Patent Description:
This section is intended to provide a background to the various embodiments of the invention that are described in this disclosure. Therefore, unless otherwise indicated herein, what is described in this section should not be interpreted to be prior art by its mere inclusion in this section.

Traditionally, cellular networks comprise a set of base stations equipped with an array of co-located antenna elements, each forming one or multiple antenna ports. When a User Equipment (UE), or terminal, has data packets to receive in the downlink or transmit in the uplink, it is first associated with one of the base stations and then it is scheduled for transmission on a block of time-frequency resources. In these resource blocks, the serving base station array forms a beam towards the UE, with a spatial signature that is selected based on the spatial position of the UEs and other UEs that are active in the same block. The beam is typically selected to balance between high received signal power at the UE and little interference towards the other UEs that are active in the same block. Each base station and the UEs that it serves constitutes a cell and all desired transmission goes on within the cell. Resource allocation tasks, such as scheduling, power control, and assignment of pilot sequences, are also implemented on a per-cell basis.

An alternative approach to network deployment is to spread out the antennas over the coverage area, using many remote-radio heads, also known as Access Points (APs) or Antenna Processing Units (APUs). Different from traditional cellular networks, where the base stations are surrounded by UEs, the UEs will be surrounded by AP antennas that may all potentially serve them simultaneously. This enables a cell-free network operation where each UE is served by its preferred set of APUs. Large-scale deployment of such networks is known as "Cell-free Massive Multiple-Input Multiple-Output (MIMO)". The physical-layer processing is partially done locally at each APU, using uplink measurements from reference signals (pilots).

One way to deploy these cell-free massive MIMO networks is to use radio stripes, where multiple APUs are deployed along the same cable and thereby shares the same fronthaul connection. This may lead to less cabling compared to a star-topology where each APU has a dedicated fronthaul connection. Within the radio stripe, antennas and the associated APUs are serially located inside the same cable, which also provides time and frequency synchronization, data transfer, and power supply via a shared bus. The APU may comprise of antenna elements and circuit-mounted chips (including power amplifiers, phase shifters, filters, modulators, A/D and D/A converters) inside a protective casing of a flexible cable or a stripe. One or more APUs may also be implemented in a non-flexible radio stick that are serially connected to other radio sticks. Each radio stripe or set of radio sticks may then be connected to one or multiple Central Processing Units (CPUs). Since the total number of distributed antennas is assumed to be large, the transmit power of each antenna can be very low, resulting in low heat-dissipation, small volume and weight, and low cost. For example, if the carrier frequency is <NUM> then the antenna size is approximately <NUM> - <NUM>, and for higher carrier frequencies the antenna elements further decreased in size. Thus, the antennas and processing hardware can be easily fitted in a cable/stripe.

<NPL>, presents a precoding approach for a radio stripe system with daisy chain topology based on the distributed computation of zero-forcing precoding vectors at each of the antenna processing units of the system.

Radio stripe systems may facilitate a flexible and cost efficient implementation of a cell-free Massive Multiple-Input Multiple-Output (MIMO) deployment.

The receive/transmit processing of an antenna in a radio stripe system is performed right next to itself. On the transmitter side, each Antenna Processing Unit (APU) receives multiple streams of input data (e.g. one stream per User Equipment (UE), or terminal, one UE with multiple streams, or some other UE-stream allocation) from the previous APU via the shared bus. In each antenna, the input data streams are scaled with a precalculated precoding vector and the sum-signal is transmitted over the radio channel to the receiver(s). By exploiting channel reciprocity, the precoding vector may be a function of the estimated uplink channels. For example, if the conjugate of the estimated uplink channel is used, Maximum Ratio (MR) precoding is obtained. This precoding requires no Channel State Information (CSI) sharing between the antennas.

On the receiver side, the received radio signal is multiplied with the combining vector previously calculated in the uplink pilot phase. The output gives data streams that are then combined with the data streams received from the shared bus and sent again on the shared bus to the next APU.

Joint transmission from multiple distributed APUs is mainly beneficial, compared with single-APU transmission, if the transmission is carried out phase-coherently, so that an array gain is obtained. Methods developed for cell-free massive MIMO and radio stripes may be divided into two categories:.

Both the distributed per-APU processing and the centralized processing methods are methods with drawbacks. The distributed per-APU processing method has low performance, relative to what is obtainable by centralized processing, and the centralized processing method has huge, unscalable fronthaul capacity requirements. Accordingly, there is a need for a scalable solution that enables transmission in a distributed MIMO system that has interference suppression capabilities.

It is in view of the above background and other considerations that the various embodiments of the present disclosure have been made.

It is proposed to provide a solution to address this problem, i.e. implementing interference suppression in a sequential manner, by the APUs computing their local precoders sequentially during downlink data transmission.

