CN108134759B - Non-orthogonal multiple access method based on interference cancellation technology - Google Patents

Non-orthogonal multiple access method based on interference cancellation technology Download PDF

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CN108134759B
CN108134759B CN201711419394.4A CN201711419394A CN108134759B CN 108134759 B CN108134759 B CN 108134759B CN 201711419394 A CN201711419394 A CN 201711419394A CN 108134759 B CN108134759 B CN 108134759B
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程崇虎
夏巍
胡洁
陆怡琪
杨洁
熊健
桂冠
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Nanjing University of Posts and Telecommunications
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
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    • H—ELECTRICITY
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    • H04B1/52—Hybrid arrangements, i.e. arrangements for transition from single-path two-direction transmission to single-direction transmission on each of two paths or vice versa
    • H04B1/525—Hybrid arrangements, i.e. arrangements for transition from single-path two-direction transmission to single-direction transmission on each of two paths or vice versa with means for reducing leakage of transmitter signal into the receiver
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    • H04—ELECTRIC COMMUNICATION TECHNIQUE
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    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
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    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
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Abstract

The invention discloses a non-orthogonal multiple access method based on interference cancellation technology, which considers an OFDMA orthogonal set system with N users as a first group of users, and simultaneously uses an MC-CDMA system as a second group to accommodate additional M users. In this scheme using N + M multi-user multiple access, the first group of N users do not interfere with each other, and the second group of M users do so, but the two groups of users interfere with each other. The transmitted symbols are finally detected by iterative decisions of successive interference cancellation. The scheme provided by the implementation of the invention can realize channel overload to relieve insufficient frequency spectrum resources, and simultaneously creatively solves the problem of insufficient number of accommodated users of a single OFDMA system. Based on this scheme, NOMA becomes a technological extension of orthogonal multiple access rather than a completely contradictory one.

Description

Non-orthogonal multiple access method based on interference cancellation technology
Technical Field
The invention relates to the technical field of communication, in particular to a non-orthogonal multiple access method based on an interference cancellation technology.
Background
In recent years, non-orthogonal multiple access (NOMA) is now a research hotspot of the future physical layer of 5G cellular networks, especially Machine Type Communication (MTC). The rise of this multiple access technique stems from an existing conclusion of multi-user information theory that orthogonal multiple access techniques are generally not optimal, while superposition coding in combination with Successive Interference Cancellation (SIC) provides an optimal solution for multiple access.
Historically, Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) have been used in various forms for a relatively long time. Two major standards for second generation mobile cellular networks in digital cellular networks are the global system for mobile communications (GSM) and IS-95 standards. The first based on TDMA technology and the second based on Code Division Multiple Access (CDMA) technology. For 3G networks, CDMA technology dominates, and wideband CDMA (wcdma) has become the standard. All these networks are based on single carrier transmission. 4G networks are multi-carrier transmission technologies based on Orthogonal Frequency Division Multiplexing (OFDM), which have been used previously for digital video broadcasting terrestrial (DVB-T), WiFi and WiMAX. In terms of multiple access, WiFi continues to use TDMA, but WiMAX employs Orthogonal Frequency Division Multiple Access (OFDMA), which allocates resources in the frequency domain of OFDM. And 3GPP Long Term Evolution (LTE) and LTE-Advanced standards that use OFDMA on the downlink and single carrier frequency division multiple access (SC-FDMA) on the uplink to reduce the peak-to-average power ratio (PAPR) of the transmitted signal.
These multiple access techniques are orthogonal and ideally ensure that there is no interference between users. TDMA is used by only one user at a time, and conventional FDMA is used by only one user at a given frequency. The orthogonality of CDMA is guaranteed by walsh-hadamard signal spreading sequences. Although the individual user signals overlap in frequency in OFDMA, orthogonality is achieved because the carrier spacing is 1/T, where T is the symbol period. Of course, the orthogonality of the uplink in these mentioned techniques requires perfect synchronization between the different user signals.
