WO2018121876A1

WO2018121876A1 - Fronthaul transmission

Info

Publication number: WO2018121876A1
Application number: PCT/EP2016/082926
Authority: WO
Inventors: Per-Erik Eriksson; Miguel Berg; Chenguang Lu; Daniel Cederholm
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2016-12-30
Filing date: 2016-12-30
Publication date: 2018-07-05

Abstract

The amount of data transmitted on a fronthaul connection between a radio equipment, RE and a radio equipment controller, REC may be reduced by transferring a signal received by the RE on a first antenna and for a signal received on a second antenna transferring a deviation from a determined relationship between signals on the first antenna and signals on the second antenna. The signal received by the RE on the second antenna is reconstructed in the REC based on the signal received by the RE on the first antenna, the relationship and the deviation.

Description

Fronthaul transmission

Technical field

The invention relates to methods, apparatuses, computer programs and computer program products for transmission of signals from Radio Equipment to a radio Equipment Controller.

Background

One of the methods to increase performance of a mobile system is to use multi-antenna techniques. Receive diversity, (Rx diversity) by using multiple receive antennas is one example. For the uplink transmission a typical scenario is that the User Equipment, UE, has one transmit antenna whereas the Radio Base Station, RBS, is equipped with multiple receive antennas, this is also defined as a SIMO (Single Input Multiple Output) system. The signals received at the RBS antennas consist (in the time domain) of the UE

transmitted signal convolved with the different impulse responses from the channels creating the multi-paths. The signals from each of the antennas are combined and different algorithms can be used to increase the performance, examples are Maximum Ratio Combining (MRC) and Interference Rejection Combining (IRC).

For the deployment scenario where the Radio Equipment, RE, (sometimes also denoted Radio Remote Unit, RRU) and the Radio Equipment Controller, REC (sometimes also denoted Base Band Unit, BBU) are separated, the signals received from different antennas have to be transported over the media that is connecting the RE with the REC as normally the signal combination is done at the REC. The interfaces that are used for the connection between the REC and the RE is called the fronthaul. The signals over the fronthaul could be complex time domain samples such as specified in the legacy Common Public Radio Interface, CPRI.

In the 5G architecture, a new frequency domain fronthaul interface will be specified. The frequency domain fronthaul is a functional split where the IFFT/FFT (Inverse Fast Fourier Transform/ Fast Fourier Transform) is moved from the REC to the RE. Frequency domain samples instead of time domain samples are sent over the fronthaul. The RE will through a communication channel have information about the resource allocation for different UEs.

The UE signals are power limited and as the path loss varies with the distance to the UE a large dynamic range is encountered when those signals are represented digitally, it may be assumed that for the complex frequency sample 10+ 10 bits will be required and in the case of MIMO (Multiple Input Multiple Output) /diversity layers the required fronthaul capacity will multiply with the number of antennas. As the capacity on the fronthaul is limited it is desired to find methods that can compress the data in order to optimize the usage of the fronthaul.

Summary

It is an object to reduce the amount of data that needs to be transmitted over a fronthaul connection, and to provide different ways to achieve such a reduction.

According to a first aspect, it is presented a method for transmitting signals from a radio equipment, RE, serving a plurality of antennas, to a radio equipment controller, REC. The method is to be performed by the RE and comprises the step of transmitting to the REC a signal received on a first antenna, using a first number of bits and the step of transmitting to the REC, for a signal received on a second antenna, a deviation of the signal received on the second antenna from a determined relationship between signals on the first and second antenna, using a second number of bits lower than the first number of bits.

According to a second aspect, it is presented a method for receiving signals from a radio equipment, RE, serving a plurality of antennas, by a radio equipment controller, REC. The method is to be performed by the REC and comprises the step of receiving a first signal received by the RE on a first antenna using a first number of bits, the step of receiving, for a second signal received by the RE on a second antenna, a deviation of the signal from a determined relationship between signals on the first antenna and the second antenna, using a second number of bits lower than the first, and the step of reconstructing the second signal based on the first signal, the relationship, and the deviation from the relationship.

In the first and second aspects, signals received on the first and second antenna may be obtained and the relationship determined based on the received signals.

Further, the determined relationship may be transmitted by the REC to the RE, received by the RE from the REC, transmitted by the RE to the REC or received by the REC from the RE.

