CN101286971A - System and method for adaptive frame length control in OFDM system - Google Patents
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Abstract
The invention relates to an adaptive frame length control system in an OFDM system and a method thereof. The fact that the quality of receiving signals is lower than a predetermined threshold value proves that the quality of the channel is poor, and therefore a command for shortening the frame length is generated to notify a sending terminal to shorten the frame length, thereby improving pilot density; when the quality of receiving signals is higher than the predetermined threshold value, the command for prolonging the frame length is generated to notify the sending terminal to prolong the frame length, thereby reducing the pilot density; meanwhile, the sending terminal indicates the frame format of the frame in every sent OFDM frame. When the quality of the channel is poor, the system and method of the invention adopt short frame and dense pilot insertion, which on one hand can fully reflect effect of increase of bit error rate caused by deep fading, and on the other hand, can reflect the change of the quality of the channel in time when the quality of the channel becomes good, thus recovering highly efficient transmission of the long frame in time.
Description
Technical field
The present invention relates to wireless communication system, be specifically related to adaptive frame length control system and method in a kind of OFDM wireless communication system.
Background technology
In recent years, in the new generation broadband wireless communication system, the orthogonal frequency division multiplex OFDM technology had replaced the single carrier spread spectrum, and CDMA for example becomes the transmission technology of main flow.The OFDM technology early is applied among digital broadcasting DAB and the Digital Television DVB.Subsequently, BWA standard IEEE 802.11g/a/n, 802.16d/e and formulating 802.20 also based on the OFDM/OFDMA technology.In addition, just also select for use the descending OFDM of OFDM and modified model thereof, up DFT-S-OFDM as basic multiple access technology probably at present in standardized 3GPP LTE Long Term Evolution and 3GPP2 AIE AIE technology.One of two alternatives of short-range communication UWB technology have also adopted multicarrier MC-OFDM.Estimate that following B3G technology also will be based on OFDM.In a word, all emerging technologies of wireless communication field are core with the OFDM technology nearly all at present.
The OFDM technology is a kind of multi-carrier transmission technology, and its main thought is the subchannel that given channel is divided into many quadratures in frequency domain, uses a subcarrier to modulate on each subchannel, and each subcarrier parallel transmission.Like this, although total channel be non-flat forms, but each subchannel is a relatively flat, and what carry out on each subchannel is narrow band transmission, signal rate is relatively low, thereby the interference between the erasure signal waveform greatly.
Wherein, the time-frequency structure of wireless OFDM frame as shown in Figure 3, the little lattice of each on time shaft are an OFDM symbol period, on frequency axis the expression each subcarrier.Comprising data and Cyclic Prefix on the carrier wave on OFDM signed theory, in actual applications, is an OFDM frame with K OFDM symbol combination often.
In ofdm system, channel estimating roughly can be divided into two kinds: a kind of auxiliary channel estimation method of pilot tone that is based on, another kind is blind algorithm for estimating.The two comparatively speaking, though based on the auxiliary channel estimation methods of pilot tone because the insertion of pilot tone has reduced the spectrum efficiency of system, because of it has relatively simply obtained extensive use in wireless communication system.
In the subcarrier allocation algorithm of existing OFDM, pilot signal at first with certain algorithm assigns to a plurality of subcarriers, and then carry out the distribution of subchannel data.There is not the mechanism that pilot tone and subchannel data are connected.
Yet in wireless communication system, particularly in mobile radio communications system, multipath fading is the major reason that causes transmission characteristic to worsen, and multipath fading changes along with the time the influence of transmission path quality.Under different transmission path quality conditions, the requirement of pilot interval is different, needs pilot tone seldom can estimate channel well when channel variation is slow, needs a lot of pilot tones could estimate channel preferably when channel variation is very fast.
