RF Transmission Chain

Abstract

A device includes: a first plurality of first RF chains; a second plurality of second RF chains, the first RF chains are configured to produce sideband emissions that are wider than the second RF chains; at least one antenna array, including antenna elements, each antenna element in the first plurality of antenna elements is coupled to a corresponding one of the first plurality of first RF chains, the first plurality of first RF chains are configured to enable transmissions to be primarily in a first frequency band within a channel, each antenna element in the second plurality of antenna elements is coupled to a corresponding one of the second plurality of second RF chains, the second plurality of second RF chains are configured to enable transmissions to be primarily in at least one second frequency band within the channel.

Description

Technical Field

[0001] Various example embodiments are directed to an apparatus having a radio frequency transmission chain. Background Art

[0002] Radio frequency transmission chains are known. Such chains are incorporated into wireless telecommunication devices and convert and / or condition baseband signals into signals to be provided to antenna elements for transmission.

[0003] Although such radio frequency transmission chains exist, they have several disadvantages. It would therefore be desirable to provide improved radio frequency transmission chains. Summary of the invention

[0004] The scope of protection sought by various embodiments of the present invention is set out by the independent claims. Embodiments and features described in this specification that do not fall within the scope of the independent claims, if any, are to be construed as examples useful for understanding various embodiments of the present invention.

[0005] According to various (but not necessarily all) embodiments of the present invention, there is provided an apparatus comprising: a first plurality of first RF chains; a second plurality of second RF chains, the first RF chains being configured to produce sideband emissions wider than the second RF chains; and at least one antenna array comprising antenna elements, each antenna element in the first plurality of antenna elements being coupled to a corresponding first RF chain in the first plurality of first RF chains, the first plurality of first RF chains being configured to enable transmission primarily in a first frequency band within a channel, each antenna element in the second plurality of antenna elements being coupled to a corresponding second RF chain in the second plurality of second RF chains, the second plurality of second RF chains being configured to enable transmission primarily in at least one second frequency band within the channel.

[0006] The first frequency band may include at least a center frequency band within the channel, and the second frequency band may include a sideband within the channel.

[0007] The first plurality of first radio frequency chains may include a first digital predistortion module, and the second plurality of second radio frequency chains may include a second digital predistortion module, which may be configured to perform a higher complexity digital predistortion than the first digital predistortion module.

[0008] The second RF chain may be configured to produce narrower sideband emissions than the first RF chain.

[0009] The first radio frequency chain may be configured to generate transmissions having greater power in a center frequency band than in at least one sideband.

[0010] The second radio frequency chain may be configured to generate transmissions in at least one sideband having greater power than in the center frequency band.

[0011] The first RF chain may be less linear than the second RF chain.

[0012] The second RF chain may have higher linearity than the first RF chain.

[0013] The center frequency band may include at least a first group of center subcarriers within the channel.

[0014] The sideband may include a second group of subcarriers adjacent to the first group within the channel.

[0015] The second plurality of second radio frequency chains may be configured to cause transmissions to be primarily within a pair of sidebands within the channel.

[0016] The second group of subcarriers may surround the first group of central subcarriers within the channel.

[0017] The first group of subcarriers may include different subcarriers than the second group of subcarriers.

[0018] Each first radio frequency chain of the first plurality of first radio frequency chains may include a non-linear precoder.

[0019] Each first radio frequency chain of the first plurality of first radio frequency chains may share a non-linear precoder.

[0020] The non-linear precoder may be configured to cause the transmission to occur outside of the channel at a power less than a second threshold amount.

[0021] The non-linear precoder may be configured to cause transmission to occur within a sideband with less than a first threshold amount of power.

[0022] The first threshold amount may be greater than the second threshold amount.

[0023] The non-linear precoder may be configured to beamform the transmission towards the intended recipient within the center frequency band.

[0024] The non-linear precoder may be configured to beamform transmissions radiated from the first plurality of radio frequency chains within sidebands away from intended recipients.

[0025] The non-linear precoder may be configured to reduce the transmission power in the sidebands compared to transmissions in the center frequency band.

[0026] Each second RF chain of the second plurality of second RF chains may include a linear precoder.

[0027] Each second RF chain in the second plurality of second RF chains may share a linear precoder.

[0028] The linear precoder may be configured to cause transmission to occur within the center frequency band at a power less than a third threshold amount.

[0029] The linear precoder may be configured to cause the transmission to occur off the channel at a power less than a second threshold amount.

[0030] The linear precoder may be configured to beamform the transmission within the sideband towards the intended recipient.

[0031] The linear precoder may be configured to beamform the transmission away from the intended recipient within a center frequency band.

[0032] The linear precoder may be configured to reduce the transmission power in the center frequency band compared to transmissions in the sidebands.

[0033] Intended recipients may include user equipment.

[0034] The apparatus may comprise a pair of antenna arrays, a first antenna array of the pair comprising a first plurality of antenna elements and a second antenna array of the pair comprising a second plurality of said antenna elements.