This general object has been addressed by the appended independent claims. Advantageous embodiments are defined in the appended dependent claims.

The various proposed embodiments herein provide a scalable solution for enabling transmission in a distributed MIMO system that has interference suppression capabilities.

These and other aspects, features and advantages will be apparent and elucidated from the following description of various embodiments, reference being made to the accompanying drawings, wherein:.

The present invention will now be described more fully hereinafter. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those persons skilled in the relevant art. Like reference numbers refer to like elements throughout the description.

In one of its aspects, the disclosure presented herein concerns a method for sequential transmit precoding in a radio stripe system.

With reference to the <FIG> and <FIG>, a first embodiment will now be described. <FIG> illustrates a message sequence chart of a process for sequential transmit precoding in a radio stripe system. <FIG> illustrates a method <NUM>, performed by a first Antenna Processing Unit (APU), for sequential transmit precoding in a radio stripe system. The radio stripe system comprises at least two APUs and a Central Processing Unit (CPU). The at least two APUs are connected in series from the CPU. The radio stripe system serves at least two User Equipment (UEs), or terminals. <FIG> illustrate an example of such radio stripe system. As seen in <FIG>, each radio stripe may comprise multiple APUs, or Access Points (APs), deployed along the same fronthaul connection to a central unit, a cloud-processor, also called the CPU <NUM>. In some embodiments, the CPU <NUM> may be called a stripe station or a network node. The radio stripe system <NUM> may be comprised in a cell-free distributed (massive) Multiple-Input Multiple-Output (MIMO) network.

The method <NUM> starts at step <NUM> with the first APU <NUM> obtaining channel estimates for channels to the at least two UEs <NUM> served by the radio stripe system <NUM>. The obtained channel estimates may be Channel State Information (CSI) obtained from reference pilot signals transmitted by the at least two served UEs <NUM>. Based on the obtained channel estimates, a precoding filter is determined in step <NUM>. The precoding filter is going to be applied to signals that are to be transmitted from the first APU <NUM> to the at least two UEs <NUM>. The precoding filter may, for example, be generated by a method selected from the group comprised of: Maximum-Ratio Transmission (MRT), Zero-Forcing (ZF) precoding and Signal-to-interference-and-Leakage Ratio (SLNR) precoding. Alternatively, the precoding filter may be generated by another method.

Thereafter the method <NUM> continues at step <NUM> with determining effective precoded channels for the signals to the at least two UEs <NUM> based on the determined precoding filter and the obtained channel estimates. The effective precoded channels represent the effective channel created after the precoding filter being applied to each channel for the at least two UEs <NUM>.

At step <NUM>, the effective precoded channels are transmitted to at least one subsequent APU <NUM>. The at least one subsequent second APU <NUM> may be located further from the CPU <NUM> than the first APU <NUM>. If it is assumed that the radio stripe system <NUM> serves K number of UEs <NUM>, K<NUM> scalar coefficients will be transmitted to the at least one subsequent second APU <NUM>. The K<NUM> scalar coefficients represent the effective K×K channels created after application of the precoding filter for each of the K served UEs <NUM>.

The proposed method <NUM> is a scalable method that enables transmission in distributed MIMO systems that have interference suppression capabilities. The method <NUM> considers downlink transmission where the APUs have obtained channel estimates and utilizes this information to transmit payload data in the downlink. The aim is to precode the signals intended for the served UEs in such way that the collectively transmitted signals by all APUs achieve interference suppression, without gathering all the channel estimates at the CPU or some other central entity in the network. The proposed method <NUM> achieves this by using the sequential topology of the fronthaul in radio stripe systems <NUM> to implement interference suppression in a sequential manner. The first APU <NUM> makes a local decision based on its locally obtained channel estimation and thereafter transmits this information to at least one second APU <NUM>. This will enable suppression of the interference that the first APU <NUM> may cause the at least one second APU <NUM>. The proposed method <NUM> overcomes the drawbacks that existing solutions have, which are that the existing solutions either are distributed, but lacks interference suppression capability, or support interference suppression, but require a centralized implementation with heavy fronthaul traffic. The proposed method <NUM> may greatly increase the achievable rates, since cell-free networks typically operate at high Signal-to-Noise-Ratio (SNR) where the system performance is interference limited.

In some embodiments, the radio stripe system <NUM> may have a tree structure. An example of such tree structure is illustrated in <FIG>. The first APU <NUM> may then be connected to two subsequent second APUs <NUM>. These two subsequent APUs <NUM> are then located further away from the CPU <NUM> than the first APU <NUM>. When the radio stripe system <NUM> has a tree structure, the effective precoded channels may then be transmitted to both these subsequent second APUs <NUM>. This enables the use of the proposed method <NUM> with flexible radio stripe systems <NUM>, which radio stripe systems <NUM> may take several different forms.

