CN101924605B - Double-hop cooperative transporting method based on physical-layer network coding - Google Patents
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Abstract
The invention provides a double-hop cooperative transporting method based on physical-layer network coding, which comprises the following steps of: each MS adds a CPI with a proper length on an OFDM signal to be transmitted when each MS transmits a first transmission signal to a selected RS, and respectively and simultaneously transmits to the RS; and the RS firstly eliminates the CP1 from the received multi-channel OPDM signal to remove a first time error existed in the system, emerges with a PNC, adds the CP2 with the proper length on the emerged signal when the RS transmits a second signal to a BS, and transmits the signal to the BS so as to help the MS to transmit the information. The BS delimitates the CP2 from the received and emerged signal to remove a second time error existed in the system, and indirectly obtains the required information by means of PNC decoding. Therefore, even if the uplink from one MS to the BS is interrupted, the BS also can obtain the transmitted information, so that the method improves the network transmission quality.
Description
Technical Field
The invention relates to the technical field of wireless transmission based on network coding and IEEE802.16j standard, in particular to a two-hop cooperative transmission method based on physical layer network coding.
Background
In order to achieve the objectives of high data rate, high spectrum efficiency, seamless coverage and the like of a future wireless communication system, the IEEE802.16j mobile multi-hop Relay standard introduces RS (Relay Station) in a network to enhance the performance of a WiMax (Worldwide Interoperability for Microwave Access) system. In the case that the mobile terminal cannot apply multiple antennas, relay cooperation can be used as a way to achieve spatial diversity to improve the performance of the system.
In the conventional relay cooperation, signal collision is not allowed at the RS, and the RS can only simply forward the received signal, so the spectrum efficiency is low. PNC (physical layer network coding) is a new method that can significantly improve the throughput of a relay network. The method is a network coding of a physical layer for processing the receiving and modulation of electromagnetic wave signals, and adopts a proper modulation and demodulation technology at RS, so that the superposition of the electromagnetic wave signals can be mapped into a Gaussian domain of data bit stream superposition, namely, the method allows a relay node to generate signal collision, thereby further improving the network throughput and reducing the information transmission time. However, previous research on PNC assumes that the channel is in an ideal channel and has strict time synchronization, while in actual environment, the channel has obvious frequency selective fading, and the system also has time error due to the distribution of nodes and the path delay of the multipath channel, and the channel fading and the time error can cause obvious system performance degradation and reduce the network transmission quality.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a two-hop cooperative transmission method based on physical layer network coding, which can effectively improve the network transmission quality.
The purpose of the invention is realized as follows: a two-hop cooperative transmission method based on physical layer network coding is carried out according to the following steps:
first, MS (Mobile Station) converts binary signal bit d (k) into N-channel parallel information bit streams through serial/parallel conversion, and the information bit streams on each branch are respectively subjected to constellation mapping to obtain complex coordinates { x ] in signal spacekComplex coordinate { x }kAn N-point IFFT (Inverse Fast fourier transform) transform is performed to perform OFDM (Orthogonal frequency division Multiplexing) modulation, and then OFDM symbols are obtained, where each OFDM symbol is added with a length TMRA first transmission signal is obtained after a first Cyclic Prefix CP1(Cyclic Prefix); then the MS selects two RSs participating in cooperative transmission according to a Signal to Noise Ratio (SNR) of a channel, and sends a first sending Signal to the RSs;
secondly, the RS removes a first cyclic prefix CP1 from the received first sending signal to obtain a first receiving signal, extracts three paths of first receiving signals to carry out XOR operation so as to carry out PNC coding to obtain a PNC merging signal, and adds a length T to the PNC merging signalRBThe second prefix CP2 obtains a second transmission signal, and transmits the second transmission signal to a BS (base Station);
thirdly, the second cyclic prefix CP2 is removed from the second sending signal received by the BS to obtain a second receiving signal, and the second receiving signal is subjected to N-point FFT (Fast Fourier Transform), PNC decoding and demodulation to recover the complex coordinate { x }k}。
Further, in a first step, the length T of the first cyclic prefix CP1MRGreater than { τd，i，j+τMRIs a maximum value of where τd，i，jIs MSiTo RSjPath delay, tau, corresponding to the d-th independent path in the channel betweenMRIs from MSiReaches the RSjTime comparison ofDelay of signals from other MSs.
Further, in a second step, the length T of the second cyclic prefix CP2RBGreater than { τd，j+τ′RBIs a maximum value of where τd，jIs RSjPath delay, τ ', corresponding to the d-th independent path in the channel to the BS'RBIs from RSjThe time of arrival of the signal at the BS is delayed compared to the signals from the other RSs.
Compared with the prior art, the invention has the beneficial effects that: the method is applied to a two-hop cooperative transmission scene defined by the IEEE802.16j standard, and effectively improves the information transmission rate and the diversity gain by utilizing the physical layer network coding in the wireless environment with the frequency selective fading channel. The method of the present invention also utilizes the cyclic prefix CP of the OFDM symbol to combat frequency selective fading and time errors, so that PNC can be smoothly implemented. Aiming at a more practical wireless channel environment with frequency selective fading, the method applies the PNC in a two-hop relay cooperative system, allows the RS to encode and combine data from different information sources so as to improve the network throughput, improve the load balance, reduce the transmission delay and save the node energy consumption, considers two time errors existing in the system, and even if an uplink from one MS to the BS is interrupted, the BS can obtain the information sent by the BS, thereby improving the network transmission quality and robustness.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention.
