If a wrong cell ID is detected, broadcast channel, data channels and control channels may not be received properly. Moreover, the UE cannot perform any reliable RRM measurements or maintain the time- and frequency synchronization. It is a problem to assure reliable detection of the cell ID especially with a low density of the associated cell identification signal.

It is a further problem to assure reliable estimation of time- and frequency errors especially with a low density of the associated reference signal.

Summary of the Invention

An object of the present invention is to provide a solution which mitigates or solves the drawbacks and problems of prior art solutions. Another object of the present invention is to provide a solution which can improve time- and frequency synchronization in cellular systems. Yet another object of the present invention is to provide an improved solution for cell ID verification.

According to a first aspect of the invention, the above mentioned objects are achieved by a method in a transmitter for transmitting signals in the downlink of a cellular wireless communication system, said transmitter being arranged to transmit at least one synchronization signal used for cell search on at least one antenna port, and said transmitter being further arranged to transmit at least one another additional signal; the method being further comprising the step of:

- transmitting said at least one additional signal on said at least one antenna port.

Different preferred embodiments of the present invention are defined in the appended dependent claims. The present method may also be performed in a transmit device arranged for executing the method which may be comprised in a computer program.

According to a second aspect of the invention, the above mentioned objects are achieved with a transmitter device arranged for transmitting signals in the downlink of a cellular wireless communication system, said transmitter device being arranged to transmit at least one synchronization signal used for cell search on at least one antenna port, and said transmitter device being further arranged to transmit at least one another additional signal; the transmitter device comprising:

- a transmit unit being arranged for transmitting said at least one additional signal on said at least one antenna port. The present invention provides a solution for generally improving the performance of a cellular wireless communication system. Especially, the probability for detecting the correct cell ID can be improved. Furthermore, time- and frequency synchronization can be improved with the use of the present invention. Further applications and advantages of the invention will be apparent from the following detailed description. Brief Description of the Drawings

The appended drawings are intended to clarify and explain different embodiments of the present invention in which:

Fig. 1 shows the detection error probability for an embodiment of the invention in which cell ID verification using CRS in LTE for a ETU channel at 3 km/h, in 6 resource blocks and 1 subframe;

Fig. 2 shows an example of a resource block pair comprising a cell identification signal on a set of resource elements;

Fig. 3 shows an example of a resource block pair comprising a frequency shifted cell identification signal on a set of resource elements;

Fig. 4 shows an example of a resource block pair comprising a cell identification signal on a set of resource elements;

Fig. 5 shows an example of a radio frame with 10 subframes which are divided into 2 subframe sets;

Fig. 6 illustrates downlink transmission of an LTE system;

Fig. 7 shows a flowchart of a method in a transmitter according to an embodiment of the present invention; and

Fig. 8 shows a flowchart of a method in a receiver according to an embodiment of the present invention.

Detailed Description of the Invention

In a 3GPP E-UTRA or LTE system, the smallest time- frequency entity that can be used for transmission is referred to as a resource element (RE), which can convey a complex-valued modulation symbol on a subcarrier. A resource block (RB) comprises a set of REs and is of 0.5 ms duration (e.g., 7 OFDM symbols) and 180 kHz bandwidth (e.g., 12 subcarriers with 15 kHz spacing). A radio frame consists of 10 subframes, or alternatively 20 slots of 0.5 ms length (enumerated from 0 to 19). The transmission bandwidth of the carrier is divided into a set of Physical RBs (PRBs). Typical LTE carrier bandwidths correspond to 6, 15, 25, 50, 75 and 100 PRBs.

In order to enable coherent demodulation of data, the transmitter has to send a pre-defined reference signal (aka pilot signal) to the receiver (UE). The reference signal may not encode any information and it is typically known to the UE. From the reference signal (using a priori information on its modulation symbols and time- frequency location), the receiver can, based on the received reference signal, obtain channel estimates (e.g., the phase and amplitude of the channel frequency response), which are used for channel equalization prior to the demodulation.