In some embodiments, the method <NUM> may further comprise step <NUM> of receiving data signals from the CPU <NUM> and the step <NUM> of applying the determined precoding filter to the received data signals to generate precoded signals. Thereafter, the method may comprise the step <NUM> of transmitting the precoded signal to the at least two UEs <NUM> served by the radio stripe system <NUM>. Thereby, with the proposed method <NUM>, there will be no interference between the served UEs <NUM> as the first APU <NUM> will transmit signals that have been applied with the determined precoding filter.

In some embodiments, each of the at least two APUs <NUM>,<NUM> in the radio stripe system <NUM> may be equipped with one antenna. In other embodiments, at least one of the at least two APUs <NUM>,<NUM> may have multiple antennas.

According to a second aspect, there is provided a method, performed by a second APU for sequential transmit precoding in a radio stripe system <NUM>.

The method <NUM> is now going to be described with reference to the <FIG> and <FIG>. As previously mentioned, <FIG> illustrates a message sequence chart of a process for sequential transmit precoding in a radio stripe system <NUM>. <FIG> illustrates the method <NUM>, performed by the second APU <NUM>, for sequential transmit precoding in a radio stripe system <NUM>. The radio stripe system <NUM> comprises at least two APUs <NUM>,<NUM> and a CPU <NUM>, the at least two APUs <NUM>,<NUM> being connected in series from the CPU <NUM>. The radio stripe system <NUM> serves at least two UEs <NUM>. <FIG> illustrate an example of such radio stripe system <NUM>.

The method <NUM> starts at step <NUM> with obtaining channel estimates for channels to the at least two UEs <NUM>. The obtained channel estimates may be CSI obtained from reference pilot signals transmitted by the at least two UEs <NUM> served by the radio stripe system <NUM>. The method continues with step <NUM> of receiving effective precoded channels from at least one preceding first APU <NUM>. The at least one preceding first APU <NUM> may be located closer to the CPU <NUM> than the second APU <NUM>. In one embodiment, there may be one preceding first APU <NUM>. In another embodiment, there may be multiple preceding first APUs <NUM>.

Thereafter, the method continues with step <NUM> of determining a precoding filter based on the obtained channel estimates and the received effective precoded channels from said at least one preceding first APU <NUM>. The precoding filter is going to be applied to signals that are to be transmitted from the second APU <NUM> to the at least two UEs <NUM> served by the radio stripe system <NUM>. The precoding filter may be generated by a method selected from the group comprised of: MRT, ZF precoding and SLNR precoding. Alternatively, the precoding filter may be generated by some other method.

According to the method <NUM>, each APU <NUM>,<NUM> makes a local decision based on its locally obtained channel estimates and fuses it with information received from previous APUs regarding how strong desired and interfering signal that these will jointly provide. When the second APU <NUM> receives input from at least one preceding first APU <NUM> that is closer to the CPU <NUM>, along the same radio stripe, the second APU <NUM> may combine the received information with its own local information. This combined information will be used when transmitting payload data in the downlink. This will enable suppression of the interference that preceding APUs are causing. If it is assumed that the second APU <NUM> comprises M antennas, the second APU <NUM> may determine an M-dimensional precoding filter, but the processing will consider an M+<NUM> antenna system. The additional dimension describes the joint effect of the precoding already applied at preceding APUs. After the processing, the second APU may inform the next, e.g. a third APU <NUM>, about the combined effect of all the precoding that has been determined so far.

The proposed method <NUM> overcomes the drawbacks that existing solutions have, which are that the existing solutions either are distributed, but lacks interference suppression capability, or support interference suppression, but require a centralized implementation with heavy fronthaul traffic. With the proposed method <NUM>, it is not necessary to gather all the channel estimates at the CPU <NUM> or some other central entity in the network. Instead, each APU will receive relevant information from preceding APUs and makes its decision based on this. The proposed method <NUM> may greatly increase the achievable rates, since cell-free networks typically operate at high Signal-to-Noise-Ratio (SNR) where the system performance is interference limited.

In some embodiments, the method <NUM> may further comprise the step <NUM> of receiving data signals from the CPU <NUM> and the step <NUM> of applying the determined precoding filter to the received data signals to generate a precoded signal. The method <NUM> may further comprise the step <NUM> of transmitting the precoded signal to the at least two UEs <NUM> served by the radio stripe system <NUM>. Thus, with the proposed method <NUM>, there will be no interference between the served UEs <NUM> as the second APU <NUM> will transmit signals that have been applied with the determined precoding filter which takes both local information and information received from at least one first APU <NUM> into account.