[0035] The apparatus may include a base station.

[0036] According to various (but not necessarily all) embodiments of the present invention, a method is provided, comprising: providing a first plurality of first RF chains, a second plurality of second RF chains, and at least one antenna array comprising antenna elements; coupling each antenna element in the first plurality of antenna elements to a corresponding first RF chain in the first plurality of first RF chains, and coupling each antenna element in the second plurality of antenna elements to a corresponding second RF chain in the second plurality of second RF chains; and configuring the first RF chain to produce a sideband emission wider than the second RF chain, the first plurality of first RF chains causing transmissions to be primarily in a first frequency band within a channel, and the second plurality of second RF chains causing transmissions to be primarily in at least one second frequency band within the channel.

[0037] The first frequency band may include at least a center frequency band within the channel, and the second frequency band may include a sideband within the channel.

[0038] The first plurality of first radio frequency chains may include a first digital predistortion module, and the second plurality of second radio frequency chains may include a second digital predistortion module, which may be configured to perform a higher complexity digital predistortion than the first digital predistortion module.

[0039] The configuring may include configuring the second radio frequency chain to produce narrower sideband emissions than the first radio frequency chain.

[0040] The configuring may include configuring the first radio frequency chain to produce transmissions having greater power in the center frequency band than in at least one sideband.

[0041] The configuring may include configuring the second radio frequency chain to generate transmissions having greater power in at least one sideband than in the center frequency band.

[0042] The first RF chain may be less linear than the second RF chain.

[0043] The second RF chain may have higher linearity than the first RF chain.

[0044] The center frequency band may include at least a first group of center subcarriers within the channel.

[0045] The sideband may include a second group of subcarriers adjacent to the first group within the channel.

[0046] The configuring may include configuring the second plurality of second radio frequency chains to transmit primarily within a pair of sidebands within the channel.

[0047] The second group of subcarriers may surround the first group of central subcarriers within the channel.

[0048] The first group of subcarriers may include different subcarriers than the second group of subcarriers.

[0049] Each first radio frequency chain of the first plurality of first radio frequency chains may include a non-linear precoder.

[0050] Each first radio frequency chain of the first plurality of first radio frequency chains may share a non-linear precoder.

[0051] The configuring may include configuring the non-linear precoder to cause the transmission to occur outside of the channel at a power less than a second threshold amount.

[0052] The configuring may include configuring the non-linear precoder so that the transmission occurs within the sideband at a power less than a first threshold amount.

[0053] The first threshold amount may be greater than the second threshold amount.

[0054] The configuring may include configuring a non-linear precoder to beamform the transmission towards the intended recipient within a center frequency band.

[0055] The configuring may include configuring the non-linear precoder to beamform transmissions radiated from the first plurality of radio frequency chains within a sideband away from an intended recipient.

[0056] The configuring may include configuring the non-linear precoder to reduce transmission power in the sidebands compared to transmissions in the center frequency band.

[0057] Each second RF chain of the second plurality of second RF chains may include a linear precoder.

[0058] Each second RF chain in the second plurality of second RF chains may share a linear precoder.

[0059] The configuring may include configuring the linear precoder so that the transmission occurs within the center frequency band at a power less than a third threshold amount.

[0060] The configuring may include configuring the linear precoder to cause the transmission to occur out of the channel at a power less than a second threshold amount.

[0061] The configuring may include configuring a linear precoder to beamform the transmission within a sideband toward an intended recipient.

[0062] The configuring may include configuring the linear precoder to beamform the transmission away from the intended recipient within the center frequency band.

[0063] The configuring may include configuring the linear precoder to reduce transmission power in the center frequency band compared to transmission in the sideband.

[0064] Intended recipients may include user equipment.

[0065] Providing may include providing a pair of antenna arrays, a first antenna array in the pair comprising a first plurality of antenna elements and a second antenna array in the pair comprising a second plurality of said antenna elements.

[0066] According to various (but not necessarily all) embodiments of the present invention, there is provided a computer program comprising instructions for causing an apparatus to perform at least the following operations: configuring a first RF chain to produce wider sideband emissions than a second RF chain, configuring the first plurality of first RF chains to transmit primarily in a first frequency band within a channel, and configuring the second plurality of second RF chains to transmit primarily in at least one second frequency band within the channel.

[0067] The computer program may include the features described above.

[0068] Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims where appropriate and may be different from the combinations explicitly set out in the claims.

