CN100456871C - Mobile communication system based on OFDM and channel switching distribution method - Google Patents

Mobile communication system based on OFDM and channel switching distribution method Download PDF

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CN100456871C
CN100456871C CNB2007100646540A CN200710064654A CN100456871C CN 100456871 C CN100456871 C CN 100456871C CN B2007100646540 A CNB2007100646540 A CN B2007100646540A CN 200710064654 A CN200710064654 A CN 200710064654A CN 100456871 C CN100456871 C CN 100456871C
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bts
time slot
data
send
sub
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CN101022643A (en
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陈霞
姚冬萍
熊磊
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Beijing Jiaotong University
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Beijing Jiaotong University
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Abstract

A mobile communication system based on OFDM is prepared as forming service region by multiple sub-network and forming each sub-network by a numbers of cells, switching-in mobile table with fixed network through radio link with BTS and BSC as well as MSC / GW, using the same downlink carrier frequency and the same uplink carrier frequency synchronous operation by all BTS in sub-network. The method for switching over channel distribution is also disclosed.

Description

A kind of mobile communication system and channel switching distribution method based on OFDM
Technical field
The present invention relates to chain digital synchronous network, channel setting and utilize this system to improve a kind of mobile communication system and the channel switching distribution method of message transmission reliability in the handover procedure, belong to railway mobile communication technology field based on OFDM based on the OFDM modulation technique.
Background technology
With respect to other land traffic specialized mobile radios of China, the development of railway mobile communication is in scale, technical all comparatively leading.At present China is building railway combined digital mobile communication GSM-R system.To Train Control (CBTC) based on communication, utilize the GSM-R system that wireless two-way communication between car-ground is provided, driverless train control (ATC) be can realize, and then existing signal and train control system replaced.
As a kind of frequency division multiplexing/time division multiple access (FDM/TDMA) system, the handover in the GSM-R network is the direct-cut operation (hard handover) that needs to change the transmission carrier frequency, thereby has handoff inevitably.During the handoff of about 400ms, channel is actually temporary close, and the most important reason of data error normally appears in the transmission course in the burst error that this mechanism causes, and is the hidden danger of traffic safety to a certain extent.At this problem, particularly in order to adapt to the demand of high-speed railway, many work has all been done by industrial quarters and academia.Aspect network design, adopt the multilayer sub-district or require wireless coverage to satisfy the strict index of railway special communication; Aspect handoff algorithms, adopt special hand-off process algorithm to guarantee handover success rate.These measures can improve the performance of handoffs of GSM-R network to a certain extent, but can not change the essence of its direct-cut operation.
Summary of the invention
The objective of the invention is weak point at the prior art existence, propose to adopt OFDM (OFDM) technology, make up the mobile communication system of a kind of suitable railway (reaching highway, water transport etc.) chain wireless coverage, utilize the characteristics of the grand diversity of ofdm signal to improve transmission performance, and a kind of mobile communication system and the channel switching distribution method based on OFDM that solves the handover interruption is provided.
The objective of the invention is to be achieved through the following technical solutions.
The railway wireless network is that the chain of downline covers, and certain overlay region is arranged between the sub-district.Whole coverage can be a big network or can be divided into a plurality of subnets of several zone establishments by the geographical position that each subnet comprises several sub-districts.On vehicle-mounted travelling carriage and the train other travelling carriages by and base station (BTS) between Radio Link, and then by base station control device (BSC) and mobile switching centre/gateway (MSC/GW) access railway fixed network or other system.
In a subnet, all BTS adopt identical descending transmission carrier frequency and identical up reception carrier frequency synchronization operation.In time, the division of signal that is sent is a frame, and every frame is divided into frame head, some time slots (TS) and postamble again, respectively comprises several OFDM symbols; On frequency, system bandwidth is divided into some (being actually overlapped) subcarrier.
For above-mentioned signal structure, in theory can by " subcarrier-time slot " carry out the time-the frequency two dimensional channel distributes.But for real system, can adopt multi-form in order to reduce complexity and/or raising transmission rate, for example, distribute (using all time slots in the frame) by subcarrier (or comprising several sub carrier group continuous or discontinuous subcarrier), perhaps distribute (using all subcarriers) by time slot.