In some embodiments, the method <NUM> may further comprise the step <NUM> of determining effective precoded channels for the signals to the at least two UEs <NUM> based on the determined precoding filter and the obtained channel estimates. The effective precoded channels represent the effective channel created after the precoding filter being applied to each channel for the at least two UEs <NUM>. The method <NUM> may further comprise the step <NUM> of transmitting the effective precoded channels to at least one subsequent third APU <NUM> located further from the CPU <NUM>.

If it is assumed that the radio stripe system <NUM> serves K number of UEs <NUM>, K<NUM> scalar coefficients will be transmitted to the at least one subsequent third APU <NUM>. The K<NUM> scalar coefficients represent the effective K×K channel results after application of the precoding filter for each of the K served UEs <NUM>. The number of variables that needs to be sent from the second APU <NUM> to the at least one subsequent third APU <NUM> may be independent of how many APUs there are on a radio stripe. If a sequence of data symbols should be transmitted in the same transmission block, the precoding may be the same for all of them. Thus, the K<NUM> effective desired and interfering scalar channels may be the same throughout the block. Accordingly, the proposed method is scalable and not dependent on how many APUs that are connected within the radio stripe system <NUM>.

In some embodiments, only a subset of the APUs <NUM>,<NUM>,<NUM> within the radio stripe system <NUM> may be transmitting to each UEs <NUM> and only this subset of APUs may determine precoding for this UE.

The above proposed methods <NUM>,<NUM> are now going to be described together with a non-limiting example arrangement. The example arrangement is illustrated in <FIG>. The arrangement in <FIG> comprises two M-antenna APUs, the first APU <NUM> and the second APU <NUM>. The arrangement further comprises two single-antenna UEs, the first UE <NUM> and the second UE <NUM>. The received signal at the first UE <NUM> is <MAT> where h<NUM> is the M-dimensional channel vector, i.e. the channel estimates, from the first APU <NUM> to the first UE <NUM> and h<NUM> is the M-dimensional channel vector from the second APU <NUM> to the first UE <NUM>, while n<NUM> is the M-dimensional noise vector.

The first APU <NUM> selects its M-dimensional transmit signal w<NUM> such that it comprises the unit-power data signal s<NUM> intended for the first UE <NUM> and the unit-power data signal s<NUM> intended for the second UE <NUM>. Power control will be achieved through scaling of the beamforming vectors. In this exemplary embodiment, linear precoding is used, where <MAT> with v<NUM> being the M-dimensional precoding vector, or precoding filter, for the first UE <NUM> and v<NUM> the M-dimensional precoding vector, or filter, for the second UE <NUM>. The first APU <NUM> may apply a local algorithm to select these vectors based on only its local channel estimates. Consequently, the received signals at the two UEs <NUM>,<NUM> will now be <MAT> <MAT> where the notation <MAT> is introduced for i, k = <NUM>,<NUM>. The first APU <NUM> may send these K<NUM> = <NUM> scalar parameters to the subsequent second APU <NUM>. The second APU <NUM> is further away from the CPU <NUM>, but located on the same radio stripe, along the same fronthaul connection. In this example, it is assumed that the first APU <NUM> may have perfect knowledge of the channel vectors, but when that is not the case, the first APU <NUM> may use estimates instead.

The second APU <NUM> may also apply linear precoding with <MAT> with v<NUM> being the M-dimensional precoding vector, or filter, for the first UE <NUM> and v<NUM> is the M-dimensional precoding vector, or filter, for the second UE <NUM>. The received signals at the two UEs <NUM>,<NUM> will now be <MAT> <MAT>.

The second APU <NUM> may now select v<NUM> and v<NUM> while knowing the contribution that the first APU <NUM> have already made. For example, it may apply a local zero-forcing solution by selecting v<NUM> such that the interference term caused to the second UE <NUM> is <MAT>. It may simultaneously select v<NUM> such that the interference term caused to the first UE <NUM> is <MAT>. With this choice of v<NUM> and v<NUM>, the received signals at the two UEs become <MAT> <MAT> and there is no interference between the two UEs <NUM>,<NUM>. The zero-forcing conditions may be satisfied by selecting any preliminary vectors ṽ<NUM> and ṽ<NUM> (e.g., using maximum ratio transmission or some other method) and then setting v<NUM>= a<NUM>ṽ<NUM> and v<NUM> = a<NUM>ṽ<NUM> where <MAT>.

When there are more than two APUs and two UEs along the radio stripe and/or strict power constraints, it might be undesirable or difficult to cancel all the interference at every APU. In that, case signal-to-interference-and-leakage ratio (SLNR) precoding or similar MMSE-like precoding methods can be used instead. In that case, the second APU <NUM> may select v<NUM> and v<NUM> to maximize <MAT> respectively, where σ<NUM> is the noise variance. In some embodiments the noise variance may be thermal noise, in some embodiments the noise variance may be an estimate of thermal noise plus interference level from other transmitters. This value may for example be derived from measurements on ZP CSI-RS assigned to the UEs.