[0069] Where an apparatus feature is described as being operable to provide a function, it will be understood that this includes apparatus features that provide that function or that are adapted or configured to provide that function. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Some example embodiments will now be described with reference to the accompanying drawings, in which:

[0071] Figure 1 An example of using nonlinear components for bandwidth extension in a radio frequency transmission chain is shown;

[0072] Figure 2Aschematically illustrates the arrangement and operation of an apparatus according to one embodiment;

[0073] Figure 2B shows the transmission of an apparatus according to one embodiment;

[0074] Figure 2C shows the transmission of an apparatus according to one embodiment;

[0075] Figure 3 is a block diagram showing a linear RF transmission chain;

[0076] Figure 4 is a block diagram showing a nonlinear radio frequency transmission chain;

[0077] Figure 5 is a flow chart illustrating a nonlinear precoding method;

[0078] Figure 6 shows the power loss versus ring width averaged over 100 instances using Algorithm 2; and

[0079] Figure 7 The relationship between the number of iterations and the ring width averaged over 100 instances using Algorithm 2 is shown. DETAILED DESCRIPTION

[0080] Before discussing example embodiments in more detail, an overview will first be provided. Some embodiments provide an arrangement that utilizes different performance attributes of transmission chains. In particular, two different sets of transmission chains are used to provide signals to associated antenna elements. One set of transmission chains is less linear than the other set, so their transmissions tend to be wider or more dispersed, and because the wider roll-off transmissions are outside the predetermined subchannels, more bandwidth is occupied. However, these transmissions tend to be more power efficient than the other set. The other set of transmissions is more linear, so their transmissions tend to be narrower, and because the narrower roll-off transmissions are outside the predetermined subchannels, less bandwidth is occupied. However, these transmissions tend to be less power efficient. When considering transmissions within the allocated bandwidth or spectrum mask of a channel, the less linear transmission chain is configured to transmit within a first region of the channel, away from at least one (multiple) guard bands at the edge of the channel. The roll-off transmissions are then contained in (multiple) sidebands within the allocated bandwidth of the channel. The more linear transmission chain is configured to transmit within a second region on at least one side of the first region of the channel. The roll-off transmissions from the more linear transmission chain are then also contained within the allocated bandwidth of the channel. This enables more efficient nonlinear transmission to occur in a first region of the channel while still containing rolled-off transmission within the allocated bandwidth of the channel, while less efficient transmission occurs in a second region. This provides an efficient and cost-effective arrangement. In some embodiments, the first region is at least a central region of the channel and the second region is at least one sideband of the channel. In the following description, nonlinear is used to refer to an arrangement in which some transmission chains are less linear than others. In particular, a nonlinear transmission chain is typically a transmission chain whose sideband emissions are larger or wider than a more linear transmission chain.

[0081] As mentioned above, the challenge with nonlinear hardware is the bandwidth extension (transmission roll-off) of the signal at the output of the nonlinear transmission chain and the corresponding out-of-band emissions at the antenna - where the radiated signal does not comply with, for example, but not limited to, 3GPP spectrum mask requirements. For example, for low-resolution digital-to-analog converters (DACs), strong quantization noise will result in out-of-band (OOB) emissions, while in the case of constant envelope precoding, the lack of amplitude modulation produces a signal with a slow roll-off outside the signal bandwidth. The additional bandwidth results in a loss of spectral efficiency and also does not comply with 3GPP requirements such as Figure 1 , which shows a roll-off transmission 5. In particular, if the nonlinear hardware is used at full power across the entire channel, the system does not comply with 3GPP requirements due to the slow roll-off. Alternatively, if the nonlinear hardware is used at full power only on the center band, a slow roll-off occurs in the guard band, and the system does comply with 3GPP. However, in this case, there is a loss in spectral efficiency because data is not communicated in the guard band.

[0082] The growth of wireless services requires the use of complex modulation schemes (high-order quadrature amplitude modulation and orthogonal frequency division multiplexing) with high peak-to-average power ratio (PAPR) signals. These signals impose strict linearity requirements on the transmission chain design, which leads to high power back-off factors and thus low energy efficiency performance. In addition, massive multiple-input multiple-output (MIMO) systems form the main cornerstone technology for fifth-generation (5G) wireless communication systems. In order to achieve practical implementation of very large antenna arrays (i.e., more than 64 antennas), simple and cost-effective transmission chains and energy-efficient power amplifiers (PAs) are required.

[0083] Conventional RF chains are linear because they are designed so that the signal radiated by the antenna is an amplified and frequency-shifted version of the signal at baseband. Therefore, the cost of conventional RF chains is dominated by the component costs of linear hardware (such as high-resolution digital-to-analog converters (DACs) that faithfully construct the analog signal) and linearization hardware (such as digital predistorters (DPDs) and feedback receivers that remove nonlinearities).

[0084] In addition to the increased cost, linear / linearized PAs operate in the less power-efficient linear region to maintain signal integrity and are therefore a major source of power loss in the RF transmission chain.

[0085] One way to reduce the cost of the RF transmission chain is to replace linear hardware with nonlinear hardware, or to remove some or all linearization components where the signal at the antenna is no longer a faithful reconstructed version of the signal at baseband. However, for massive MIMO base stations with several antennas, the "dirty" signals from each transmit antenna can be combined "over the air" to create a clean signal at the user equipment. However, combining the over-the-air signal with the nonlinear hardware is not simple and straightforward, but requires specialized baseband algorithms that compensate for some of the effects of the nonlinear hardware, which is called nonlinear precoding.