Until the advent of the multi-user information theory, orthogonality of the signals of different users has been considered to be the most desirable property. Analysis of channel capacity has found that orthogonal multiple access is not always optimal for opening new perspectives and new research directions for future networks. The prior art has the problems of multi-user transmission, insufficient frequency spectrum resources and insufficient number of accommodated users of a single OFDMA system under the condition of short frequency spectrum resources.
Disclosure of Invention
The technical problem to be solved by the invention is to overcome the defects of the prior art and provide a non-orthogonal multiple access method based on an interference cancellation technology, so that the utilization rate of a frequency spectrum is improved.
The invention adopts the following technical scheme for solving the technical problems:
the invention provides a non-orthogonal multiple access method based on interference cancellation technology, which comprises the following steps:
step one, setting N + M users, using an OFDMA orthogonal set system with N users as a first group OFDMA signal set, and outputting N-dimensional QAM symbol vectors { anN is 1,2,3nRepresents an OFDMA symbol allocated to the nth user;
using the MC-CDMA signal set as a second group to accommodate M users, the MC-CDMA signal set being an M-dimensional symbol vector b m1,2,3mRepresenting symbols allocated to the mth MC-CDMA user, spreading the MC-CDMA symbol vector over N carriers to obtain an N-dimensional vector added to the OFDMA symbol vector, i.e. a transmission signal xnComprises the following steps:
Figure BDA0001522656230000021
wherein the signal is spread wm,n＝(wm,1,wm,2........wm,N) Is a Walsh-Hadamard sequence; the OFDMA signal set and the MC-CDMA signal set are transmitted by adopting an orthogonal multiple access technology, and the OFDMA signal set and the MC-CDMA signal set are transmitted by adopting a non-orthogonal multiple access technology;
detecting symbols of the OFDMA signal and the MC-CDMA signal through iterative decision of serial interference elimination; the method comprises the following specific steps:
the received signal is rn，rn＝Xn+un，unIs additive noise, rnConversion to frequency domain by N-point DFT and alignment of OFDMA symbols, i.e. a, by threshold detectornCarrying out first iteration judgment on the symbol value to obtain a symbol judgment value
Figure BDA0001522656230000022
Figure BDA0001522656230000023
Is anA symbol decision value;
Figure BDA0001522656230000024
subtracting the decision on the basis of the DFT operator to output
Figure BDA0001522656230000025
ynM-dimensional symbol vector b of MC-CDMA signal set by Walsh-Hadamard despreader and threshold detectormM1, 2,3.. then M } makes a first iteration decision; bmThe result of the decision is
Figure BDA0001522656230000026
Is a Walsh-Hadamard sequence,
Figure BDA0001522656230000027
is bmSymbol decision value, { r }nN is 1,2,3
Figure BDA0001522656230000028
Is passed through a threshold detector pair anMaking second symbol value decision to obtain second time anA symbol decision value of; finally, rnMinus a second time anThe obtained signal is a Walsh-Hadamard sequence and continues through the threshold detector pair bmMaking second symbol value decision to obtain second bmThe symbol decision value of (2).
As a further optimization scheme of the non-orthogonal multiple access method based on the interference cancellation technique, in step one, the first set of OFDMA signals has N carriers, and each carrier is assigned to a separate user, and a QAM symbol is provided for each user in each OFDM symbol.
As a further optimization scheme of the non-orthogonal multiple access method based on the interference cancellation technology, in the first step, the bandwidth of the multiple access channel of the OFDMA signal set with N users is N · W hz, and if the signals of the OFDMA signal set are transmitted separately, W represents the bandwidth required for transmitting the signal of a single user.
As a further optimization scheme of the non-orthogonal multiple access method based on the interference cancellation technology, the invention limits the number of MC-CDMA users to be less than or equal to
Figure BDA0001522656230000031
As a further optimization scheme of the non-orthogonal multiple access method based on the interference cancellation technology, the MC-CDMA symbol vectors are spread to N carriers by adopting Walsh-Hadamard spreading.
Compared with the prior art, the invention adopting the technical scheme has the following technical effects:
the invention can improve the utilization rate of frequency spectrum, realize channel overload to relieve the shortage of frequency spectrum resources, and solve the problem of the shortage of the number of users accommodated by a single OFDMA system; NOMA has become a technological extension of orthogonal multiple access, rather than a completely contradictory technique, increasing the number of users while increasing spectrum utilization.