According to a third aspect of the present disclosure, there is provided a first APU <NUM> for performing the method <NUM> according to the first aspect.

The first APU <NUM> is now going to be described with reference to <FIG>. The first APU <NUM> may be used in, but are not limited to, a radio stripe system <NUM> such as illustrated in <FIG>.

The first APU <NUM> is configured for sequential transmit precoding in a radio stripe system <NUM>. The radio stripe system <NUM> comprises at least two APUs <NUM>,<NUM> and a CPU <NUM>. The at least two APUs <NUM>,<NUM> are connected in series to the CPU <NUM>. The radio stripe system <NUM> serves at least two UEs <NUM>. As illustrated in <FIG>, the first APU <NUM> comprises a processing circuitry <NUM> and a memory circuitry <NUM>.

Additionally, or alternatively, the first APU <NUM> may further comprise a transmitter, or a transmitting circuitry <NUM>, configured to transmit data to other apparatuses, such as the at least one subsequent second APU <NUM>.

Additionally, or alternatively, the first APU <NUM> may further comprise a receiver, or a receiving circuitry <NUM>, configured to receive data from other apparatuses, such as the CPU <NUM> or at least one UE.

The memory circuitry <NUM> stores computer program code which, when run in the processing circuitry <NUM>, causes the first APU <NUM> to obtain channel estimates for channels to the at least two UEs and to determine a precoding filter based on the obtained channel estimates. The obtained channel estimates may be CSI obtained from reference pilot signals transmitted by the at least two UEs <NUM> served by the radio stripe system <NUM>. The precoding filter is going to be applied to signals that are to be transmitted from the first APU <NUM> to the at least two UEs <NUM>. The precoding filter may be generated by a method selected from the group comprised of: MRT, ZF precoding and SLNR precoding. The first APU <NUM> is further caused to determine effective precoded channels for the signals to the at least two UEs <NUM> based on the determined precoding filter and the obtained channel estimates. The effective precoded channels represent the effective channel created after the precoding filter being applied to each channel for the at least two UEs. Thereafter, the first APU <NUM> is caused to transmit the effective precoded channels to at least one subsequent second APU <NUM>. The at least one subsequent second APU <NUM> may be located further from the CPU <NUM> than the first APU <NUM> located further from the CPU <NUM>.

In some embodiments, each of the at least two APUs <NUM>,<NUM> in the radio stripe system <NUM> may be equipped with one antenna.

In some embodiments, the radio stripe system <NUM> may have a tree structure and the first APU <NUM> is connected to two subsequent second APUs <NUM> located further away from the CPU <NUM>. The effective precoded channels may be transmitted to both subsequent second APUs <NUM>.

According to a fourth aspect, there is provided a second APU <NUM> for implementing the method according to the second aspect.

The second APU <NUM> is now going to be described with reference to <FIG>. The second APU <NUM> may be used in, but are not limited to, a radio stripe system <NUM> such as illustrated in <FIG>.

The second APU <NUM> is configured for sequential transmit precoding in a radio stripe system <NUM>. The radio stripe system <NUM> comprises at least two APUs <NUM>,<NUM> and a CPU <NUM>, the at least two APUs <NUM>,<NUM> being connected in series to the CPU <NUM>. The radio stripe system <NUM> serves at least two UEs <NUM>. As illustrated in <FIG>, the second APU <NUM> comprises a processor, or a processing circuitry <NUM>, and a memory, or a memory circuitry <NUM>.

Additionally, or alternatively, the second APU <NUM> may further comprise a transmitter, or a transmitting circuitry <NUM>, configured to transmit data to other apparatuses, such as the at least one subsequent third APU <NUM>.

Additionally, or alternatively, the second APU <NUM> may further comprise a receiver, or a receiving circuitry <NUM>, configured to receive data from other apparatuses, such as the at least one preceding first APU <NUM>.

The memory circuitry <NUM> storing computer program code which, when run in the processing circuitry <NUM>, causes the second APU <NUM> to obtain channel estimates for channels to the at least two UEs <NUM>. The obtained channel estimates may be CSI obtained from reference pilot signals transmitted by the served UEs <NUM>. The second APU <NUM> is further caused to receive, from at least one preceding first APU <NUM>, effective precoded channels. The at least one preceding first APU <NUM> may be located closer to the CPU <NUM> than the second APU <NUM>. The memory circuitry <NUM> further stores computer program code which, when run in the processing circuitry <NUM>, causes the second APU <NUM> to determine a precoding filter based on the obtained channel estimates and the received effective precoded channels from said at least one preceding first APU <NUM>. The precoding filter is going to be applied to signals that are to be transmitted from the second APU <NUM> to the at least two UEs <NUM>. The precoding filter may be generated by a method selected from the group comprised of: MRT, ZF precoding and SLNR precoding.