[0086] Linear / non-linear arrays

[0087] Some embodiments utilize hybrid arrays, i.e., arrays containing a mix of linear and nonlinear (or less linear and even less linear) RF transmission chains, such as Figure 2A As shown schematically in Figure 2AAs shown, an antenna array 10 for use, for example, in a base station includes a plurality of antenna elements 20A and a plurality of antenna elements 20B, which are arranged in an n×m array in this example. Several linear radio frequency transmission chains 30A are also provided, each coupled to a corresponding antenna element 20A. Similarly, a plurality of nonlinear radio frequency transmission chains 30B are provided, each coupled to a corresponding antenna element 20B. Although this embodiment shows that the antenna elements 20A, 20B are juxtaposed, this is not necessary, and they may be located at different locations within the cell and coupled to the base station. In addition, although this embodiment shows a separate linear radio frequency chain 30A for each antenna element 20A, a single linear radio frequency transmission chain 30A shared by all antenna elements 20A may be provided. Similarly, although this embodiment shows a separate nonlinear radio frequency transmission chain 30B for each antenna element 20B, a single nonlinear radio frequency transmission chain 30B shared by all antenna elements 20B may be provided.

[0088] Linear transmission chain

[0089] Figure 3 An example linear RF transmission chain 30A coupled to an antenna element 20A is schematically shown, wherein a linear / linearization component is provided with a linear precoder, a digital predistorter (DPD), a digital-to-analog converter (DAC), and an analog-to-digital converter (ADC). In some embodiments, the linear precoder is shared by all antenna elements 20A. In other embodiments, a linear precoder is provided for each antenna element 20A. Other components include a crest factor reduction (CFR), a power amplifier (PA) driver power amplifier (PA), and a bandpass filter (BPF) as part of a digital front end (DFE), as is known in the art.

[0090] Nonlinear transmission chain

[0091] Figure 4 An exemplary non-linear radio frequency transmission chain 30b is schematically shown with a non-linear precoder 70, which will be explained in more detail below. The non-linear radio frequency transmission chain 30B is coupled to the antenna element 20B.

[0092] transmission

[0093] Since the nonlinear RF transmission chains 30B generate excessive OOB emissions compared to the linear RF transmission chain 30A, their transmission bandwidth is selected to be smaller than the overall signal or channel bandwidth B. S In other words, antenna element 20B transmits over the entire signal or channel bandwidth B S The center part B NL The signal in the channel is such that the bandwidth expansion caused by the nonlinear RF transmission chain 30B occurs over the entire signal or channel bandwidth BS Therefore, OOB emissions from these nonlinear RF transmission chains 30B become in-band emissions, occurring in the guard band or sideband B. L / 2 and is represented as signal portion 60.

[0094] The guard band or sideband B emitted by antenna element 20A in the band including antenna element 20B L / 2. This is Figure 2B In the whole signal or channel bandwidth B S The center has a bandwidth B NL The frequency band 40 corresponds to the signal radiated by the antenna element 20B from the nonlinear RF transmission chain 30B and has a bandwidth B L Band 50 (on the roll-off band of band 40) corresponds to the signal radiated from linear RF transmission chain 30A by antenna element 20A. The combined signal then meets the 45dB adjacent channel leakage ratio (ACLR) specified by 3GPP.

[0095] However, such an arrangement has a number of disadvantages, such as the subcarriers radiated by the linear RF transmission chain 30A have a smaller array gain than the subcarriers radiated by the nonlinear RF transmission chain 30B. Therefore, the spectral efficiency of these subcarriers is smaller. In addition, the overall spectral efficiency decreases with the sideband B. L In addition, the in-band emission from the nonlinear RF transmission chain 30B will interfere with the sub-carriers radiated by the linear RF transmission chain 30A and increase the error vector magnitude in these sub-carriers.

[0096] Figure 2C A guard band or sideband B is shown where antenna element 20A transmits only within the band containing antenna element 20B. L This is an example of radiation on Figure 2C is shown in Figure 2, where the total signal or channel bandwidth B S One side has bandwidth B NL The frequency band corresponds to the signal radiated by the antenna element 20B from the nonlinear RF transmission chain 30B and has a bandwidth B L The sidebands of correspond to the signals radiated from the linear RF transmission chain 30A by the antenna element 20A. The combined signal then meets the 45dB adjacent channel leakage ratio (ACLR) specified by 3GPP.

[0097] Nonlinear Precoding

[0098] Some of these shortcomings can be addressed by using nonlinear precoding within the nonlinear precoder 70 to exploit the spatial degrees of freedom and radiate the distortion components of the "dirty" signal from each nonlinear radio chain 30B away from the user equipment (UE), while ensuring that the signal component in the beam direction of the UE is "clean".