In the prior art 3 GPP LTE system, multiple transmit and receive antennas are supported and the notion of Antenna Port (AP) is used. Each downlink antenna port is associated with a unique reference signal. An antenna port may not necessarily correspond to a physical antenna and one antenna port could be mapped to more than one physical antenna. In any case, the reference signal can be used for channel estimation for data that is transmitted on the same antenna port. Channel estimation therefore needs to be performed for all antenna ports that are used for the data transmission.

An antenna port could alternatively be explained by the use of a precoding operation, wherein a modulation symbol is precoded for antenna port i with w- ^k) = - ^k)■ e ^J<f ' ' , i.e., an amplitude scaling and a phase shift. The precoding weights could be either zero or a complex-valued number subject to the transmit power constraints in the transmitter. For example, if for a given antenna port i , all values of are zero except for k = k ₀ , then there is a direct mapping between antenna port i and the physical antenna k = k ₀ . For a system comprising k = 0, ... ,M - 1 physical antennas the precoding operation could be defined by a set of precoding vectors W = mitted on antenna port i would thu · s .

The precoding vector for associating antenna port to physical antennas is typically not defined in the standard specification and is left as a choice for implementation. For the CRS, the precoding vector would be cell-specific and time- and frequency invariant. This allows the UE to interpolate channel estimates over time and frequency. In the LTE system, the transmit power of a CRS modulation symbol may be different than the transmit power of a data modulation symbol. This could alternatively be interpreted as that the amplitude scaling factors are different for the two types of modulation symbols. Thus, the CRS modulation symbol would at least experience the same phase shift as any data modulation symbol transmitted on the same antenna port. However, there also exists higher-layer signaling from which the receiver will be able to deduce the transmit power of the data modulation symbols in relation to that of the CRS modulation symbols. With this information, the receiver would be able to utilize channel estimates obtained from the CRS to deduce the channel for the data modulation symbols.

The PSS is transmitted in two different subframes in a radio frame. The specification mandates that the UE cannot assume that the antenna port for the PSS is the same as for any reference signal. There is no specified relation between the antenna ports with CRS and the antenna ports with PSS and SSS. This implies that there is no reference signal which can be used for estimating the channel for the PSS, which therefore has to be non-coherently detected. Moreover, the UE cannot assume that the same antenna port is assumed for different instances of the PSS transmission. This allows the eNodeB to perform standard transparent precoding to achieve transmit diversity, i.e., it may use different precoding vectors (i.e., creating different antenna beams) at different PSS transmission instances. Thus the PSS could experience a new channel at every transmission instance. The SSS is also transmitted in two different subframes in a radio frame. The SSS uses the same antenna port as the PSS in the given subframe, i.e., is using the same precoding vector and antenna mapping. This implies that, once the PSS is detected, the PSS can be used as a reference signal for estimating the channel for the SSS, allowing for coherent detection of the SSS.

To achieve the aforementioned and other objects, the present invention relates to a method in a transmitter for transmitting signals in the downlink of a cellular wireless communication system and to a transmitter device thereof. The present transmitter is arranged to transmit at least one synchronization signal used for cell search, such as e.g., PSS and SSS in LTE systems. The synchronization signal is transmitted on at least one antenna port of the transmitter as described above. Moreover, the present transmitter is further arranged to transmit at least one another additional signal, wherein the additional signal is transmitted on said at least one antenna port.

In one embodiment of the invention, the notion of using the same antenna port implies that the same phase shift and/or the same amplitude is used for precoding a modulation symbol for the given signal and the modulation symbols of the synchronization signals. An advantage of using the same phase shift and the same amplitude for the precoding, is that the channel estimates from the synchronization signals can be used to detect any modulation format assumed for the additional signal, e.g., different kind of multi- level modulation formats such as 16-QAM (Quadrature Amplitude Modulation). An advantage of using the same phase shift but different amplitude for the precoding, is that it allows a larger freedom for the transmitter to execute transmit power control as it may choose to allocate different power to synchronization signals and to the additional signal. Furthermore, even if the amplitude is different, detection of the additional signal can still be improved for modulation formats which use constant amplitude, such as Phase Shift Keying (PSK) schemes (e.g., BPSK, QPSK, 8-PSK, etc.).