In some embodiments, the memory circuitry <NUM> storing computer program code which, when run in the processing circuitry <NUM>, may further cause the second APU <NUM> to determine effective precoded channels for the signals to the at least two UEs <NUM> based on the determined precoding filter and the obtained channel estimates. The effective precoded channels represent the effective channel created after the precoding filter being applied to each channel for the at least two UEs. The second APU <NUM> may further be caused to transmit the effective precoded channels to at least one subsequent third APU <NUM> located further from the CPU <NUM>.

In some embodiments, each of the at least two APUs <NUM>,<NUM> in the radio stripe system <NUM> may be equipped with one antenna. In other embodiments, at least one of the at least two APUs <NUM>,<NUM> may be equipped with multiple antennas.

According to a fifth aspect, there is provided a computer program comprising instructions which, when executed on a processing circuitry, cause the processing circuitry to carry out the method according to the first aspect and/or the second aspect.

According to an sixth aspect, there is provided a carrier containing the computer program of the fifth aspect, wherein the carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.

In the following, a non-limiting example of the proposed solution will now be described.

A radio stripe with <NUM> single-antenna equal-spaced APUs may be deployed on a <NUM>-meter-long wall. There may be a fronthaul connection between each of the APUs. A CPU may be located at one of the ends of the stripe, the setup is symmetric so it could be at any of the ends. Three UEs may be located <NUM> meters from the wall and may be <NUM> or <NUM> meters from each other, as shown in <FIG>. The propagation channels may be computed using free-space line-of-sight propagation modelling at a <NUM> carrier frequency with no reflections. Each APU may have perfect knowledge of the channel from each of the UEs, but no channel state information is communicated to or from the other APUs.

In the following, the proposed solution is compared with two baseline methods during downlink data transmission. These two baseline methods are known in the art and are:.

These baseline methods may be compared with the proposed solution, wherein in this example each APU may use local SLNR precoding. The proposed solution is here referred to as "sequential SLNR" or "seq-SLNR.

Error! Reference source not found. shows the achievable rates with the proposed solution and the two previously described known methods, as a function of the average downlink SNR. As seen in <FIG>, the performance with the distributed MRT method saturates at high SNR, due to its lack of interference suppression capabilities. In contrast, the centralized method provides an achievable rate that grows linearly with the SNR in dB-scale, as expected from a method that provides interference suppression and has access to high-quality CSI. Note that in this short-range scenario, an SNR of <NUM>-<NUM> dB is achieved even when the transmit power is very low.

The proposed solution combines the benefits of the two baseline methods. The proposed solution has a distributed implementation but provides a rate comparable to that of the centralized method, up to a rate of <NUM> bit/s/Hz. With a fine-tuned local precoding selection, the rate can increase linearly with the SNR in dB-scale.

Although the subject matter described herein may be implemented in any appropriate type of system using any suitable components, the embodiments described herein relate to a wireless network, such as the example wireless communication network illustrated in <FIG>. For simplicity, the wireless communication network of <FIG> only depicts network <NUM>, network nodes <NUM> and 1060b, and Wireless Devices (WDs) <NUM>, 1010b, and 1010c. The wireless communication network may further include any additional elements suitable to support communication between wireless devices or between a wireless device and another communication device, such as a landline telephone. Of the illustrated components, network node <NUM> and wireless device (WD) <NUM> are depicted with additional detail. The illustrated wireless communication network may provide communication and other types of services to one or more wireless devices to facilitate the wireless devices' access to and/or use of the services provided by the wireless communication network.

The wireless communication network may comprise and/or interface with any type of communication, telecommunication, data, cellular, and/or radio network or other similar type of system. In some embodiments, the wireless communication network may be configured to operate according to specific standards or other types of predefined rules or procedures. Thus, particular embodiments of the wireless communication network may implement communication standards, such as Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE), and/or other suitable <NUM>, <NUM>, <NUM>, or <NUM> standards; wireless local area network (WLAN) standards, such as the IEEE <NUM> standards; and/or any other appropriate wireless communication standard, such as the Worldwide Interoperability for Microwave Access (WiMax), Bluetooth, and/or ZigBee standards.

These components may work together in order to provide network node and/or wireless device functionality, such as providing wireless connections in a wireless network. In different embodiments, the wireless network may comprise any number of wired or wireless networks, network nodes, base stations, controllers, wireless devices, relay stations, and/or any other components that may facilitate or participate in the communication of data and/or signals whether via wired or wireless connections.