[0099] essentially, Figure 2A The method is combined with a nonlinear precoding algorithm within the nonlinear precoder, which helps ensure that out-of-band (in this example, sideband) emissions from the nonlinear RF transmission chain 30B are optionally beamformed away from the beam direction corresponding to the UE, while the subcarriers radiated by the linear RF transmission chain 30A in the frequency band are beamformed in the beam direction. Alternatively or additionally, the nonlinear precoding algorithm helps ensure that the power of the out-of-band (in this example, sideband) emissions from the nonlinear RF transmission chain 30B is reduced compared to the subcarriers radiated by the linear RF transmission chain 30A in the frequency band. Therefore, B L The nonlinear precoder helps to ensure that the bandwidth of the in-band transmission is reduced, so that the nonlinear RF transmission chain 30B supports a larger proportion of the system bandwidth B. S This results in an overall higher spectral efficiency because the proportion of subcarriers with higher array gain and diversity increases.

[0100] To provide a large utilization of the entire channel bandwidth B S This approach allows most of those linear RF transmission chains 30A to be replaced by lower-cost, simpler non-linear RF transmission chains 30B while still meeting the 3GPP spectrum mask.

[0101] The linear RF transmission chain 30A and the nonlinear RF transmission chain 30B radiate on different subcarriers (or on non-overlapping bandwidths) to solve the problem of out-of-band emissions (OOB emissions from the nonlinear RF transmission chain 30B are converted to in-band emissions) while reducing the overall cost and complexity of the base station containing the device. The nonlinear precoding algorithm limits the in-band emissions and optionally radiates them in a spatial direction away from the UE beam so that the in-band emissions from the nonlinear RF transmission chain 30B do not increase the bandwidth B. L The approach thus exploits a hybrid array architecture to address the OOB emissions inherent in the non-linear RF transmission chain 30B. The combination of this approach is a base station architecture that provides higher spectral efficiency per dollar / euro compared to traditional linear approaches while meeting the 3GPP spectrum mask.

[0102] In summary, the method divides the array 10 into two parts radiating on different subcarriers to improve the out-of-band emission problem while reducing the overall cost of the base station. The algorithm for nonlinear precoding generates constant envelope or nearly constant envelope signals in all nonlinear RF transmission chains 30B so that they are combined at the UE into, for example, a desired orthogonal frequency division multiplexing (OFDM) signal or other type of signal. Optionally, the nonlinear precoding algorithm also ensures that the in-band emission is radiated away from the UE beam direction so that the bandwidth B L The EVM does not increase.

[0103] Nonlinear precoding algorithm

[0104] We consider an OFDM system with N subcarriers, which are divided into subcarrier sets N u (Used by non-linear chains to carry data), (empty, not used by the nonlinear chain for data). By oversampling by a factor of θ, our process can efficiently work with a total of Nθ subcarriers. The additional N(θ-1) / 2 subcarriers on either side of the center N subcarriers are given by Denotes (out-of-band subcarrier). Given the subcarrier spacing f s , the total system bandwidth is B S =f s N, the bandwidth covered by the nonlinear RF chain is And the bandwidth covered by the linear RF chain is

[0105] Some constraints will be defined on groups of subcarriers (e.g., groups of 20 adjacent subcarriers) to be enforced. The set of subcarriers in group g will be denoted G(g) and will constitute the subcarrier set and The set of subcarrier groups will be represented as and That is, for example,

[0106] The nonlinear portion of the array has M RF chains, and we communicate to K user equipments (UEs) simultaneously. We describe the baseband processing that determines the complex time domain signal to be sent on each RF chain during one OFDM symbol.

[0107] We define the following symbols:

[0108] H[n]: KxM downlink matrix from M antennas to K UEs for each subcarrier n.

[0109] u[n]: Kx1 vector, modulated constellation data to be sent on subcarrier n.

[0110] x[n]: Mx1 vector, frequency domain signal to be transmitted on M antennas on (extended) subcarrier n.

[0111] X = [x[1],…,x[n]]: Mx(Nθ) matrix, extended frequency domain precoded signal.

[0112] A = XF: MX(Nθ) oversampled time domain signal, where F is the IFFT matrix of (Nθ) x (Nθ), normalized to a Hermitian matrix, so that F H F=FF H =I.

[0113] For each The nonlinear precoding block is given an estimate of the channel H[n] and the constellation data. The output of this block is the matrix A, which gives the complex time domain values ​​to be modulated on the M nonlinear RF chains on the Nθ time samples of the oversampled OFDM symbol.

[0114] The oversampling factor θ determines the additional bandwidth on either side of the system bandwidth which is “monitored” by the nonlinear processing to ensure that out-of-band signal energy remains below strict requirements. Frequencies exceeding this bandwidth should be Figure 3 and Figure 4 The final bandpass filter (BPF) control depicted in FIG.

[0115] Figure 5 Indicates the general process for non-linear precoding.

[0116] In block S10, for a subcarrier set The initial data provided are the channel matrix H[n] and the user data vector u[n].

[0117] At block S20, a linear precoding method known in the art is used to obtain a frequency domain signal X[0] which is suitable for transmitting data on these channels but may not have the desired time domain properties.

[0118] At block S30, the signal is converted to the time domain using an inverse Fourier transform.