In the zone, the public information of down link (comprising regional broadcast information, system timing information etc.) is occupied fixing channel.Because adopt OFDM and Synchronization Network technology, downlink common channel covers whole zone.Therefore, for common signal channel, does not only disturb the field intensity overlay region of neighbor cell, because grand diversity effect, neighbor cell signal also can obtain diversity gain in the constructive addition in reception antenna place, transmission performance is compared with single sub-district made moderate progress.The transmission channel of other downlink informations and uplink information (being Traffic Channel) during then according to message transmission the residing position dynamic of travelling carriage distribute.For fear of co-channel interference, suppose that certain channel is assigned with a travelling carriage and takies, can not distribute to other travelling carriages in several sub-districts of neighbor cell even vicinity before then this channel is released.For certain jamming margin, this channel can use in other sub-districts that are separated by enough far away.
When a vehicle-mounted travelling carriage (handoff algorithm the railway mobile communication system is simplified because of the characteristics that chain covers) when a sub-district switches to neighbor cell, system is the allocation of communications switching channels of travelling carriage according to the requirement and the disturbed conditions at that time such as the traffic of service quality and Target cell.
Because use same carrier in the same subnet, no matter adopt which kind of channel setting, do not need in the handoff procedure to change carrier frequency (soft handover), so the phenomenon of handoff can not occur.On the other hand, in handoff procedure, receiving terminal (for downlink information is travelling carriage, is the base station for uplink information) can receive the total data (comprising other user's data) of channel all the time.Therefore, whether handover success is that can travelling carriage find correct channel position in time from the data that receive, and recovers one's own data thereby extract.
The method for channel allocation of handover has following several:
(1) soft time slot switches
Vehicle-mounted travelling carriage is from base station control device BSC 1The base station BTS of following former sub-district 1Switch to the base station BTS of Target cell 2Process be following soft handover:
1. switch in the preceding former sub-district BTS 1At i time slot (TS i) the transmission data;
2. in switching in the overlay region
A. by BTS 1Continuation is at time slot TS iThe interior data that send,
B. selected BTS 2Available targets switch the time slot j (individual time slot (TS of j ≠ i) j),
C. when switching index and reach requirement, BTS 1Discharge time slot TS i, BTS 2At time slot TS jThe interior data that send;
3. switch and finish in the Target cell BTS 2At time slot TS jSend data.
(2) the simple macro diversity is switched
1. switch in the preceding former sub-district BTS 1At time slot TS iSend data;
2. in switching in the overlay region, BTS 1And BTS 2All at time slot TS iThe interior data that send;
3. switch and finish in the Target cell BTS 2At time slot TS iSend data.
(3) the grand diversity of enhancement mode is switched
1. switch in the preceding former sub-district BTS 1At time slot TS iSend data;
2. in switching in the overlay region, BTS 1And BTS 2All at time slot TS iWith time slot TS j(send data in the j ≠ i);
3. switch and finish in the Target cell BTS 2At time slot TS jSend downlink data.
The invention has the beneficial effects as follows: adopt OFDM (OFDM) modulation technique, make up the mobile communication system of a kind of suitable railway (reaching highway, water transport etc.) chain wireless coverage, utilize the characteristics of the grand diversity of ofdm signal to improve transmission performance, and the method for channel allocation that solves the handover interruption is provided, the advantage of its method has:
1. soft handover;
2. for down channel, can utilize the time diversity (if in former sub-district and Target cell, using different time-gap) or the grand diversity of ofdm signal (if in former sub-district and Target cell, using identical time slot) that realize with two down wireless links of former sub-district and Target cell;
3. for up channel, realize the Combined Treatment of neighbor cell base station received signal if having ready conditions, then can utilize travelling carriage and two wireless links of former sub-district and Target cell to realize time diversity (if in former sub-district and Target cell, using different time-gap) or reception space diversity (if in former sub-district and Target cell, using identical time slot);
4. can select the channel switching distribution method of available performance the best according to Target cell channel occupancy situation.
Description of drawings
Fig. 1 is that the chain that the present invention is based on the OFDM technology covers (railway) mobile communication system schematic diagram;
Fig. 2 be the present invention is based on OFDM the time-frequency 2D signal structure chart;
Fig. 3 is the soft time slot handoff procedure of a present invention channel allocation schematic diagram;
Fig. 4 is a simple macro diversity handoff procedure channel allocation schematic diagram of the present invention;
Fig. 5 is the grand diversity handoff procedure of an enhancement mode of the present invention channel allocation schematic diagram.
Embodiment
The invention will be further described below in conjunction with the drawings and specific embodiments.
Below with Frequency Division Duplexing (FDD) (FDD) pattern, distribute the situation of (using all subcarriers), down link to describe as example by time slot.
As shown in Figure 1, the railway wireless network is that the chain of downline covers, whole coverage can be a big network or can be divided into a plurality of subnets of several zone establishments by the geographical position that each subnet comprises several sub-districts, and certain overlay region is arranged between the sub-district.Vehicle-mounted travelling carriage and base station (BTS 1, BTS 2) between Radio Link, be by base station control device (BSC 0, BSC 1, BSC 2) and mobile switching centre/gateway (MSC/GW) access railway fixed network or other system.In a subnet, all BTS adopt identical descending transmission carrier frequency f 1With identical up reception carrier frequency f 2Synchronous operation.A vehicle-mounted travelling carriage is from BSC 1The BTS that following former sub-district is 1. interior 1Switch to the 3. interior BTS of Target cell 2Process be only to change the soft handover of time slot, 2. be former sub-district 1. with Target cell overlay region 3..
