A METHOD OF TRANSMITTING MBMS DATA

TECHNICAL FIELD The present invention relates to a method and a device for transmitting Multimedia

Broadcast Multicast Service (MBMS) data in a TDD (Time Division Duplex) system such as TD-SCDMA (Time Division-Synchronous Code Division Multiple Access) system and its evolved systems.

BACKGROUND

Multimedia Broadcast Multicast Service (MBMS) is a broadcasting service that can be offered via existing GSM and UMTS and other cellular networks. The infrastructure offers an option to use an uplink channel for interaction between the service and the user, which is not a straightforward issue in usual broadcast networks, as for example conventional digital television is only a one-way (unidirectional) system. MBMS uses multicast distribution in the core network instead of point-to-point links for each end device.

The MBMS feature is split into the MBMS Bearer Service and the MBMS User Service. The MBMS Bearer Service includes a Multicast- and a Broadcast Mode. The MBMS Bearer Service uses IP Multicast addresses for the IP flows. The advantage of the MBMS Bearer Service compared to legacy UMTS bearer services (interactive, streaming, etc) is, that the transmission resources in the core- and radio network are shared. One MBMS packet flow is replicated by GGSN, SGSN and RNCs. MBMS may use an advanced counting scheme to decide, whether or not zero, one or more dedicated (i.e. unicast) radio channels lead to a more efficient system usage than one common (i.e. broadcast) radio channel. UTRAN MBMS offers up to 256kbit/s per MBMS Bearer Service and between 800kbit/s and 1.7Mbit/s per cell/band. The actual cell capacity depends on the UE capabilities. GERAN MBMS offers between 32kbit/s and 128kbit/s. Up to 4 GSM Timeslot may be used for one MBMS bearer in downlink direction. The actual data rate per Traffic Slot depends on

network dimensioning. Furthermore, the type of the channel used by MBMS can be P-t-P (Po\nt-to-Point) channel or P-t-M (Point-to Multi point) channel according to the number of service users.

MBMS has been standardized in various groups of 3GPP (Third Generation Partnership Project), and the first phase standards are found in UMTS release 6. It is developed based on WCDMA (Wide-band CDMA) and WCDMA HSDPA (High Speed Downlink Packet Access) step by step and some devices have been released.

For WCDMA and its evolutions, due to the radio resource limit, MBMS is focused on the implement in WCDMA HSDPA, HSP A+ and LTE (Long Term Evolution). For example, WCDMA HSDPA provides a streaming type and high power bearer in order to guarantee QoS (Quality of Service) and ensure the broadcasted data can be transmitted to the cell boarder. Typically, a fixed MCS (Modulation Coding Scheme) is selected to match different data rate for different applications.

However, for TD-SCDMA (Time Division-Synchronous Code Division Multiple Access), which is another standard in the IMT-2000 family, the traffic channel is different than for example WCDMA. To give some examples, traffic data in TD-SCDMA is transmitted by a beam-formed but omni-directional pattern. Also data traffic is transmitted in time slots. Therefore, MBMS when applied in TD-SCDMA will face a number of problems such as time slot change for MBMS, power allocation and optimization, channelization code number as well as some other TD-SCDMA specific problems related to the N-carrier technique employed by TD-SCDMA. The solution should be based on the principle of minimizing system interference and optimizing the radio resource assignment.

As stated above, to implement MBMS in air-interface for a certain standard, the radio resource utilization should be taken into account from the beginning. Compared WCDMA, TD-SCDMA adopts a TDD + TDMA mode, and also FDMA, CDMA with short channelization code. All of these will bring flexibility but complexity for radio resource utilization. For example, it is a challenge to deal with the relationship of broadcasted MBMS implement and beam-formed dedicate channels in TD-SCDMA.

Hence, there exist a need for a method and a system that implements MBMS in TD- SCDMA and which at the same time is efficient in terms of radio resource utilization and which can be used in other future radio systems.

SUMMARY

It is an object of the present invention to overcome or at least reduce some of the problems associated with the Introduction of MBMS technology into a TDD system such as TD- SCDMA and its evolved systems.

It is another object of the present invention to provide a method and a device that is capable of providing MBMS technology in TD-SCDMA and its evolved systems, for example the TD-SCDMA systems take into account N-carrier and TD-HSDPA as well as LTE TDD.