Application number is 200810103091.6 and is called that the Chinese patent application of " a kind of OFDM automatic closed-loop transmitting scattered pilot insertion control method " has provided the scattered pilot that the bit error rate (BER) of utilizing receiving terminal to estimate in a kind of ofdm system comes dynamic auto closed loop to adjust transmitting terminal and inserted method at interval, when the BER that receives data surpasses set-point, just by increasing order, the notice transmitting terminal increases the pilot number of inserting; When the BER that receives data was lower than set-point, just by reducing order, the notice transmitting terminal reduced the pilot number of inserting; Transmitting terminal is all indicated the frame format of this frame in the OFDM of each transmission frame; Frame format information is inserted in the back of first pilot cell.
The technical scheme of above-mentioned application has been inserted the pilot tone of a greater number when channel quality degrades, the received signal quality when being actually with being reduced to cost and realizing poor channel quality of efficiency of transmission stable.Long frame is when channel quality is good, and is not obvious to the influence of efficiency of transmission.But long frame can not effectively be resisted the deep fade that occurs in the frame, and the long frame error rate can not fully reflect the multipath deep fade of burst.Long frame is slow to the reaction of variations, and when channel quality improved gradually, the efficient of long frame transmission can not in time improve, and is a kind of waste to Radio Resource.Therefore when bad channel quality, should adopt short frame transmission.
Summary of the invention
The object of the present invention is to provide the adaptive frame length control system in a kind of ofdm system.
Another object of the present invention is to provide the control method of the adaptive frame length in a kind of ofdm system.
Adaptive frame length control system in a kind of ofdm system provided by the invention, comprise transmitting terminal and receiving terminal, wherein said transmitting terminal comprises coding unit, interleave unit, digital modulation unit, inserts pilot tone and frame format control unit, IFFT unit, parallel serial conversion unit, adds cyclic prefix unit;
Described receiving terminal comprises to be separated coding unit, deinterleaving unit, digital demodulation unit, extraction pilot tone and frame format unit, FFT unit, serial to parallel conversion unit, goes cyclic prefix unit;
Wherein, described transmitting terminal comprises that also one inserts pilot tone and frame format control unit, and described insertion pilot tone and frame format control unit determine the OFDM frame length, and pilot tone and the frame format information corresponding with described frame length are inserted in the wireless OFDM frame;
Described receiving terminal also comprises frame length control command unit, the received signal quality of the more described receiving terminal in described frame length control command unit and reservation threshold value, if described received signal quality is lower than and subscribes threshold value then produce and send the first frame length control command to transmitting terminal, if described received signal quality is higher than and subscribes threshold value then produce and send the second frame length control command to transmitting terminal;
Described insertion pilot tone and frame format control unit are transferred the frame length of OFDM frame to increase to 1/2 of current frame length after receiving the described first frame length control command;
Described insertion pilot tone and frame format control unit are transferred 2 times that increase to current frame length with the frame length of OFDM frame after receiving the described second frame length control command.
Aforesaid adaptive frame length control system, wherein, described insertion pilot tone and frame format control unit if the frame length of current OFDM frame is to be minimum frame length, then do not carry out the frame length adjustment after receiving the described first frame length control command; Described insertion pilot tone and frame format control unit if the frame length of current OFDM frame is to be maximum frame length, then do not carry out the frame length adjustment after receiving the described second frame length control command.
Aforesaid adaptive frame length control system, wherein, described insertion pilot tone and frame format control unit insert pilot tone at first OFDM symbol of OFDM frame, insert described frame format information at first subcarrier of second OFDM symbol.
Aforesaid adaptive frame length control system, wherein, described first, second frame length control command is by the transmission of the control channel from the receiving terminal to the transmitting terminal.
Aforesaid adaptive frame length control system, wherein, described received signal quality is characterized by decoded signal Block Error Rate BER.
Aforesaid adaptive frame length control system, wherein, during described adaptive frame length control system initialization, described insertion pilot tone and frame format control unit are set at maximum frame length with the frame length of OFDM frame.
Adaptive frame length control method in a kind of ofdm system provided by the invention comprises the steps:
A, receiving terminal receive and decode from the signal of transmitting terminal;
Whether B, the receiving terminal relatively received signal quality of this signal are lower than the reservation threshold value, if then produce and send the first frame length control command, execution in step C to transmitting terminal; Otherwise, produce and send the second frame length control command, execution in step D to transmitting terminal;
C, transmitting terminal are adjusted into 1/2 of current frame length with the frame length of OFDM frame, return steps A;
D, transmitting terminal are adjusted into 2 times of current frame length with the frame length of OFDM frame, return steps A.