The relationship may based on channel estimates for the first an second antennas, or it may be based on an estimate of a relative channel between the first and second antenna, or it may be based on a correlation between signals on the first and second antenna.

The signal received on the first antenna may be transmitted to the REC or received from the RE in equalized form. The deviation may be a difference between an equalized form of the signal received on the first antenna and an equalized form of the signal received on the second antenna or the deviation may be a linear combination of the signal received o the first antenna and the signal received on the second antenna, the linear combination having weights based on the relative channel between the first and second antenna or the deviation may be a linear combination of the signal received on the first antenna and the signal received on the second antenna, the linear combination having weights based on a correlation between signals received on the first and second antennas.

The antenna of the plurality having a best signal quality may be selected to be the first antenna.

The method may be triggered by a determination that a signal quality on the first antenna and a signal quality on the second antenna are better than a predetermined threshold.

Where multiple user equipments, UEs, are active, the relationship may be obtained and used on a per-UE basis.

According to a third aspect, it is presented a radio equipment, RE, for serving a plurality of antennas and transmitting signals to a radio equipment controller, REC. The RE has means for transmitting to the REC a signal received on a first antenna, using a first number of bits and means for transmitting to the REC, for a signal received on the second antenna, a deviation of the signal from a determined relationship between signals on the first and second antenna, using a second number of bits lower than the first.

According to a fourth aspect it is presented a radio equipment, RE, having a processor and memory configured to perform the method steps of the first aspect.

According to a fifth aspect, it is presented a radio equipment controller, REC, for receiving signals from a radio equipment, RE, serving a plurality of antennas. The REC has means for receiving a first signal received by the RE on a first antenna using a first number of bits, means for receiving, for a second signal received by the RE on a second antenna, a deviation of the signal from a determined relationship between signals on the first antenna and the second antenna, using a second number of bits lower than the first, and means for reconstructing the second signal based on the first signal, the relationship, and the deviation from the relationship.

According to a sixth aspect, it is presented a radio equipment controller, REC, having a processor and memory configured to perform the method steps of the second aspect.

According to a seventh aspect, it is presented a computer program for controlling transmission of signals from a radio equipment, RE, serving a plurality of antennas, to a radio equipment controller, REC. The program comprises instructions which when executed by the RE causes the RE to perform the steps of transmitting to the REC a signal received on a first antenna, using a first number of bits and transmitting to the REC, for a signal received on the second antenna, a deviation of the signal from a determined relationship between signals on the first and second antenna, using a second number of bits lower than the first.

According to an eight aspect, it is presented a computer program product comprising a program according to the seventh aspect and computer readable media on which the program is stored.

Brief description of the drawings

Figure 1 is a schematic diagram of a part of a communications network wherein embodiments presented herein may be applied.

Figure 2a and 2b are flow charts illustrating embodiments of methods for reducing the amounts of data that needs to be transmitted over a fronthaul connection.

Figure 3 is a schematic diagram illustrating an RE wherein embodiment illustrated in figure 5 may be applied.

Figure 4 is a schematic diagram illustrating an REC wherein embodiments illustrated in figure 6 may be applied.

Figure 5 is a flow chart illustrating embodiments of methods presented herein that may be applied in the RE of figure 3.

Figure 6 is a flow chart illustrating embodiments of methods presented herein that may be applied in the REC of figure 4.

Figure 7 is a schematic diagram illustrating an RE wherein embodiments illustrated in figure 9 may be applied.

Figure 8 is a schematic diagram illustrating an REC wherein embodiments illustrated in figure 10 may be applied.

Figure 9 is a flow chart illustrating embodiments presented herein that may be applied in the RE of figure 7.

Figure 10 is a flow chart illustrating embodiments presented herein that may be applied in the REC of figure 8.

Figure 1 1 is a schematic diagram illustrating a computer implementation of a network node comprising an RE.

Figure 12 is a schematic diagram illustrating a computer implementation of a network node comprising an REC. Figure 13 is a flowchart illustrating a method for determining and transfering a relationship.

Figure 14 is a flowchart illustrating a method for selecting whether to use other methods of the invention or not.