According to another embodiment of the invention the additional signal uses the same modulation sequence as a CRS of the cellular system. The additional signal may also use at least one time- frequency resource as a CRS of the cellular system. By keeping such resemblance to the CRS in the LTE system, many functions in the receiver could be reused, thereby decreasing its cost and complexity. In the LTE system, the PSS and SSS do not utilize the same antenna port as the CRS and there is no other signal that uses the same antenna port as the PSS/SSS (the PSS and SSS on the other hand uses the same antenna port). Thus, in the third step of the cell search procedure, there are no available channel estimates that could be utilized when detecting the CRS for the cell ID verification. Only non-coherent CRS detection is therefore possible. In this invention, it is realized that, e.g., the cell ID verification could be improved by allowing cell ID verification from a signal which is using the same antenna port as the PSS/SSS. Thereby coherent detection could be applied in the cell ID verification resulting in performance improvement.

Fig. 1 shows the detection performance of antenna port p=0 in the 6 central RBs in one subframe in the LTE system using either coherent detection (enabled by this invention) or non-coherent detection (required in prior art solution). It is observed that significant gains are obtainable with the present invention. The coherent detection benefits from that channel estimates can be obtained from using both the PSS and the SSS as a reference signal for obtaining channel estimates. This assures robust channel estimation, since the PSS and SSS occupy 62 resource elements each in the 6 central PRBs of the LTE carrier. This embodiment hence discloses a method for transmitting a cell identification signal on the same antenna port as a primary- and a secondary synchronization signal. The additional signal is characterized by that one or several of its properties (e.g., modulation sequence, time- frequency resources) are depending on the cell ID. In particular, if the additional signal uses a same modulation sequence and/or using a similar time-frequency pattern for its resource elements as a CRS signal, it would be possible to utilize the additional signal for similar purposes as the CRS, albeit with a lower signal density, while leveraging on existing receiver implementations. Just like the CRS in the prior art system, the signal may serve one or several purposes, such as being a reference signal for cell ID verification, being a reference signal for time- and frequency synchronization, being a reference signal for radio resource management (RRM) measurements (e.g., RSRP and RSRQ) and being a reference signal for radio link monitoring (RLM) measurements. A further advantage of this embodiment is that standard transparent transmit diversity schemes (i.e., the precoding vector is not known to the receiver) can be applied to the cell identification signal, which is not possible in the prior art LTE system, since the CRS has a time-invariant precoder. This could be achieved by using the same precoding vector on the cell identification signal as for the synchronization signals, which may be time-variant. This further balances the detection performance among the synchronization signals and the cell identification signal. That is, in a subframe where the precoding vector resulted in large received power of the synchronization signals for a UE, the cell identification signal would also be received with large received power. Thus it could be avoided that the synchronization signals will be received with high signal quality while the cell identification signal will be received with low signal quality and vice versa. This would be beneficial for reliable detection of the cell ID as well as robust time- frequency synchronization.

Another advantage is that if the received signal power is similar for the synchronization signals and the additional signal, it would be possible to begin time- and frequency synchronization immediately once the synchronization signals have been detected. Furthermore, by utilizing the synchronization signals as reference signals for obtaining channel estimates, it would be possible to improve the time- and frequency synchronization. According to another embodiment of the invention, at least one property or parameter of the additional signal is dependent on a cell identity. The property may relate to a frequency shift of the additional signal or a modulation sequence of the additional signal. This would make it possible to verify a cell ID from the additional signal.