As shown in Figure 2, in a subnet, all N BTS (BTS 1BTS N) all adopt identical descending transmission carrier frequency f 1With identical up reception carrier frequency f 2The signal that is sent is synchronous in time, and abscissa is the time, and ordinate is a frequency, and signal is divided into frame in time, and every frame is divided into frame head, some time slots (TS) and postamble again, respectively comprises several OFDM symbols; System bandwidth is divided into some (being actually overlapped) subcarrier sub carrier group of experimental process carrier wave (or comprise) on the frequency.
As shown in figures 1 and 3, in the handover procedure:
1. before switching former sub-district 1. in, BTS 1At i time slot (TS i) the transmission data;
2. in switching the overlay region 2. in
A. by BTS 1Continuation is at time slot TS iThe interior data that send,
B. selected BTS 2Available targets switch the time slot j (individual time slot (TS of j ≠ i) j) the interior data that send,
C. (when switching index, signal power for example is when reaching requirement) BTS 1Discharge time slot TS i, BTS 2At time slot TS jThe interior data that send;
3. switch finish Target cell 3. in, BTS 2At time slot TS jSend data.
In this handoff procedure, transfer of data receives does not interrupt, original " direct-cut operation " problem is converted into " data extract and integrity checking " problem, and to the processing procedure of the data of " comprising information needed (and wherein the position of useful data should be very not peculiar) certainly ", more strongly disturbing Signal Processing is easier accomplishes than realization " safety " handoff procedure with to having, and therefore can improve reliability of information transmission.
Wherein, " BTS in the overlay region 1Discharge time slot TS i" and " BTS 2At time slot TS jThe interior data that send " need not be strict with in time, because these two signals can phase mutual interference (ideally being quadrature).In fact, BTS 2When sending earlier, travelling carriage receives two versions of identical information, and the two overlapping in time part is equivalent to the effect of " time-space transmit diversity ", can improve the reliability of transmission.
In the overlay region, if can in time open switching channels, postpone simultaneously to discharge original channel, then can access a soft switching process---do not have the carrier frequency change,, can obtain time diversity from this two paths of signals for the travelling carriage specific information.
But said process is also underused the advantage of OFDM Synchronization Network.If the operating position that can make channel in the handoff procedure is as shown in Figure 4:
1. before switching former sub-district 1. in, BTS 1At time slot TS iSend data;
2. in switching the overlay region 2. in, BTS 1And BTS 2All at time slot TS iThe interior data that send;
3. switch finish Target cell 3. in, BTS 2At time slot TS iSend data.
The dedicated channel of travelling carriage just can be utilized the OFDM synchronizing network as common signal channel macro diversity further improves radio transmission performance.But the mode among Fig. 4 requires sub-district on the way all must provide same time slot when this travelling carriage arrives own scope, this may cause in the Target cell ongoing communication need redistribute channel or communication disruption, even the travelling carriage of transmission Train Control information and base station communication interruption.For fear of this situation, can with the scheme of Fig. 3 as the time slot of expectation the alternative when occupied.
If in the handoff procedure, the time slot TS of former sub-district jTime slot TS with Target cell iAll be available, then can use " enhancement mode " scheme as shown in Figure 5:
1. before switching former sub-district 1. in, BTS 1At time slot TS iSend data;
2. in switching the overlay region 2. in, BTS 1And BTS 2All at time slot TS iWith time slot TS j(send same data in the j ≠ i);
3. switch finish Target cell 3. in, BTS 2At time slot TS jSend downlink data;
Wherein, time slot TS jBe handoff procedure selected BTS when being triggered 2The available channel of sub-district.This scheme also has other benefits except that the advantage with preceding two scheme soft handovers:
1. the signal of two time slots can obtain the OFDM macro diversity respectively;
2. the merging of the signal of two time slots can obtain the time diversity gain.
Above-mentioned description about three kinds of channel switching distribution methods at be Frequency Division Duplexing (FDD) (FDD) pattern, the situation of distributing (using all subcarriers), down link by time slot.For up link, realize the Combined Treatment of neighbor cell base station received signal if having ready conditions, in former sub-district and Target cell, use different time-gap (during soft time slot switches and the grand diversity of enhancement mode switches) can utilize two wireless links of travelling carriage and former sub-district and Target cell to realize time diversity, and in former sub-district and Target cell, use identical time slot (the grand diversity switching of switching of simple macro diversity and enhancement mode) can realize receiving space diversity.
Realization in order to demonstrate the invention; above-mentioned specific embodiment has been described; but other variations of the present invention and modification; it will be apparent to those skilled in the art that, all belong to claim protection range of the present invention in the essence of the disclosure of invention and any modification/variation in the basic principle scope or equivalent transformation.

