Label: MBMS TD System
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Multimedia business is a very promising application that is currently favored by the industry. Multimedia broadcasting mainly refers to a mobile terminal user receiving digital audio and video content (such as a television, etc.) in a broadcast form in the form of a channel or channel on an intelligent mobile terminal having an operating system and a video function. There are two main types of multimedia broadcasting: one is the large-area terrestrial broadcast mobile TV standard for broadcasting and television systems, and the other is the MBMS (Multimedia Broadcasting and Multicast Technology) standard based on the cellular system of mobile communication systems. Here, we mainly discuss how to implement MBMS technology to carry multimedia broadcast services in TD networks.
MBMS standard progress
Multicast and broadcast are the way to transfer packets from one data source to multiple destinations. In the traditional mobile network, the cell broadcast service (CBS) can also perform the broadcast function, but only allows the low bit rate data to be sent to all users through the cell shared broadcast channel, which can basically only be used to send text information, which is difficult to satisfy. Today's people's request. Therefore, in the R6 version, 3GPP implements the broadcast and multicast functions for multimedia services.
3GPP introduces the MBMS function in R6, which is a service for multicast and broadcast. It is implemented based on WCDMA/GSM packet network. By adding some new functional entities, the broadcast multicast service center BM-SC is broadcast and the existing packets are grouped. Domain functional entities, such as SGSN, GGSN, RNC, UE, etc., implement MBMS function support to implement, and define a new logical shared channel on the air interface to implement air interface resource sharing.
The MBMS service defined by 3GPP is relatively complicated, especially for multicast services. In March 2006, Vodafone proposed an enhancement to the broadcast mode to facilitate the use of broadcast mode to carry mobile TV services. The proposal of this program is conducive to accelerating the industrialization of MBMS.
3GPP continues to research enhanced MBMS services in R7. The research focus mainly includes two aspects. The first is MBMSSFN. In R6, MBMS services and other non-MBMS services, such as voice services, data services, etc., operate on the same carrier. In R7, 3GPP proposes to run MBMS services on a single carrier because MBMS is a pure downlink service and does not require uplink feedback. This idea is called the MBMSSFN scheme. Another research focus is on dual receivers, which are for the terminal. For FDD terminals, if the MBMS is running on a separate carrier, the terminal can receive both non-MBMS services and MBMS services simultaneously through only two receivers. For the R6 version of the MBMS design is too complicated, the R7 has also been simplified.
At present, FDD, HCRTDD and TD have basically completed the standard development process of their respective MBMSSFN.
MBMS network architecture
MBMS has two working modes: broadcast and multicast. The broadcast mode means that multimedia data is sent from one service source to all UEs in the broadcast service area in one direction; the multicast mode is very similar to the broadcast mode, but the reception range is limited to UEs that have applied for the multimedia service.
In the MBMS network, the edge of the MBMS bearer service is the Gmb and Gi reference point, that is, the interface between the broadcast multicast service center BM-SC and the GGSN. The Gmb interface provides the control plane function, and the Gi interface provides the user plane bearer function. The only new interface here is the Gmb interface, and the others are interfaces that already exist on the WCDMA/GSM network.
For traditional packet domain services, each user establishes a point-to-point link with the core network. The MBMS service is a point-to-multipoint business. On the Iu link, only one bearer is established and shared by multiple users. Moreover, on the air interface, the public channel, that is, the PTM mode, or the dedicated channel, that is, the PTP mode, can be used to provide services for the user according to the amount of the service user in the cell.
MBMS is an end-to-end service involving CN, RAN and terminals. The CN changes are the same for WCDMA and TD. The changes on the RAN side and the changes above the MAC layer, WCDMA and TD are also the same, the only difference between the two is the physical layer.
UTN technical solution
The spectrum efficiency of the MBMS based on the R6 version of 3GPP is relatively low, only 0.02 bps/Hz to 0.2 bps/Hz, and the implementation complexity of the service flow and the access network is high. So in the R7 version, 3GPP proposed the idea of ​​carrying MBMS services on dedicated carriers, namely the concept of MBMSSFN.
At present, the TD-MBMS technology solution being researched and developed in CCSA, based on R6MBMS, draws on some ideas in R7 and combines the characteristics of TD itself. The main idea is to provide MBMS services in the N-band TD-SCDMA using the same time slot of the same frequency point, which is referred to as UTN (simultaneous slot network). UTN technology requires all neighboring base stations to simultaneously transmit the same wireless signal. In UTN mode, the UE can treat signals from different base stations as multipath signals, thus greatly improving spectrum utilization.
The UMN mode can be used to implement the multimedia broadcast service, that is, the entire network adopts UTN for networking; and the regional multimedia broadcast service can also be implemented, that is, the UTN networking is adopted in the regional area. The frequencies used by a certain UTN network can be reused in different regions (non-adjacent areas) to form another regionalized UTN network, but the two regionalized UTN networks must have a certain degree of physical isolation.
In the existing N-band system of TD, the UTN mode can be time division multiplexed or frequency division multiplexed with the N frequency point mode.
In the UTN networking mode, the same MBMS service is synchronously transmitted on the same carrier and on the same time slot. The time slots and frequency points occupied by the UTN mode can be flexibly allocated. The number of slots occupied by an MBMS service depends on its rate. A UTN time slot of one frequency point can provide a data transmission rate of 128 kbps to 192 kbps, and more frequency points or time slots can be configured according to service requirements.
Since the base station synchronization has been implemented by GPS in the TD, content synchronization in the same RNC is realized, and no additional processing is required for the TD.
SFN technical solution
From the perspective of improving large coverage and spectrum utilization, 3GPP carries out technical research on carrying MBMS on a single carrier, which is referred to as MBMSSFN technology. Since the MBMS SFN is carried on a separate carrier, its frame structure can be redesigned to some extent. From the perspective of improving spectrum utilization, since the MBMS service is a pure downlink service, its frame structure can be designed to be full downlink.
The slot structure of the TD-MBMS SFN adopts a full downlink time slot, and combines three special time slots into one downlink control time slot for transmitting MBMS control signaling. The DwPTS, UpPTS, and 96chip GPs in the original frame structure no longer appear, but are combined into one short time slot, which has a length of 0.275 ms, which is an MBSFN control time slot.
Compared with the frame structure of the traditional unicast service, the frame structure of the TD-MBMS SFN is unchanged for the 5ms subframe and the 10ms multiframe, the slot length is unchanged, the frame structure is aligned, and the backward compatibility is obtained.
Since the MBMS SFN is carried on a single carrier, the terminal should implement dual receiver functions in order not to affect the terminal's reception of signaling and services carried on the original carrier.
At present, although the standard of MBMS R6 has been completed, due to its complexity and low frequency utilization, its market application prospects are not very clear, so various system manufacturers have been unable to launch corresponding commercial products. Due to the characteristics of the whole network synchronization, TD has a natural advantage in implementing MBMS services in its network. At the same time, the clarification of TD-MBMS technology and the formulation of standards are very beneficial to the advancement of the TD industry and enhance the competitiveness of TD technology.
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