Method wherein, after transmitting terminal receives the described first frame length control command, if the frame length of current OFDM frame is to be minimum frame length, is not then carried out the frame length adjustment as mentioned above; Transmitting terminal if the frame length of current OFDM frame is to be maximum frame length, does not then carry out the frame length adjustment after receiving the described second frame length control command.
Method as mentioned above, wherein, transmitting terminal inserts pilot tone at first OFDM symbol of OFDM frame, inserts described frame format information at first subcarrier of second OFDM symbol.
Method as mentioned above, wherein, described first, second frame length control command is by the transmission of the control channel from the receiving terminal to the transmitting terminal.
Method as mentioned above, wherein, described received signal quality is characterized by decoded signal Block Error Rate BER.
Method as mentioned above, wherein, during the ofdm system initialization, transmitting terminal is according to largest frames long hair feed signals.
When frame length shortens, because the quantity of pilot cell does not change, so the also corresponding increase of pilot density; During the frame length lengthening, because the quantity of pilot cell does not change, so the also corresponding reduction of pilot density.
System and method for of the present invention is when bad channel quality, adopt short frame, intensive pilot tone to insert, can fully reflect the influence of the error rate rising that deep fade causes on the one hand, on the other hand, when channel quality improves, also can in time reflect the variation of channel quality, imitate transmission thereby in time recover long vertical frame dimension.
Description of drawings
Fig. 1 is the schematic diagram of described adaptive frame length control system;
Fig. 2 is the flow chart of described adaptive frame length control method;
Fig. 3 is N=3, during M=3, and the schematic diagram of long frame format;
Fig. 4 is N=3, during M=3, and the inferior schematic diagram of long frame format;
Fig. 5 is N=3, during M=3, and the schematic diagram of short frame format.
Embodiment
Describe the specific embodiment of the present invention in detail below in conjunction with accompanying drawing.
Fig. 1 is the schematic diagram that the OFDM self adaptation becomes the frame length control system.Wherein, described system comprises transmitting terminal and receiving terminal.Transmitting terminal comprises coding, interweaves, digital modulation, insertion pilot tone and frame format control, IFFT, parallel serial conversion, add CP unit such as (Cyclic Prefix), and insertion pilot tone wherein and frame format control unit are the functional units directly related with the present invention.Receiving terminal comprises to be separated coding, deinterleaving, digital demodulation, extraction pilot tone and frame format, FFT, serial to parallel conversion, goes to CP unit such as (Cyclic Prefix).Transmitting terminal is inserting pilot tone and frame format control unit decision frame length, and pilot tone and frame format information are inserted in the radio frames, launches through a day bundle of lines OFDM frame at last, passes through space channel then, and the OFDM frame is received termination and receives.After the FFT conversion, behind extraction pilot tone and the frame format information, through decoding, receiving terminal just can be learnt the error rate BER value γ of the OFDM frame of reception again.This BER value γ that receives in real time and given BER value Ψ are compared in frame length control command unit, become the control command T that the generation of frame length control command unit becomes frame length according to comparative result by producing:
When γ＞Ψ, T=1, the order transmitting terminal shortens to original 1/2 with frame length;
When γ≤Ψ, T=0, the order transmitting terminal increases to original 2 times with frame length.
Order T transmits by the control channel from the receiving terminal to the transmitting terminal.
After transmitting terminal receives the T order of receiving terminal, just whether reached the length that maximum or minimum frame length are controlled the OFDM frame according to this control command and current frame length.And at the back of first subcarrier pilot cell insertion frame format information.