Fig. 7 shows a flowchart of a method in a transmitter (steps T1-T2) and Fig. 8 shows a flowchart of a method in a receiver (steps R1-R3) according to an embodiment of the present invention. The cellular system is in this example a 3 GPP E-UTRA or LTE system which implies that the transmitter is an eNodeB (or a relay node) and the receiver is a UE but it is realized that other corresponding network nodes can act as transmitter/receiver. The system is illustrated in Fig. 6 in which the eNodeB transmit downlink signals to the UE in the cellular system.

Tl

In step Tl the eNode B processes and transmits the PSS and SSS on an antenna port AP in the downlink of the system.

T2

In step T2 the eNodeB further processes and transmits the additional signal on the same antenna port AP as for the PSS and SSS. In this example, the additional signal uses the same modulation sequence and similar time- frequency pattern for its resource elements as a CRS signal so as to be a reference signal for cell ID verification.

Rl

In step Rl the UE receives and detects the PSS and thereafter the SSS in the downlink from the eNodeB. The UE further obtains a candidate cell ID in step Rl .

R2

In step R2 the UE receives the additional signal having the properties as described above in step T2. R3

In step R3 the UE uses the additional signal for cell ID verification of the candidate cell ID. Thereby, the performance can be substantially improved, see Fig. 1. It should further be realized that the frequency shift of the additional signal may be fixed and independent of a cell identity, or configurable by the cellular system according to other embodiments of the invention. This would still make it possible to verify a cell ID from the additional signal, as long as some other signal property depends on the cell ID. The ability to control the frequency shift and let it be independent of the cell ID is particularly relevant for systems where joint transmissions can be performed from multiple cells. Thereby inter-cell interference due to reference signals could be reduced by controlling the frequency shifts.

It should be noted that the additional signal may use the same modulation sequence as at least one cell-specific reference signal of the cellular system, e.g., as the CRS transmitted on antenna port 0 in LTE systems. An advantage is that some of the existing procedures in the receiver that are based on the CRS can be reused.

Moreover, the additional signal may use the same time- frequency resources as at least one cell-specific reference signal of the cellular system, e.g. as the CRS transmitted on antenna port 0 in LTE systems. An advantage with this is that some of the existing procedures in the receiver that are based on the CRS can be reused.

Fig. 2 shows an example of 2 RBs (12 subcarriers and 14 OFDM symbols) and the time- frequency resources used by the cell identification signal. Fig. 3 shows a frequency shifted version of the cell identification signal.

In a further example of the present invention, the additional signal uses at least one time- frequency radio resource of a first cell-specific reference signal in a set of resource blocks, and may according to another example further use at least one time- frequency radio resource of another second cell-specific reference signal within a subset of the mentioned set of resource blocks. The time- frequency radio resource of the first cell-specific reference signal may relate to a first bandwidth of the system and the time- frequency radio resource of the second cell-specific reference signal may relate to a second bandwidth of the system, the first and second bandwidths being different (e.g., the first bandwidth can be the whole system bandwidth and the second bandwidth be only a part of the system bandwidth). For example, the additional signal could comprise time- frequency resources according to the CRS of antenna port 0 in all resource blocks, while further comprising time-frequency resource blocks according to the CRS of one or several of antenna port p=\, p=2 or p=3 on a subset of the resource blocks (e.g., the 6 central resource blocks of the carrier). Fig. 4 shows 2 RBs where time- frequency resources from the cell identification signal on antenna port p=0 and p=\ are utilized. Such RBs may be used in the central part of the carrier while RBs depicted in Fig. 2 may be used otherwise. For example, in the initial access process, the UE may only utilize signals which are present in the 6 central RBs of the carrier. Thus, in order to assure sufficient performance while still not transmitting the additional signal in all RBs, it would be beneficial to increase the signal density in the 6 central RBs in those subframes comprising the additional signal. Furthermore, the present invention is also applicable to the case where the synchronization signals (e.g., PSS and SSS) are transmitted multiple times in a radio frame. The invention includes that the transmission of the additional signal is made in at least one subframe in a radio frame and that for each such subframe, there is a predefined association to which primary- and secondary synchronization signals for which the signal can be assumed to use the same antenna port. The number of subframes containing the synchronization signals may be larger than the number of subframes containing the cell identification signal.