Drawings
Fig. 1 is a prior art TDMA/OCDMA based scheme.
Figure 2 is a combined OFDMA/MC-CDMA scheme designed by the present invention.
Fig. 3 is a block diagram of a NOMA transmitter incorporating an OFDMA/MC-CDMA scheme.
Fig. 4 is a block diagram of a NOMA receiver incorporating an OFDMA/MC-CDMA scheme.
FIG. 5 is a schematic design of the inventive solution.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in detail with reference to the accompanying drawings and specific embodiments.
In order to improve the utilization rate of frequency spectrum, the invention adopts a non-orthogonal multiple access scheme based on interference cancellation technology. To illustrate the basic principle of NOMA, we focus on discussing the uplink channels of two users in a cellular network. Let us assume that user 1 has a stronger signal power P1 and user 2 has a weaker signal power P2.In these conditions, the receiver can detect that the signal of user 1 is interfered by user 2, and subtract the signal of user 1 from the received signal to obtain the signal of user 2 without interference. Assuming that the channel is an additive white gaussian noise channel with normalized bandwidth W of 1Hz, user 1 capacity R1In Hz
Figure BDA0001522656230000032
Wherein N is0Is the noise spectral density (noise power at unit bandwidth W ═ 1 Hz). After detecting the signal transmitted by subscriber 1, the receiver may subtract the signal from the received signal to obtain a non-interfering subscriber 2 signal. User 2 capacity R2Is that
Figure BDA0001522656230000041
Thus, the total capacity of two users is
Figure BDA0001522656230000042
The above formula can be abbreviated as
Figure BDA0001522656230000043
P＝P1+P2Which indicates that the capacity of the multi-user channel is the same as the capacity of a single-user channel with the same total power.
The reality is multiple access (OWMA) with orthogonal waveforms. Without loss of generality, consider that user 2 uses the OFDMA scheme. Again, P1 represents the user 1 signal power, P2 represents the power of the user 2 signal, and P1+ P2 is the total power. We specify that P1 ═ α P and P2 ═ 1- α) P, 0 ≦ α ≦ 1. The signal power is evenly distributed on the N carriers of the OFDMA signal, and the bandwidth allocation of the two users is the same as the signal power allocation proportion. In other words, we W1 ═ α W and W2 ═ 1- α) W, and bandwidth W1 is allocated to user 1 and W2 is allocated to user 2. The capacity equation for two users is given as follows
Figure BDA0001522656230000044
Figure BDA0001522656230000045
The total capacity R-R1 + R2 is the same as the NOMA capacity given by equation (4). In summary, both NOMA and OMA can achieve single-user channel capacity when there is no relative attenuation of the user signal, and the two multiple access techniques do not differ in capacity.
The difference between the two multiple access techniques occurs when one user signal is subject to different attenuation compared to the other. Suppose that the user 2 signal is attenuated by 6dB and the user 1 signal is not attenuated. In this case, the OFDMA capacity becomes
Figure BDA0001522656230000046
NOMA capacity of
Figure BDA0001522656230000051
Comparing these two capacities, assuming α is 0.8 and P/No is 15, the single user channel is 4bit/hz, and equation (7) will be ROFDMA3.65, (8) is RNOMAComparing these two capacities we can see that in the specific example NOMA increases the two user channel capacity by 3.5%, and further comparison shows that the advantage of NOMA is particularly significant when parameter α is reduced and user 2 signal attenuation is further increased, but the increase in NOMA capacity is not completely gratuitousAnd (4) performing secondary iteration decision until the performance is close to the interference-free transmission. This process is better when there is a strong imbalance between the two user signals, but when the two signals have similar powers, convergence problems can occur.
The current state of NOMA development is closely related to the emergence of 5G cellular network research projects. The main research subjects defined by the physical layer of the 5G network are massive MIMO, waveform design and millimeter wave technology, and multiple access is a key component of waveform design. A number of NOMA papers have been published in the last few years, and a series of papers over 15 years have been based on NOMA. The term NOMA was not used in this early document, but the concept of NOMA and the physical part of using this technology began to emerge in 2000. The following is a brief summary of this work, the basic principle of which is to use two sets of orthogonal signal waveforms. We focus here on techniques that combine TDMA and OCDMA, with the TDMA signal set to a full signal and the OCDMA signal set to a partial signal.