With (N, M) the longest frame of expression comprises 2
NIndividual OFDM symbol, 2
MOne framing format of individual subcarrier.A framing format like this comprises the frame format of the different frame lengths of N kind, promptly by the shortest OFDM frame that comprises 2 OFDM symbols to comprising 2
NThe longest OFDM frame of individual OFDM symbol, between the adjacent frame format, frame length differs 2 times.When using such OFDM frame bearer data, pilot cell can be inserted in first symbol place of each subcarrier, and second time frequency unit of first subcarrier can be frame format information.With the longest OFDM frame is example, and it comprises 2 on time span
NIndividual OFDM symbol, its sequence number are 0,1 ..., n ..., 2
N-1, have 2 at frequency domain
MIndividual subcarrier, its sequence number are 0,1 ..., m ..., 2
M-1, whole OFDM frame always has 2
N+MIndividual time frequency unit always has 2
MIndividual pilot cell, pilot cell are at first symbol place of each subcarrier, and frame format information needs | log
2 N| the frame format that bit representation is all, second time frequency unit of first subcarrier is frame format information, remaining all is a data cell.
Fig. 2 when the ofdm system init state, adopts the longest frame format to send data for the flow chart of OFDM adaptive frame length control method of the present invention, promptly adopts form as described in Figure 3 to transmit data.
After receiving terminal received data, the error rate BER value γ of the OFDM frame of the reception that decoding is learnt and given BER value Ψ compared, and become the control command T that frame length control command unit produces the change frame length according to comparative result by producing:
When γ＞Ψ, T=1 promptly orders transmitting terminal that frame length is shortened to original 1/2;
When γ≤Ψ, T=0 promptly orders transmitting terminal that frame length is increased to original 2 times.
Order T transmits by the control channel from the receiving terminal to the transmitting terminal.
If transmitting terminal receives T=1, transmitting terminal judges whether current frame format is the shortest frame format, if, then do not carry out the form adjustment, if not, then frame length is shortened to original 1/2.
If transmitting terminal receives T=0, transmitting terminal judges whether current frame format is the longest frame format, if, then do not carry out the form adjustment, if not, then frame length is increased to original 2 times.
With N=3, one group of OFDM frame format of M=3 is an example below in conjunction with Fig. 3 to 5, and OFDM adaptive frame length control method of the present invention is described.
Fig. 3 is N=3, during M=3, and the schematic diagram of long frame format, the black square represents that the square that pilot cell, white square represent that data cell, grid are filled represents frame format information among the figure.As shown in Figure 3, pilot cell is positioned at first OFDM symbol.In the back of first subcarrier pilot cell is exactly the frame format information unit.In the present embodiment, because N=3, need 2 and represent frame format information, establish that the frame format information of long frame format is 11, receiving terminal has extracted first pilot cell, and then the parse for frame format information knows it is the longest frame format, just can receive entire frame.
Fig. 4 is N=3, during M=3, and the schematic diagram of inferior the longest frame format, the black square represents that the square that pilot cell, white square represent that data cell, grid are filled represents frame format information among the figure.When transmitting terminal during in the longest frame format, receive the T=1 order that receiving terminal is sent, just next frame is changed into time the longest frame format, simultaneously at the back insertion frame format information 10 of first subcarrier pilot cell of next frame.Compare with the longest frame format, the OFDM symbolic number under the longest inferior frame format has descended half, 1/2 when having only the longest frame format.Receiving terminal has extracted first pilot cell, and then the parse for frame format information knows it is time the longest frame format, just can receive entire frame.
Fig. 5 is N=3, during M=3, and the schematic diagram of short frame format, the black square represents that the square that pilot cell, white square represent that data cell, grid are filled represents frame format information among the figure.Having only first OFDM symbol is pilot tone, and the back of first subcarrier pilot cell is a frame format information, in the present embodiment, establishes that the frame format information of short frame format is 00, and remaining time frequency unit all is a data cell in the OFDM frame.When receiving the T=0 order that receiving terminal is sent, transmitting terminal just doubles the OFDM symbolic number of next frame, and promptly the frame length of OFDM frame doubles.
Adopt this self adaptation of the present invention to become the frame length control method, can in time dynamically adjust the length of OFDM frame, both satisfied the requirement of system, farthest improved spectrum efficiency again the BER value according to channel situation.