[0119] In block S40, if the time domain properties of the signal are satisfactory, the algorithm ends and returns the current time domain solution.

[0120] Otherwise, at block S50, a clipping operation or a non-linear operation similar to clipping is applied to improve the time domain signal properties.

[0121] At block S60, the signal is converted to the frequency domain using a Fourier transform.

[0122] At block S70, operations such as projection or approximate projection are used to improve the frequency domain properties of the signal, for example, to ensure that the out-of-band signal energy is sufficiently low.

[0123] The process is then repeated starting from the inverse Fourier transform block S30 until a satisfactory solution is found, or until a maximum number of iterations is reached.

[0124] Although multiple solutions are possible, two embodiments of this nonlinear precoding process will now be described. In the first embodiment, the goal is to minimize the maximum amplitude of the time domain signal. In the second embodiment, the goal is to minimize the maximum amplitude of the time domain signal under the constraint of the maximum-minimum ratio of the signal envelope. As previously described, in both cases, the amplitude characteristics of the precoded time domain signal make the signal suitable for RF chain hardware with reduced cost or increased efficiency.

[0125] Algorithm 1: Minimize the maximum signal amplitude under spectral constraints

[0126] The objectives and constraints motivating Algorithm 1 are:

[0127]

[0128]

[0129]

[0130]

[0131] X=AF

[0132] In other words, the goal is to minimize the peak amplitude of the time domain signal A under the frequency domain constraint. The signal H received by the kth UE k [n]x[n] should be close to the expected data constellation value u k [n]. For empty tone groups and out-of-band tone groups The average transmitted energy on the tone groups shall not exceed ε e and ε o Here, the out-of-band energy requirement is very small in order to enforce the regulator constraint on ACLR.

[0133] To solve the specified convex optimization problem, Algorithm 1 adopts iterations as shown below.

[0134] enter : For each For every n∈G(g), given H[n],u[n].

[0135] initialization : For each For every n∈G(g), let x[n]=H[n] H (H[n]H[n] H) -1 u[n], and for all other subcarriers, let x[n] = 0. Then we form X = [x[1], x[2], ... x[Nθ]], and A[0] = XF, i.e., A[0] is the inverse Fourier transform of the linear zero-forcing solution.

[0136] Iteration : Repeat the following iterative steps, starting from k = 0:

[0137] 1) B[k] = prox f2 (A[k]); (i.e., projection in the frequency domain to satisfy the constraint)

[0138] 2) Select the proximal scaling factor γ[k] (see explanation below)

[0139] 3) C[k]=prox γ[k]f1 (2B[k]-A[k]); (i.e., using the impulse parameter γ[k] to apply the limit)

[0140] 4) A[k+1]=A[k]-B[k]+C[k]

[0141] After some iterations, the output is B[k], a valid solution that satisfies the frequency domain constraints.

[0142] Note that A[k], B[k], and C[k] are complex-valued matrices Mx(Nθ).

[0143] To define the function prox mentioned in step 3) γ[k]f1 , we first define the clipping function for complex a and non-negative c Then B = prox γf1 (A) means applying the clipping function to each element of A, i.e. B mt =h(A mt , c * ), where the clipping threshold c is selected * To meet ∑ m，t |A mt -h(A mt , c * )|=γ.

[0144] The operator B=prox mentioned in step 1 f2 (A) Implement frequency domain constraints. They are defined as follows.

[0145] Given an input matrix A

[0146] Calculate the Fourier transform X = AF H

[0147] For each antenna m and each definition Then set (Energy constraints are enforced for empty tone groups)

[0148] For each antenna m and each definition Then set (Energy constraints are enforced for out-of-band tone groups)

[0149] For each user k and each definition Then set H[n] + =H[n] H (H[n]H[n] H ) -1 , z[n]=H[n] + u[n], then for each antenna m, let y m [n] = x m [n]-(1-λ g )H[n] + H[n](x m [n]-z m [n]) (implementing sufficiently small reception error).

[0150] Finally, calculate the inverse Fourier transform B=YF.

[0151] Choice of scaling factor γ[k]: In step 2, we choose the scaling factor to be applied in step 3. Different approaches can be used, and this may affect the speed at which the algorithm converges. One possibility is to use a fixed step size. One approach that seems to lead to fast convergence is

[0152]

[0153] Algorithm 2: Minimize the maximum signal amplitude under spectrum constraints and envelope ratio constraints.

[0154] The goal of the algorithm can be specified as follows:

[0155] The objectives and constraints motivating Algorithm 1 are:

[0156]

[0157]

[0158]

[0159]

[0160]

[0161] X=AF

[0162] The only difference between this problem and the one solved by Algorithm 1 is the requirement that the ratio of the maximum and minimum amplitudes does not exceed a specified ratio β. This requirement makes the problem non-convex. However, modifications to Algorithm 1 seem to be able to find reasonable solutions in practice. By setting β = 1, the algorithm looks for a constant envelope signal, suitable for RF chains with only phase modulation. Setting β > 1 produces a signal with a limited dynamic range envelope.