Consider a simple TDMA system with N users, each of which obtains one data symbol in N symbols per frame. The bandwidth of the multiple access channel is NW hertz, W representing the bandwidth required to transmit the signal of a single user if they are transmitted separately. Thus, the scheme can accommodate N users without any interference. A second set of signals is used to accommodate additional users (e.g., M users, M < N). The second signal set is also an orthogonal set, but the two sets are not orthogonal to each other. Specifically, as used herein, the second set of OCDMA signals are Walsh Hadamard (WH) sequences of length N. In using an N + M multi-user multiple access scheme, the N users of the first group do not interfere with each other and the same applies to the M users of the second group, but each user of the first group interferes with each user of the second group and vice versa. That is, we have a NOMA scheme where two groups of users interfere with each other, and iterative detection of successive interference cancellation is required to detect the transmitted symbols.
Fig. 1 shows the basic principle of NOMA, illustrating the superposition of M OCDMA symbols together to form the symbols of N TDMA. In the abscissa of this figure, time is represented, Tc is the OCDMA chip duration and also the TDMA symbol duration, and T ═ N · Tc is the OCDMA symbol duration, which is also the TDMA symbol block duration. The instantaneous power transmitted per OCDMA symbol is P and the instantaneous power transmitted per TDMA is N x P, so the TDMA symbol and OCDMA symbol energy is e.n.p.tc.p.t. This picture clearly shows that a preliminary decision can be made on the TDMA symbols as long as M remains smaller than N.
Let us write an equation for the transmitted signal. The time index n indicates the position of the symbol in the TDMA block, symbol anN is less than or equal to 1 and less than or equal to N in the user # N of the TDMA, symbol bmM is more than or equal to 1 and less than or equal to M in the OCDMA user. We write the WH sequence as the signal extension wm,n＝(wm,1,wm,2........wm,N) M, using this symbol, the transmitted signal can be written as
Figure BDA0001522656230000061
Figure BDA0001522656230000062
(9) In the formula
Figure BDA0001522656230000063
Is used to preserve symbol energy during signal transmission. The received signal can be written as follows: r isn＝Xn+un，unN. n. presupposing that the number of OCDMA M users is not very large, (9) the interference term is still small compared to the TDMA symbol power, and the received signal samples r are additive noise, n 1,2nTo a threshold detector, at which the symbol a is transmittednMake a decision on all anMaking a decision on the symbol for the first iteration based on
Figure BDA0001522656230000064
Subtracting the symbol estimate from the received signal samples, each n and
Figure BDA0001522656230000065
all represent symbol anThe decision made above returns to (9) and y is sentnIs rewritten into
Figure BDA0001522656230000066
Suppose that
Figure BDA0001522656230000067
(10) Can be simplified into
Figure BDA0001522656230000068
The second step of the receiver is to despread the signal and make decisions on the OCDMA symbols, the signal despreading consisting of:
Figure BDA0001522656230000069
the second term parametric noise term has the same variance as the original noise. The decision for the first iteration of the OCDMA symbol will be at z through the threshold detectorkIt is started.
Once for { bmAnd M is 1,2,3mWhen the second iteration decision is made, the interference can be eliminated, namely M1, 2,3. The process is as follows: for each n, calculate
Figure BDA00015226562300000610
Wherein each m and
Figure BDA00015226562300000611
is used to decide bm. Suppose that
Figure BDA00015226562300000612
According to formula (9), we obtain vn＝an+un. This signal will next be sent to a threshold detector, pair a without interferencenA decision is made. The second iteration judgment is obviously more reliable than the first iteration judgment process, and still carries out the judgment on the b as the first iteration judgment stepmAnd performing second iteration judgment on the M-1, 2,3However, additional iterations may further improve performance in some cases, but the results show that when m is smaller, two iterations are sufficient.