As used herein, network node refers to equipment capable, configured, arranged and/or operable to communicate directly or indirectly with a wireless device and/or with other network nodes or equipment in the wireless communication network to enable and/or provide wireless access to the wireless device and/or to perform other functions (e.g., administration) in the wireless communication network. Examples of network nodes include, but are not limited to, access points (APs) (e.g., radio access points), base stations (BSs) (e.g., radio base stations, Node Bs, and evolved Node Bs (eNBs)). As another example, network node <NUM> may be a virtual network node as described in more detail below. More generally, however, network nodes may represent any suitable device (or group of devices) capable, configured, arranged, and/or operable to enable and/or provide a wireless device with access to the wireless communication network or to provide some service to a wireless device that has accessed the wireless communication network.

In <FIG>, Network node <NUM> includes processing circuitry <NUM>, device readable medium <NUM>, interface <NUM>, user interface equipment <NUM>, auxiliary equipment <NUM>, power source <NUM>, power circuitry <NUM>, and antenna <NUM>. Although network node <NUM> illustrated in the example wireless communication network of <FIG> may represent a device that includes the illustrated combination of hardware components, other embodiments may comprise network nodes with different combinations of components. It is to be understood that a network node may comprise any suitable combination of hardware and/or software needed to perform the tasks, features, functions and methods disclosed herein.

In certain embodiments, some or all of the functionality described herein as being provided by a network node, base station, eNB or other such network device may be provided by processing circuitry <NUM> executing instructions stored on device readable medium <NUM> or memory within processing circuitry <NUM>.

Interface <NUM> is used in the wired or wireless communication of signaling and/or data between network node <NUM>, network <NUM>, and/or WDs <NUM>. As illustrated, interface <NUM> comprises port(s)/terminals(s) <NUM> to send and receive data, for example to and from network <NUM> over a wired connection.

As used herein, wireless device (WD) refers to a device capable, configured, arranged and/or operable to communicate wirelessly with network nodes and/or other wireless devices. Unless otherwise noted, the term WD may be used interchangeably herein with user equipment (UE). Communicating wirelessly may involve transmitting and/or receiving wireless signals using electromagnetic waves, radio waves, infrared waves, and/or other types of signals suitable for conveying information through air. In some embodiments, a WD may be configured to transmit and/or receive information without direct human interaction. For instance, a WD may be designed to transmit information to a network on a predetermined schedule, when triggered by an internal or external event, or in response to requests from the network. Examples of a WD include, but are not limited to, a smart phone, a mobile phone, a cell phone, a voice over IP (VoIP) phone, a wireless local loop phone, a desktop computer, a personal digital assistant (PDA), a wireless cameras, a gaming console or device, a music storage device, a playback appliance, a wearable terminal device, a wireless endpoint, a mobile station, a tablet, a laptop, a laptop-embedded equipment (LEE), a laptop-mounted equipment (LME), a smart device, a wireless customer-premise equipment (CPE), a vehicle-mounted wireless terminal device, etc.. A WD may support device-to-device (D2D) communication, for example by implementing a 3GPP standard for sidelink communication, vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), vehicle-to-everything (V2X) and may in this case be referred to as a D2D communication device. As yet another specific example, in an Internet of Things (IoT) scenario, a WD may represent a machine or other device that performs monitoring and/or measurements, and transmits the results of such monitoring and/or measurements to another WD and/or a network node. The WD may in this case be a machine-to-machine (M2M) device, which may in a 3GPP context be referred to as an MTC device. As one particular example, the WD may be a UE implementing the 3GPP narrow band internet of things (NB-IoT) standard. Particular examples of such machines or devices are sensors, metering devices such as power meters, industrial machinery, or home or personal appliances (e.g. refrigerators, televisions, etc.) personal wearables (e.g., watches, fitness trackers, etc.). In other scenarios, a WD may represent a vehicle or other equipment that is capable of monitoring and/or reporting on its operational status or other functions associated with its operation. A WD as described above may represent the endpoint of a wireless connection, in which case the device may be referred to as a wireless terminal. Furthermore, a WD as described above may be mobile, in which case it may also be referred to as a mobile device or a mobile terminal.

UE <NUM> may be any UE identified by the 3rd Generation Partnership Project (3GPP), including a NB-IoT UE, a machine type communication (MTC) UE, and/or an enhanced MTC (eMTC) UE. UE <NUM>, as illustrated in <FIG>, is one example of a WD configured for communication in accordance with one or more communication standards promulgated by the 3rd Generation Partnership Project (3GPP), such as 3GPP's GSM, UMTS, LTE, and/or <NUM> standards.

Network connection interface <NUM> may be configured to provide a communication interface to network 1143a. Network 1143a may encompass wired and/or wireless networks such as a local-area network (LAN), a wide-area network (WAN), a computer network, a wireless network, a telecommunications network, another like network or any combination thereof. For example, network 1143a may comprise a Wi-Fi network.