[0163] The algorithm is as follows.

[0164] enter : For each For every n∈G(g), given H[n],u[n].

[0165] initialization : For each For every n∈G(g), let x[n]=H[n] H (H[n]H[n] H ) -1 u[n], and for all other subcarriers, let x[n] = 0. Then we form X = [x[1], x[2], ... x[Nθ]], and A[0] = XF, i.e., A[0] is the inverse Fourier transform of the linear zero-forcing solution.

[0166] Iteration : Repeat the following iterative steps, starting from k = 0:

[0167] 1) B[k] = prox f2 (A[k]); (i.e., projection in the frequency domain to satisfy the constraint)

[0168] 2) Set Set minimum threshold

[0169] 3) Select the proximal scaling factor γ[k] (same as Algorithm 1).

[0170] 4) Let Z[k] = 2B[k] - A[k], and limit the lower part to

[0171] 5) C[k]=prox γ[k]f1 (Z′[k]); (i.e., applying the above clipping using the impulse parameter γ[k])

[0172] 6) A[k+1]=A[k]-B[k]+C[k]

[0173] 7) Check feasible solutions: Set threshold Clipping is performed in the upper and lower parts.

[0174] Check whether X′=A′F is satisfied by ε u , ε e , and ε o Spectral constraints specified. If yes, exit with solution A'.

[0175] 8) Otherwise, ignore A'[k+1] and continue from step 1 using A[k+1]

[0176] This process continues until the maximum number of iterations is reached without finding a feasible solution, or until step 7 exists with a feasible solution A'.

[0177] result

[0178] Figure 6 and Figure 7 It is shown that when M=64 RF chains, K=8 UEs, the oversampling factor is θ=8, and the null tone fraction is Numerical results are obtained for the case of . Algorithm 2 is applied to 100 channel and user data instances with different envelope ratio constraints β and the power penalty is calculated for each instance. The power penalty is given as the ratio of the average transmit power of the nonlinear precoded signal A compared to the linear precoded signal A[0]. This power penalty is plotted as a ring width between 0.05 and 0.5 function, that is, ranging from 1.1 to 3 The number of iterations of Algorithm 2 required to obtain a feasible solution is Figure 7 As shown. A larger ring width w helps the algorithm converge faster while making the hardware implementation more challenging. By varying parameters such as the null fraction and the ring width, a trade-off between optimizing performance and efficiency can be sought in hybrid array design.

[0179] As an additional embodiment, we consider a configuration in which the linear RF transmission chain 30A operates over the full signal bandwidth and therefore utilizes the same subcarriers as the nonlinear RF transmission chain 30B. The RF transmission chain 30A can contribute to data traffic (i.e., PDCCH and PDSCH) together with the nonlinear linear RF transmission chain 30B because the linear RF transmission chain 30A can mimic the signal generated by the nonlinear linear RF transmission chain 30B. In this embodiment, the nonlinear precoder 70 treats the linear RF transmission chain 30A as an additional nonlinear RF transmission chain 30B and jointly precodes on both of them.

[0180] In addition, the linear RF transmission chain 30A may also be used to transmit synchronization and broadcast channels as required for initial access and basic network operation. These signals and channels must be transmitted using wider beams to ensure good cell coverage and therefore cannot utilize array gain from an array with many antennas. In this embodiment, the nonlinear precoder 70 ensures that signals sent by the nonlinear RF transmission chain 30B do not interfere with signals from the linear RF transmission chain 30A. It will be appreciated that the nonlinear RF transmission chain 30B may be implemented using a linear precoder paired with a digital predistortion module (in the nonlinear Tx RF chain) having a lower complexity than the digital predistortion module of the linear RF transmission chain 30A to provide a lower complexity digital predistortion than the linear RF transmission chain 30A.

[0181] Furthermore, as mentioned above, it is important to note that the two arrays (i.e., the linear array and the non-linear array) do not have to be co-located; that is, the linear array can be mounted on a cell tower, while the non-linear array (serving as a data booster) can be deployed at some other location inside the same cell. This setup makes sense especially for situations where the cell tower is already overcrowded.

[0182] Those skilled in the art will readily recognize that the steps of various above-described methods can be performed by a programmed computer. Herein, some embodiments are also intended to cover program storage devices, such as digital data storage media, which are machine or computer readable and encode machine-executable or computer-executable instruction programs, wherein the instructions perform some or all of the steps of the above-described methods. The program storage device can be, for example, a digital memory, a magnetic storage medium such as a disk and tape, a hard drive, or an optically readable digital data storage medium. The embodiments are also intended to cover computers programmed to perform the steps of the above-described methods.

[0183] Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as set forth in the claims.

[0184] Features described in the preceding description may be used in combinations other than those explicitly described.

[0185] Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

[0186] Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

[0187] While an effort has been made in the foregoing description to call attention to those features of the present invention regarded as particularly important, it should be understood that the applicants claim protection for any patentable feature or combination of features hereinbefore mentioned and / or illustrated in the accompanying drawings, whether or not particular emphasis has been placed thereon.