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In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be further described in detail with reference to the accompanying drawings:
fig. 1 is a network topology model diagram of a two-hop cooperative transmission method based on physical layer network coding according to the present invention;
fig. 2 is a time division duplex uplink subframe frame structure diagram of a two-hop cooperative transmission method based on physical layer network coding according to the present invention;
fig. 3 is a system architecture diagram of a two-hop cooperative transmission method based on physical layer network coding according to the present invention.
Detailed Description
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
Fig. 1 shows a network topology model of a two-hop cooperative transmission method based on physical layer network coding, which includes a plurality of source nodes U, a plurality of relay nodes R and a destination node. In an embodiment of the present invention, the source node U is an MS, the relay node R is an RS, and the destination node may be a BS or an RS, each node being in a half-duplex mode. The MS and the RS adopt a TDM (Time Division Multiplex) operation mode. The RS can utilize PNC technology to assist multiple users in two-hop transmission simultaneously.
In order to further improve the spectrum efficiency and reduce the transmission delay, the network adopts a mode of sending information with the same frequency. This requires information synchronization and the RS and BS can correctly estimate the channel state information. The uplink subframe structure of the method is shown in fig. 2, the whole frame is divided into two equal-length time slots, the MS sends the signal to the RS in the first time slot, the RS sends the signal to the BS in the second time slot, and the MS can also directly send the signal to the BS. When the frame structure is adopted, the RS does not need to transmit and receive data at the same time, and compared with the RS in a full-duplex mode, the RS in a half-duplex mode has greatly reduced requirements on hardware and is relatively simple to realize.
In the two-hop cooperative transmission scenario defined by the ieee802.16j standard, the first-hop transmission is used for the MS to send signals to the MSAnd RS, wherein the first hop transmission is that the RS sends a signal to the BS, and the first hop transmission and the second hop transmission can be distinguished by time. There are three links in the network: a "BS-RS" link, a "BS-MS" link, and a "RS-MS" link, wherein the "BS-RS" link is a first hop transmission, and the "BS-MS" link and the "RS-MS" link are a second hop transmission. Assuming that all three links are frequency selective rayleigh fading channels and are quasi-static, i.e. slowly fading, from the MSiTo RSjHas D independent propagation paths, and the channel impulse response is expressed as:
wherein,
indicating MSiTo RSjAmong the channels in between, the channel coefficient of the d-th independent path. Suppose that
Are independent and identically distributed random variables, τd，i，jIs the corresponding path delay, each channel coefficientAll are complex Gaussian random variables with zero mean and variance of
The other two channels are similar.
The OFDM technique increases the length of a symbol time by adding a CP to the start position of each symbol to further resist ISI (inter symbol Interference) and reduce a timing offset error at a receiving end. It is the CP of the OFDM symbol that is utilized in the present invention to resist frequency selective fading and time error. OFDM modulation is adopted in each MS, IFFT/FFT or PNC decoding is not needed in RS, and only simple operation is needed to be carried out on the received mixed signals.
Fig. 3 shows an implementation flow of the two-hop cooperative transmission method based on physical layer network coding according to the present invention, specifically:
first, MS converts binary signal bit d (k) into N paths of parallel information bit streams through serial/parallel conversion, and the information bit streams on each branch are respectively subjected to constellation mapping to obtain complex coordinates { x ] in signal spacekComplex coordinate { x }kOFDM modulation (namely N-point IFFT transform) is carried out to obtain OFDM symbols, and the sum of each OFDM symbol and the length is TMRObtaining a first transmission signal after the first cyclic prefix CP 1; then, the MS selects two RSs participating in cooperative transmission according to the channel SNR and transmits a first transmission signal to the RSs.
In the first step, MS1 converts binary signal bit d (k) into N parallel bit streams through serial/parallel conversion, and performs constellation mapping on the information bits on each branch to obtain complex coordinate { x ] in signal spacekIs sent to an OFDM modulator, and frequency domain data symbols x are processed by an N-point IFFT operationkTransforming into time-domain data symbols sk。
After IFFT, each OFDM symbol plus a length TMRCP1 of the first transmission signal. If the signal from the MS1 arrives at the Relay RSjIs delayed by tau from the signals from the other MSsMRThere is a first time error. To combat frequency selective fading channel and time error of the RS receiving the first transmitted signal, CP1 has a length TMRShould be greater than { τd，i，j+τMRThe maximum value of.