Claims (3)

1. mobile communication system based on OFDM, the wireless network that the chain of downline covers, certain overlay region is arranged between the sub-district, whole coverage can be a big network or can be divided into a plurality of subnets of several zone establishments by the geographical position, each subnet comprises several sub-districts, it is characterized in that: comprise other travelling carriages, base station (BTS) and base station control device (BSC), mobile switching centre/gateway (MSC/GW), railway fixed network or other system on vehicle-mounted travelling carriage and the train; Radio Link between travelling carriage and the base station (BTS) inserts railway fixed network or other system by base station control device (BSC) and mobile switching centre/gateway (MSC/GW); In a subnet, all base stations (BTS) all adopts identical descending transmission carrier frequency and identical up reception carrier frequency synchronization operation; The signal that is sent is divided into frame in time, and every frame is divided into frame head, some time slot TS and postamble again, respectively comprises several OFDM symbols; System bandwidth is divided into the experimental process carrier wave on frequency; When its signal structure can be undertaken by subcarrier-time slot-distribution of frequency two dimensional channel; Perhaps distribute, use all time slots of plurality of continuous in the frame or discontinuous subcarrier by sub carrier group; Perhaps press time slot and distribute, use all subcarriers of plurality of continuous in the frame or discontinuous time slot.
2. a kind of mobile communication system based on OFDM according to claim 1 is characterized in that: adopt OFDM and Synchronization Network technology, downlink common channel is the same with digit broadcasting system to cover whole zone.
3. the handover method for channel allocation of claim 1 or 2 described a kind of mobile communication system based on OFDM, when a vehicle-mounted travelling carriage when a sub-district switches to neighbor cell, Target cell requires according to the traffic and service quality etc. and disturbed condition at that time, allocation of communications switching channels for travelling carriage, whether handover success is that can travelling carriage find correct channel position in time from the demodulating data that receives, recover one's own data thereby extract, it is characterized in that: following several handover method for channel allocation:
(1) soft time slot switches: Vehicle mounted station is from base station control device BSC 1Following base station BTS 1Switch to base station BTS 2Process be following soft handover;
1. switch in the preceding former sub-district BTS 1At i time slot TS iSend data;
2. in switching in the overlay region:
A. by BTS 1Continuation is at time slot TS iThe interior data that send,
B. selected BTS 2Available targets switch j time slot TS of time slot jThe interior data that send, j ≠ i,
C. when switching index and reach requirement, BTS 1Discharge time slot TS i, BTS 2At time slot TS jThe interior data that send;
3. switch and finish in the Target cell BTS 2At time slot TS jSend data;
(2) the simple macro diversity is switched;
1. switch in the preceding former sub-district BTS 1At time slot TS iSend data;
2. in switching in the overlay region, BTS 1And BTS 2All at time slot TS iThe interior data that send;
3. switch and finish in the Target cell BTS 2At time slot TS iSend data;
(3) the grand diversity of enhancement mode is switched;
1. switch in the preceding former sub-district BTS 1At time slot TS iSend data;
2. in switching in the overlay region, BTS 1And BTS 2All at time slot TS iWith time slot TS jInterior data, the j ≠ i of sending;
3. switch and finish in the Target cell BTS 2At time slot TS jSend downlink data.
CNB2007100646540A 2007-03-22 2007-03-22 Mobile communication system based on OFDM and channel switching distribution method Expired - Fee Related CN100456871C (en)

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