These objects and others are obtained by the method and system as set out in the appended claims. Thus, by implementing MBMS in the existing TD-SCDMA framework and its evolution systems without modifying the existing basic physical frame structure, an efficient TD-SCDMA system with MBMS capabilities is obtained. Thus, the MBMS system is enabled to assign dedicate frequency band(s) or dedicate time slot(s) or other Radio Resource Units to an MBMS service.

The solution as described herein is compatible with current common TD-SCDMA solution like N-carrier technique. The solution is based on the principle of minimizing system interference and optimizing the radio resource assignment in order to get optimized coverage and capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which:

- Fig. I is a general view of a TD-SCDMA system,

- Fig. 2 is a view of a protocol for transmitting MBMS data in accordance with a first embodiment,

. - Fig. 3 is a view of a protocol for transmitting MBMS data in accordance with a second embodiment, - Fig. 4 is a view of a protocol for transmitting MBMS data in accordance with a third embodiment,

- Fig. 5 is a view of a protocol for transmitting MBMS data in accordance with a fourth embodiment,

- Fig. 6 is a view of a protocol for transmitting MBMS data in accordance with a fifth embodiment and

- Fig. 7 is a flow chart illustrating some procedural steps performed when setting up a requested service in a TD-SCDMA system.

DETAILED DESCRIPTION In Fig.l a general view of a TD-SCDMA system 100 is shown. The system 100 comprises a base station (Node B) 101. The base station 101 serves a number of mobile terminals, sometimes termed User Equipment (UE) 103, located within the area covered by the base station 101. The base station 101 is also connected to a controller unit, such as a radio network controller node (RNC), 105. The RNC is further adapted to receive MBMS data through the core network as schematically shown by the block 107.

The RNC can adopt a number of different schemes for scheduling MBMS data transmission. In the following a number of embodiments will be described in more detail. In the transmission schemes TS denotes Time Slot and an upward pointing arrow denotes uplink traffic whereas a downward pointing arrow denotes downlink traffic for a particular time slot.

In Fig. 2 a protocol for transmitting MBMS data in a TD-SCDMA system in accordance with a first embodiment is shown, In accordance with this first embodiment the MBMS traffic, both the MBMS Indicator Channel (MICH) and the MBMS Transport Channel (MTCH) is allocated to the TSO (Time Slot 0) and broadcast it like common control channels PCCPCH (Primary Common Control Physical CHannel), SCCPCH (Secondary

Common Control Physical CHannel) in TD-SCDMA. In N-carrier TD-SCDMA, secondary carriers are used for the transmission of MICH data and MTCH data.

TD-SCDMA classifies TSO as a broadcasted slot and TSl to TS7 as beam-formed slots. That is to say, in the TSO all of physical channels in the same or other carriers could use the same weight set to generate a broadcasted signal transmission pattern. In the remaining time slots, the traffic slots, all channels are delivered by a beam-formed pattern to reduce MAI (Multiple Access Interference). For N-carrier TD-SCDMA and its evolution systems, only the TSO in primary carrier is busy and other TSO in secondary carriers are spare to implement frequency reuse among adjacent cells to reduce inter-frequency interference. So, this MBMS scheme can use the free TSO in secondary carriers to heighten radio resource utilization ratio but at the cost of increasing inter-frequency interference. Due to the isolation effect of frequency guard gap, the increased inter-frequency interference is not significant.

In Fig 3, a transmission scheme in accordance with another embodiment is shown. Thus, in the case of unsymmetrical slot number configuration for uplink and downlink, like 1 :5 or 2:4 for TD-HSDPA, a specific dedicate downlink time slot to for the MBMS bearer is allocated. Hereby the MBMS service physical layer bearer is allocated a dedicate channel, so in this scheme one or more dedicate slot(s) is assigned as MBMS bearer. In accordance with the embodiment depicted in Fig. 3 the MICH data is transmitted in time slot 0 of the primary carrier of an N-carrier TD-SCDMA, whereas MTCH data is transmitted in dedicated channels such as Time Slot 6 of a first secondary carrier and Time slot S and 6 of a second secondary carrier. This scheme can will facilitate handling of transmission power and coverage balance for common control channels, beam-formed dedicate channels and broadcasted dedicate channels. In a preferred embodiment the broadcast dedicate channels are transmitted using the same transmission power setting and beam-forming weights as for the common control channels to get the same coverage. Compared with the scheme described above in conjunction with Fig. 2, this solution has no impact to TSO so that common control information transmission and reception is guaranteed.