The MS1 determines whether to perform cooperative transmission and which relay nodes participate in cooperative transmission according to the collected channel SNR information. In the initial selection phase, the RS participating in the cooperative transmission is selected, and it is ensured that the channel quality gap between the cooperative RS is not large, that is, the receiving power gap is not large. The IEEE802.16j standard specifies that a threshold may be set according to an actual environment, the MS1 may select multiple RSs with channel SNRs greater than the threshold to perform cooperative transmission, and the IEEE802.16j standard also proposes to select two RSs to perform cooperative transmission. Finally, the first transmission signal is transmitted to the selected two RSs. The other MSs send their own signals to the RS in the same manner as the MS 1.
Secondly, the RS removes a first cyclic prefix CP1 from the received first sending signal to obtain a first receiving signal, extracts three paths of first receiving signals to carry out XOR operation so as to carry out PNC coding to obtain a PNC merging signal, and adds a length T to the PNC merging signalRBThe second cyclic prefix CP2 obtains a second transmission signal, and transmits the second transmission signal to the BS.
Assuming that each RS can reliably obtain signals from three MSs, the BS may also receive signals transmitted by the MSs over the direct link. The RS first removes the CP1 for each received first transmission signal to obtain a first received signal. Thus, ISI caused by frequency selective fading of the first transmitted signal and time errors between the three MSs and any one RS are removed. Then, the three paths of signals without CP are directly subjected to exclusive OR operation under the condition of no demodulation so as to carry out PNC coding, and a PNC combined signal is obtained. RS1, signal processing at RS2 is as in equation (2), and similarly at other RSs:
wherein x isR1、xR2Is a three-way first received signal x1、x2、x3Or x1、x4、x5The result of PNC encoding.
After PNC, RS needs to use an amplified signal with a specific power in order to ensure that the average power of any RS transmission signal is maintained at a fixed level. Then, a new length T is added to the PNC combined signalRBAnd a second cyclic prefix CP 2. If the signal from the RS is transmitted to the RS in three MS processes or the MS transmits the signal to the BS through the direct linkjIs delayed by τ 'from the signals from the other RSs by the time of arrival at the BS'RBThere is a second time error. To combat the frequency selective fading channel and the time error of the second transmitted signal, the CP2 has a length TRBMaximum value τ that should be greater thanl，i，j+τ′RB}. Finally, the processed second transmission signal is transmitted to the BS.
Thirdly, the second cyclic prefix CP2 is removed from the second sending signal received by the BS to obtain a second receiving signal, and the complex coordinate { x ] is recovered after N-point FFT conversion, PNC decoding and demodulation are carried out on the second receiving signalk}。
At the BS, the CP2 is first removed from the received mixed signal, so that ISI caused by frequency selective fading in the second transmitted signal and time error between the RS and the BS are removed. After CP2 is removed, the received second received signal is subjected to N-point FFT conversion, and then the BS performs PNC decoding according to the received signal, and then demodulates the signal to obtain the required information.
For example, if the BS cannot get the information x sent by U1 through the direct link1To correctly obtain xR1And direct path or decoding the combined signal to x2And x3Or xR2，x4And x5PNC decoding and demodulation can be performed by an exclusive or operation:
or
Recover x1The mobile station MS1 ends the two-hop-at-a-time cooperative transmission process to the base station BS.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and it is apparent that those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.
Claims (1)
1. A two-hop cooperative transmission method based on physical layer network coding is characterized by comprising the following steps:
first, MS converts binary signal bit d (k) into N paths of parallel information bit streams through serial/parallel conversion, and the information bit streams on each branch are respectively subjected to constellation mapping to obtain complex coordinates { x ] in signal spacekThe complex coordinate { x }kCarrying out N-point IFFT to obtain OFDM symbols after OFDM modulation, wherein the sum of each OFDM symbol and the length is TMRObtaining a first transmission signal after the first cyclic prefix CP 1;then the MS selects two RSs participating in cooperative transmission according to the channel SNR and sends the first sending signal to the RSs;
secondly, the RS removes the first cyclic prefix CP1 from the received first sending signal to obtain a first receiving signal, extracts three paths of the first receiving signals to carry out exclusive OR operation so as to carry out physical layer network coding to obtain a physical layer network coding merging signal, and adds the length T to the physical layer network coding merging signalRBThe second cyclic prefix CP2 obtains a second transmission signal, and transmits the second transmission signal to the BS;
thirdly, the second cyclic prefix CP2 is removed from the second sending signal received by the BS to obtain a second receiving signal, and the complex coordinate { x ] is restored after N-point FFT conversion, physical layer network coding and decoding and demodulation are carried out on the second receiving signalk}；
A length T of the first cyclic prefix CP1MRGreater than { τd，i，j+τMRIs a maximum value of where τd，i，jIs MSiTo RSjPath delay, tau, corresponding to the d-th independent path in the channel betweenMRIs from MSiReaches the RSjTime of the signals from the other MSs;
a length T of the second cyclic prefix CP2RBGreater than { τd，j+τ′RBIs a maximum value of where τd，jIs RSjPath delay, τ ', corresponding to the d-th independent path in the channel to the BS'RBIs from RSjThe time of arrival of the signal at the BS is delayed compared to the signals from the other RSs.
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| CN104836643A (en) * | 2015-04-21 | 2015-08-12 | 中国人民解放军军械工程学院 | Communication method based on MIMO-OFDM and physical layer network coding |
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