Claims

1. A device for communication, comprising: a first plurality of first radio frequency chains (30B); a second plurality of second radio frequency chains (30A), the first radio frequency chains (30B) being configured to produce wider sideband emissions than the second radio frequency chains (30A); At least one antenna array (10) comprising antenna elements (20A, 20B), Each antenna element of the first plurality of antenna elements (20B) is coupled to a corresponding first radio frequency chain of the first plurality of first radio frequency chains (30B), the first plurality of first radio frequency chains (30B) being configured to transmit primarily in the channel (B S ) in the first frequency band (40), Each antenna element of the second plurality of antenna elements (20A) is coupled to a corresponding one of the second plurality of second radio frequency chains (30A), the second plurality of second radio frequency chains (30A) being configured to transmit primarily over the channel (B S ) in at least one second frequency band (50).

2. The apparatus according to claim 1, wherein the first frequency band (40) includes at least the channel (B S ) within the center frequency band (B NL ), and the second frequency band includes the channel (B S ) within the sideband (B L ).

3. The apparatus of claim 2, wherein the first plurality of first radio frequency chains (30B) comprises a first digital predistortion module, and the second plurality of second radio frequency chains (30A) comprises a second digital predistortion module, the second digital predistortion module being configured to perform digital predistortion of higher complexity than the first digital predistortion module.

4. The device according to claim 2, wherein the center frequency band (B NL ) includes at least the channel (B S ) within the first group of central subcarriers, and the sidebands (B L ) includes the channel (B S ) within the first group of adjacent second group of subcarriers.

5. The apparatus according to claim 4, wherein the second plurality of second radio frequency chains (30A) are configured to transmit mainly in the channel (B S ) within a pair of sidebands (B L / 2), and the second group of subcarriers is within the channel (B S ) around the first group of center subcarriers within.

6. The apparatus of claim 5, wherein the first set of subcarriers comprises different subcarriers than the second set of subcarriers.

7. The apparatus of claim 5, wherein each of the first plurality of first radio frequency chains (30B) comprises a nonlinear precoder (70), and wherein the nonlinear precoder (70) is configured to cause transmission to occur at a power level less than a second threshold amount over the channel (B S ), and the nonlinear precoder (70) is configured to cause the transmission to be transmitted at a power less than a first threshold amount at the sideband (B L ) within. The apparatus of claim 7 , wherein the first threshold amount is greater than the second threshold amount.

9. The apparatus according to claim 7, wherein the nonlinear precoder (70) is configured to provide a desired receiver with a signal in the center frequency band (B NL ) for transmission, and the nonlinear precoder (70) is configured to be away from the intended recipient in the sideband (B L / 2) to perform beamforming on the transmission.

10. The apparatus according to any one of claims 5 to 9, wherein each of the second plurality of second radio frequency chains (30A) comprises a linear precoder, and wherein the linear precoder is configured to cause transmission to be transmitted at a power less than a third threshold amount in the center frequency band (B NL ), and the linear precoder is configured to cause the transmission to be performed at a power less than the second threshold amount on the channel (B S ) outside.

11. The apparatus of claim 10, wherein the linear precoder is configured to provide a linear precoder to an intended recipient at the sideband (B L / 2) to perform beamforming on the transmission.

12. The apparatus according to any one of claims 5 to 9, comprising a pair of antenna arrays (10), a first antenna array in the pair of antenna arrays (10) comprising the first plurality of antenna elements (20B), and a second antenna array in the pair of antenna arrays (10) comprising the second plurality of antenna elements (20A).

13. A base station comprising an apparatus according to any preceding claim.

14. A method for communication, comprising: Providing a first plurality of first radio frequency chains (30B), a second plurality of second radio frequency chains (30A) and at least one antenna array (10) comprising antenna elements (20A, 20B); coupling each antenna element of the first plurality of antenna elements (20B) to a corresponding first radio frequency chain of the first plurality of first radio frequency chains (30B), and coupling each antenna element of the second plurality of antenna elements (20A) to a corresponding second radio frequency chain of the second plurality of second radio frequency chains (30A); and The first radio frequency chain (30B) is configured to generate a wider sideband emission than the second radio frequency chain (30A), and the first plurality of first radio frequency chains (30B) enable transmission mainly in the channel (B S ) within a first frequency band (40), and the second plurality of second radio frequency chains (30A) enable transmission primarily within the channel (B S ) in at least one second frequency band (50).

15. A computer program product, comprising instructions for causing an apparatus to perform at least the following operations: The first radio frequency chain (30B) is configured to generate a wider sideband emission than the second radio frequency chain (30A), and the first plurality of first radio frequency chains (30B) are configured to transmit mainly in the channel (B S ), and configuring the second plurality of second radio frequency chains (30A) so that transmission is mainly in the first frequency band (40) within the channel (B S ) in at least one second frequency band (50).

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