The above concepts are not only applicable to multiple access. It is equally applicable to single-user transmission and so the term "channel overload" is used to describe it. The basic idea is to overload by superimposing the second signal to the first signal once the channel is fully loaded using an orthogonal signaling scheme (orthogonal transmission of multi-user channels or orthogonal multiple access of multi-user channels). The optimal joint detection is too complex to be achieved and the receiver actually employs an iterative receiver with interference cancellation. For multiple access, recent NOMA papers focus primarily on the superposition of two user signals, but the above TDMA and OFDMA signals are actually processed more deeply and are the superposition of two user group signals.
Since OFDMA has become the basic multiple access scheme for 4G cellular systems, and is also being applied by 3GPP for mobile broadband (EMB) services in 5G, we will describe a frequency domain NOMA scheme involving a first group of users using OFDMA and a second group of users using multi-carrier CDMA (MC-CDMA). The principle is the same as described before, and as shown in fig. 2, the time dimension is replaced by the frequency dimension. In the figure, 1/N is the carrier spacing, the Power Spectral Density (PSD) of OFDMA symbols is N × D watts/hz, and the power spectral density of MC-CDMA symbols superimposed on them is D watts/hz.
Further describing the NOMA technique, consider that an OFDMA system has N carriers and assume without loss of generality that each carrier is allocated to a separate user. This system accommodates N users, each provided with a QAM symbol in each OFDM symbol. Using the concepts described in the earlier models, we superimpose a set of MC-CDMA signals carrying a second set of user information on this OFDMA signal. The concept of the early model mathematical equation remains unchanged, in the range 1<＝n<N-N specifies the number of carriers, x given by (9)nIndicates that the transmission is in nthA signal on a carrier wave. Fig. 3 shows a simple block diagram of a transmitter. The output of OFDMA user module is an N-dimensional QAM symbol vector { a }nN1, 2,3Is an m-dimensional vector symbol vector bmM1, 2,3. Walsh-Hadamard spreading spreads the MC-CDMA symbols over N carriers and outputs an N-dimensional vector that is summed with the OFDMA symbol vector. The resulting signal block is passed to an N-point DFT inverse, and this N-point DFT inverse inserts Cyclic Prefixes (CPs) between successive inverse DFT blocks.
A sketch of a corresponding receiver is shown in fig. 4. In the time domain after the CP is removed, the signal is converted to the frequency domain by an N-point DFT. Operator of output { rnN1, 2.. N } makes a first iteration decision on the OFDMA symbol directly through a threshold detector. The result is a first timenSymbol decision value of
Figure BDA0001522656230000071
These decisions based on the subtraction DFT operator will output
Figure BDA0001522656230000072
And this signal is passed through a Walsh-Hadamard despreader. The output of the despreader is sent to the m-dimensional symbol vector b of the set of MC-CDMA signals for the threshold detectormM1, 2,3. First time bmIs determined by the decision value of
Figure BDA0001522656230000081
Figure BDA0001522656230000082
Is a Walsh-Hadamard sequence, { rnN1, 2,3.. N } will subtract this sequence output block, and the resulting signal passes through the threshold detector pair { anN1, 2.. times, N } makes a second decision. Finally, the decisions are subtracted from the decision input, the output signal is a Walsh-Hadamard sequence and continues through the threshold decision pair { b }mAnd M is 1,2,3. This process can make multiple further decisions if needed, and it is actually anticipated that if M is small, two decisions will be sufficient.
In this case, the performance of the composition can be improved without significant deteriorationThe planning of the number of MC-CDMA user signals superimposed on the OFDMA signal is also important. The WH sequence used for signal spreading is a binary sequence consisting of ± 1. Due to the use of multiplication terms in signal propagation
Figure BDA0001522656230000083
Interference of each MC-CDMA user to OFDMA user
Figure BDA0001522656230000084
In the form of (1). When the number of MC-CDMA users reaches
Figure BDA0001522656230000085
The peak interference amplitude reaches 1 and the OFDMA signal eye pattern closes. In this case, even in the absence of noise, the first iteration decision of the OFDMA symbol is erroneous, which means that the corresponding bit error rate curve (BER) has a level of error. Therefore, we limit the number of MC-CDMA users to
Figure BDA0001522656230000086
Although this does not represent a strict constraint. In fact, an iterative receiver that uses soft decisions instead of hard decisions would help accommodate more MC-CDMA users.