Fig. 5 shows an example of a radio frame of 10 subframes where primary- and secondary synchronization signals are transmitted in subframes 0 and 5. In the example, subframe 0, 1, 2, 3 and 4 comprise a first set and subframe 5, 6, 7, 8 and 9 comprise a second set. A predefined rule may be to associate the signal with the antenna port of PSS/SSS in subframe 0, if the signal is transmitted on a subframe within the first subset, and vice versa for the second subset according to a preferred embodiment of the invention. In one example of the invention, the cell identification signal is transmitted in subframes which always contain at least one of the primary- or secondary synchronization signals, e.g., subframe 0 and 5. This is advantageous in a time -varying channel, as the channel estimates from the synchronization signals become more reliable if the additional signal is transmitted in the vicinity of the synchronization signals.

Furthermore, as understood by the person skilled in the art, any method according to the present invention may also be implemented in a computer program, having code means, which when run by processing means causes the processing means to execute the steps of the method. The computer program is included in a computer readable medium of a computer program product. The computer readable medium may comprises of essentially any memory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive.

Moreover, the present invention also relates to a transmitter device comprising the necessary functions in the form of means, units, elements, etc. to execute any method according to the present invention. Mentioned means, units, elements, may, e.g., be memory, processing circuitry, coupling means, antenna means, precoding unit, amplifier unit, etc. The present transmitter may according to an embodiment be an eNodeB (i.e., a base station) or a relay node in a LTE system as shown in Fig. 6. The transmitter device is arranged for transmitting signals in the downlink and is further arranged to transmit at least one synchronization signal used for cell search on at least one antenna port which implies that the device comprises a signal processing unit coupled to a memory, and also having input and output means. The transmitter processes different communication signals, such as the additional signal and other signals, and transmits the signals on antenna ports over physical antennas using a transmit unit which e.g., comprises one or more of processing units, precoder, MIMO antennas, amplifier(s), etc. The transmitter device may also comprise a receiver unit for receiving uplink signals from UEs.

The mentioned cellular wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a transmitter, also referred to as a radio network node or base station, e.g., a Radio Base Station (RBS), "eNB", "eNodeB", "NodeB" or "B node", depending on the technology and terminology used. Sometimes, also the expression cell may be used for denoting the transmitter/radio network node itself. However, the cell is also, or in normal terminology, the geographical area where radio coverage is provided by the transmitter/radio network node at a base station site. One transmitter, situated on the base station site, may serve one or several cells. The transmitters communicate over the air interface operating on radio frequencies with the receivers within range of the respective transmitter.

A receiver, also known as UE in LTE systems, mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a cellular wireless communication system. The receiver may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The receivers in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle- mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity. In some radio access networks, several transmitters may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural transmitters connected thereto. In 3rd Generation Partnership Project (3 GPP) Long Term Evolution (LTE), transmitters, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks.

Furthermore, the processing circuitry may comprise, e.g., one or more instances of a Central Processing Unit (CPU), a processing unit, a processing circuit, a processor, an Application Specific Integrated Circuit (ASIC), a microprocessor, or other processing logic that may interpret and execute instructions. The expression "processing circuitry" may thus represent a processing circuitry comprising a plurality of processing circuits, such as, e.g., any, some or all of the ones mentioned above. The processing circuitry may further perform data processing functions for inputting, outputting, and processing of data comprising data buffering and device control functions, such as call processing control, user interface control, or the like. Finally, it should be understood that the present invention is not limited to the embodiments described above, but also relates to and incorporates all embodiments within the scope of the appended independent claims.