In Fig 4, a transmission scheme in accordance with yet another embodiment is shown. Thus in accordance with the embodiment shown in Fig. 4 the MBMS data traffic, i.e. the MTCH, and other services are transmitted in the same traffic slot at different channels. This means that beam-formed and broadcasted information is delivered at the same time, but different weights setting with regard to transmission pattern and transmission power may be employed as is described below.

This solution can be regarded as an advanced algorithm of the scheme described above in conjunction with Fig. 3. In the scheme depicted in Fig. 4, MBMS channels and other dedicate channels can be put into the same time slot in order to avoid the resource waste in the scheme shown in Fig. 3.

The transmission power and channelization code allocation priority for MBMS channels and other dedicate channels should be set and balanced. Since the MBMS bearer is broadcasted, the beam-formed other bearers will endure a higher interference than other traffic slots without MBMS bearer. In other words it is preferred that the slots with MBMS channels are handled separately from the slots without MBMS channels for radio resource management. This will add some complexity to the Radio Resource Management algorithms.

In addition to the transmission schemes described above the, MBMS bearer can also be implemented by using a dedicated carrier or a dedicated time slot in TD-SCDMA. The advantage of such an approach is that it is easy to handle and can get a higher data rate, but it may result in resource waste in some cases. In Fig. 5 a transmission scheme where a dedicated time slot is used for the MBMS transport Channel(s) (MTCH) is depicted and in Fig. 6 a transmission scheme where a dedicated secondary carrier is used for the MBMS Transport Channels (MTCH) is depicted.

In Fig. 7 a flow chart illustrating some procedural steps performed when setting up a requested service in a TD-SCDMA system is shown. First in a step 701 the system receives a request for a particular service from a UE and determines what type of service that is requested. If an MBMS service is requested the procedure proceeds to a step 703. Otherwise the procedure proceeds to a step 707. In step 703 the procedure determines if the requested

service will result in a P-t-P or P-t-M transmission of MBMS data. If the result of the determination in step 703 is that there will be a P-t-M transmission of MBMS data the procedure proceeds to a step 705. Otherwise the procedure proceeds to a step 707.

In step 705 the procedure allocates a MBMS specific time slot and carrier. In particular one of the transmission schemes and selections of time slot and carrier as described above in conjunction with Figs. 2 - 6 can be employed. Moreover in a particular region the same time slot(s) and carriers) may be used to simplify the receiver in terms of reception requirements and combination complexity of the receiver. Next in the step 707, radio resource unit allocation to bearer data is performed. For TD-SCDMA, all of carrier, timeslot, code is set, so that TD-SCDMA can adjust the assigned radio Resource Unit (RU) to a particular UE based on the changed radio link quality measurement, i.e. DCA (Dynamic Channel Allocation). The procedure then proceeds to a step 709. In step 709 transmission power is allocated to provide acceptable signal strength for all users in the system. In a typical set-up an, N-carrier TD-SCDMA uses 3 carriers and 6 timeslot to bearer data in the uplink and downlink. The total transmitted power from the base station is split between the data on these 3 carriers in one timeslot. For example, if the total power is 42dBm, the power for each carrier is about 37dBm = 42 - 10 * log (3) when the load in each carrier is the same.

Because legacy services are transmitted in a beam-formed manner but the MBMS service will have to be broadcasted in a uniform, Omni directional, pattern, the MBMS services will require more power to get the same coverage as a corresponding beamformed service. This difference in power requirement between regular data and MBMS data will impact the power split balance in TD-SCDMA with MBMS.

Finally in a step 71 1 load control is performed to ensure that the system does not exceed any threshold levels set to guarantee system performance and MBMS data is transmitted using the selected radio resource units.

The system as described herein is hence adapted to assign a dedicated frequency band or a dedicated time slot to MBMS service. Once having selected a suitable Radio Resource Units

such as frequency, time slots etc to transmit the MBMS data, the MBMS data is transmitted is transmitted in accordance therewith.

Using the method and the system as described herein will Implement MBMS in TD- SCDMA and its evolution systems without modifying the basic physical frame structure. The solution is compatible with current common TD-SCDMA solution as possible like N- carrier technique. The solution is based on the principle of minimizing system interference and optimizing the radio resource assignment in order to get optimized coverage and capacity. This is obtained by allocating a suitable carrier and timeslot for MBMS bearer, and to do power split for the slots with broadcasted MBMS and beamformed other bearers.