In <FIG>, processing circuitry <NUM> may be configured to communicate with network 1143b using communication subsystem <NUM>. Network 1143a and network 1143b may be the same network or networks or different network or networks. Communication subsystem <NUM> may be configured to include one or more transceivers used to communicate with network 1143b.

Network 1143b may encompass wired and/or wireless networks such as a local-area network (LAN), a wide-area network (WAN), a computer network, a wireless network, a telecommunications network, another like network or any combination thereof. For example, network 1143b may be a cellular network, a Wi-Fi network, and/or a near-field network. Power <NUM> source <NUM> may be configured to provide alternating current (AC) or direct current (DC) power to components of UE <NUM>.

Virtualization environment <NUM>, comprises general-purpose or special-purpose network hardware devices <NUM> comprising a set of one or more processors or processing circuitry <NUM>, which may be commercial off-the-shelf (COTS) processors, dedicated Application Specific Integrated Circuits (ASICs), or any other type of processing circuitry including digital or analogue hardware components or special purpose processors.

NFV may be used to consolidate many network equipment types onto industry standard high-volume server hardware, physical switches, and physical storage, which can be located in data centers, and customer premise equipment.

In some embodiments, some signaling can be affected with the use of control system <NUM> which may alternatively be used for communication between the hardware nodes <NUM> and radio units <NUM>.

Access network <NUM> comprises a plurality of base stations 1312a, 1312b, 1312c, such as NBs, eNBs, gNBs or other types of wireless access points, each defining a corresponding coverage area 1313a, 1313b, 1313c. Each base station 1312a, 1312b, 1312c is connectable to core network <NUM> over a wired or wireless connection <NUM>. A first UE <NUM> located in coverage area 1313c is configured to wirelessly connect to, or be paged by, the corresponding base station 1312c. A second UE <NUM> in coverage area 1313a is wirelessly connectable to the corresponding base station 1312a.

Connection <NUM> may be direct, or it may pass through a core network (not shown in <FIG>) of the telecommunication system and/or through one or more intermediate networks outside the telecommunication system.

It is noted that host computer <NUM>, base station <NUM> and UE <NUM> illustrated in <FIG> may be similar or identical to host computer <NUM>, one of base stations 1312a, 1312b, 1312c and one of UEs <NUM>, <NUM> of <FIG>, respectively.

Wireless connection <NUM> between UE <NUM> and base station <NUM> is in accordance with the teachings of the embodiments described throughout this disclosure. One or more of the various embodiments improve the performance of OTT services provided to UE <NUM> using OTT connection <NUM>, in which wireless connection <NUM> forms the last segment. More precisely, the teachings of these embodiments may improve the data rate and thereby provide benefits such as better responsiveness.

These functional units may be implemented via processing circuitry, which may include one or more microprocessor or microcontrollers, as well as other digital hardware, which may include Digital Signal Processors (DSPs), special-purpose digital logic, and the like. The processing circuitry may be configured to execute program code stored in memory, which may include one or several types of memory such as Read-Only Memory (ROM), Random-Access Memory (RAM), cache memory, flash memory devices, optical storage devices, etc. Program code stored in memory includes program instructions for executing one or more telecommunications and/or data communications protocols as well as instructions for carrying out one or more of the techniques described herein.

Claim 1:
A method (<NUM>), performed by a first Antenna Processing Unit, APU (<NUM>) for sequential transmit precoding in a radio stripe system (<NUM>), wherein the radio stripe system (<NUM>) comprises at least two APUs (<NUM>,<NUM>) and a Central Processing Unit, CPU (<NUM>), the at least two APUs (<NUM>,<NUM>) being connected in series from the CPU (<NUM>), wherein the radio stripe system (<NUM>) serves at least two User Equipment, UEs, (<NUM>), the method (<NUM>) comprising:
- obtaining (<NUM>) channel estimates for channels to the at least two UEs (<NUM>);
- determining (<NUM>) a precoding filter based on the obtained channel estimates, wherein the precoding filter is going to be applied to signals that are to be transmitted from the first APU (<NUM>) to the at least two UEs (<NUM>);
- determining (<NUM>) effective precoded channels for the signals to the at least two UEs(<NUM>) based on the determined precoding filter and the obtained channel estimates, wherein the effective precoded channels represent the effective channel created after the precoding filter being applied to each channel for the at least two UEs (<NUM>); and
- transmitting (<NUM>) the effective precoded channels to at least one subsequent second APU (<NUM>);
the method (<NUM>) being characterized in that the radio stripe system (<NUM>) have a tree structure, and wherein the first APU (<NUM>) is connected to two subsequent second APUs (<NUM>) located further away from the CPU (<NUM>), and wherein the effective precoded channels are transmitted to both subsequent second APUs (<NUM>), and
wherein the precoding filter is generated by signal-to-interference-and-leakage ratio, SLNR, precoding method.