In the present invention, we have described the NONA technology as one of the powerful technology representatives of the future 5G cellular system machine communication. Having described the basic principles of this technology, we point out that this technology actually dates back to 2000, and recent scholars seem to be unaware of this fact. This concept appears in a series of papers published at that time describing multiple access using two orthogonal signal sets and successive interference cancellation for iterative detection. We first review this technique using TDMA for a first group of users and OCDMA for a second group of users. Focusing on the 5G cellular system, we describe a practical Noma scheme combining OFDMA and MCCDMA, which provides an attractive solution for 5G machine communication, as shown in fig. 5. In this scheme, when OFDMA occupies all channel resources, NOMA can be seen as a means for OFDMA to achieve channel overload and accommodate a large number of users. Alternatively, OFDMA and MC-CDMA may be used as a method to accommodate two sets of users with different configuration schemes and data rate requirements.
The above description is only for the specific embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.

Claims (5)

1. A non-orthogonal multiple access method based on interference cancellation technique is characterized by comprising the following steps:
step one, setting N + M users, using an OFDMA orthogonal set system with N users as a first group OFDMA signal set, and outputting N-dimensional QAM symbol vectors { anN is 1,2,3nRepresents an OFDMA symbol allocated to the nth user;
using the MC-CDMA signal set as a second group to accommodate M users, the MC-CDMA signal set being an M-dimensional symbol vector bm1,2,3mRepresenting symbols allocated to the mth MC-CDMA user, spreading the MC-CDMA symbol vector over N carriers to obtain an N-dimensional vector added to the OFDMA symbol vector, i.e. a transmission signal xnComprises the following steps:
Figure FDA0002632512920000011
wherein the signal is spread wm,n＝(wm,1,wm,2........wm,N) Is a Walsh-Hadamard sequence; the OFDMA signal set and the MC-CDMA signal set are transmitted by adopting an orthogonal multiple access technology, and the OFDMA signal set and the MC-CDMA signal set are transmitted by adopting a non-orthogonal multiple access technology;
detecting symbols of the OFDMA signal and the MC-CDMA signal through iterative decision of serial interference elimination; the method comprises the following specific steps:
the received signal is rn，rn＝Xn+un，unIs additive noise, rnConversion to frequency domain by N-point DFT and alignment of OFDMA symbols, i.e. a, by threshold detectornCarrying out first iteration judgment on the symbol value to obtain a symbol judgment value
Figure FDA0002632512920000012
Figure FDA0002632512920000013
Is anA symbol decision value; r isnSubtracting the decision on the basis of the DFT operator to output
Figure FDA0002632512920000014
ynM-dimensional symbol vector b of MC-CDMA signal set by Walsh-Hadamard despreader and threshold detectormM1, 2,3.. then M } makes a first iteration decision; bmThe result of the decision is
Figure FDA0002632512920000015
Is a Walsh-Hadamard sequence,
Figure FDA0002632512920000016
is bmSymbol decision value, { r }nN is 1,2,3
Figure FDA0002632512920000017
Is passed through a threshold detector pair anMaking second symbol value decision to obtain second time anA symbol decision value of; finally, rnMinus a second time anThe obtained signal is a Walsh-Hadamard sequence and continues through the threshold detector pair bmMaking second symbol value decision to obtain second bmThe symbol decision value of (2).
2. The method of claim 1, wherein in step one, the first set of OFDMA signals has N carriers, and each carrier is assigned to a separate user, and each user is provided with a QAM symbol in each OFDM symbol.
3. The method of claim 1, wherein in step one, the bandwidth of the multiple access channel with the OFDMA signal set of N users is N · whz, and W represents the bandwidth required to transmit the signal of a single user if the signals of the OFDMA signal set are transmitted separately.
4. The method of claim 1, wherein the number of MC-CDMA users is limited to less than or equal to
Figure FDA0002632512920000021
5. The method of claim 1, wherein the MC-CDMA symbol vectors are spread over N carriers using Walsh-Hadamard spreading.
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