The method of the foregoing description can be applied to N, M and become the frame length control procedure greater than the self adaptation of any ofdm system of 1.The above is preferred embodiment of the present invention only, is not limited to the present invention, all any modifications of being made within the present invention spirit and principle, is equal to replacement and improvement etc., all is contained within protection scope of the present invention.
Claims (10)
1, the adaptive frame length control system in a kind of ofdm system comprises transmitting terminal and receiving terminal, and wherein said transmitting terminal comprises coding unit, interleave unit, digital modulation unit, IFFT unit, parallel serial conversion unit, adds cyclic prefix unit;
Described receiving terminal comprises to be separated coding unit, deinterleaving unit, digital demodulation unit, extraction pilot tone and frame format unit, FFT unit, serial to parallel conversion unit, goes cyclic prefix unit;
It is characterized in that:
Described transmitting terminal comprises that also one inserts pilot tone and frame format control unit, and described insertion pilot tone and frame format control unit determine the OFDM frame length, and pilot tone and the frame format information corresponding with described frame length are inserted in the wireless OFDM frame;
Described receiving terminal also comprises frame length control command unit, the received signal quality of the more described receiving terminal in described frame length control command unit and reservation threshold value, if described received signal quality is lower than and subscribes threshold value then produce and send the first frame length control command to transmitting terminal, if described received signal quality is higher than and subscribes threshold value then produce and send the second frame length control command to transmitting terminal;
Described insertion pilot tone and frame format control unit are transferred the frame length of OFDM frame to increase to 1/2 of current frame length after receiving the described first frame length control command;
Described insertion pilot tone and frame format control unit are transferred 2 times that increase to current frame length with the frame length of OFDM frame after receiving the described second frame length control command.
2, adaptive frame length control system as claimed in claim 1, it is characterized in that: described insertion pilot tone and frame format control unit are after receiving the described first frame length control command, if the frame length of current OFDM frame is to be minimum frame length, then do not carry out the frame length adjustment; Described insertion pilot tone and frame format control unit if the frame length of current OFDM frame is to be maximum frame length, then do not carry out the frame length adjustment after receiving the described second frame length control command.
3, adaptive frame length control system as claimed in claim 1, it is characterized in that: described insertion pilot tone and frame format control unit insert pilot tone at first OFDM symbol of OFDM frame, insert described frame format information at first subcarrier of second OFDM symbol.
4, adaptive frame length control system as claimed in claim 1 is characterized in that: described first, second frame length control command is by the transmission of the control channel from the receiving terminal to the transmitting terminal.
5, adaptive frame length control system as claimed in claim 1 is characterized in that: during described adaptive frame length control system initialization, described insertion pilot tone and frame format control unit are set at maximum frame length with the frame length of OFDM frame.
6, the adaptive frame length control method in a kind of ofdm system is characterized in that comprising the steps:
A, receiving terminal receive and decode from the signal of transmitting terminal;
Whether B, the receiving terminal relatively received signal quality of this signal are lower than the reservation threshold value, if then produce and send the first frame length control command, execution in step C to transmitting terminal; Otherwise, produce and send the second frame length control command, execution in step D to transmitting terminal;
C, transmitting terminal are adjusted into 1/2 of current frame length with the frame length of OFDM frame, return steps A;
D, transmitting terminal are adjusted into 2 times of current frame length with the frame length of OFDM frame, return steps A.
7, method as claimed in claim 7 is characterized in that: after transmitting terminal receives the described first frame length control command, if the frame length of current OFDM frame is to be minimum frame length, then do not carry out the frame length adjustment; Transmitting terminal if the frame length of current OFDM frame is to be maximum frame length, does not then carry out the frame length adjustment after receiving the described second frame length control command.
8, method as claimed in claim 7 is characterized in that: transmitting terminal inserts pilot tone at first OFDM symbol of OFDM frame, inserts described frame format information at first subcarrier of second OFDM symbol.
9, method as claimed in claim 7 is characterized in that: described first, second frame length control command is by the transmission of the control channel from the receiving terminal to the transmitting terminal.
10, method as claimed in claim 7 is characterized in that: during the ofdm system initialization, transmitting terminal is according to largest frames long hair feed signals.
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