Detailed description

Figure 1 shows a typical setting relevant to the invention for a Radio

Equipment, RE, and a Radio Equipment Controller, REC in a mobile

communications network. An RE 300 has at least two antennas 331 , 332 and is connected to an REC 400 via a fronthaul connection 380. The REC 400 is connected to a core network 1 10 and possibly to other RECs (not shown) via one or more backhaul or crosshaul connections 480. Radio signals received on the antennas 331 , 332 are processed by FFT in the RE and transferred over the fronthaul connection 380 in the frequency domain to the REC for further processing such as Maximum Ratio Combining, MRC or Interference Rejection Combining, IRC. The fronthaul connection 380 and backhaul connection 480 may be point-to-point links or networks, and may be shared with other traffic.

With reference to figure 2a, 2b, 3 and 4, general methods for reducing the amount of data to be transferred on the fronthaul connection 380 are described. A UE 310 transmits signals which are received by the antennas of an RE 300, processed there and forwarded in a compressed form to the REC for still further processing.

The radio channels 321 and 322 between the antenna of UE 310 and each of the RE antennas 331 and 332 will normally be different, but for stationary or slow-moving UEs the channel coherence time is typically many times longer than the duration of individual signals sent on the channel. Thus, for such conditions the channel will usually not change quickly. Since each antenna receives the same signal from the UE but subjected to the specific radio channel for that antenna, each antenna will receive a different signal, but there is a relationship between the signals received on the different antennas which does not change quickly.

This can be used to compress the data that is to be transmitted on the fronthaul connection 380 from the RE 300 to the REC 400.

The signals received on the antennas may be subject to noise which is different for each antenna, and it may not be possible to determine the relationship between the signals on the antennas exactly, but nevertheless it is possible for the REC to determine fairly accurate values for signals on several antennas from knowledge of just one of them, based on the relationship. The deviation of the so determined values from the actually received values will typically be small and thus sending a further small amount of data to transfer also the deviation is sufficient to reconstruct the received signals completely.

In other words, it is normally possible for the REC to determine values for signals on several antennas from knowledge of just one of them with an accuracy that is good enough that a deviation of the true signal from the so determined value is small enough to be represented with fewer bits than the true signal itself.

Hence, by sending from the RE to the REC the full signal for one antenna and sending the deviations from the relationship for the other antennas, the true received signals for all antennas can be reconstructed by the REC. Since the deviation can normally be represented using fewer bits than the full signal, data compression is achieved on the fronthaul connection.

The relationship between the signals may be determined separately by the RE and the REC, determined by the RE and sent to the REC or determined by the REC and sent to the RE. It may have various forms, for example channel estimates for all antennas, relative channel estimates between antennas or a determined correlation between signals on antennas.

Typically one antenna will be selected as reference antenna, for example one with the strongest or strong enough signal or best or good enough signal quality. Relative channels, or correlations may be determined with respect to the reference antenna for each of the other antennas, or an absolute cannel may be determined for each antenna. The signal on the reference antenna is below denoted "reference signal". For clarity, the special reference signals defined in the LTE (Long Term Evolution) resource grid, some of which may be used for channel estimation, are denoted below as "LTE reference signals". It is understood that corresponding reference signals will also exist in a 5G or other system, and may be used in the same or similar fashion.

Since the relationship between the signals on the antennas does not change quickly, compression and/or performance gain may be achieved by using a determined relationship for successively received signals during a time period, for example for many OFDM (Orthogonal Frequency-Division Multiplexing symbols or LTE subframes in a 4G system. That is, a relationship determined for a particular received OFDM symbol may be used also for later symbols. The longer the channel coherence time, the longer is the time that may accurately be covered by a single estimate.

This idea may instead be applied to the frequency domain when the

relationship does not change quickly with frequency and thus a relationship determined for a particular frequency may be used also for nearby frequencies. For example by determining an estimate of the relationship for every OFDM symbol, but only for certain subcarriers, or as average estimates for groups of subcarriers. The larger the coherence bandwidth of the channel, the larger is the frequency span or number of subcarriers that can be accurately covered by a single estimate. The two principles may also be combined, so that estimates of the relationship are made less often than every OFDM symbol and not specifically for all subcarriers.

Thus, with reference to figures 1 and 2a, an embodiment of a method

performed by an RE 300 for transmitting data from the RE 300 to an REC 400 is as follows.

In a first transmit step 210, a signal received by the RE 300 on a first antenna 331 is transmitted from the RE 300 to the REC 400 over a fronthaul 380 using a first number of bits.

In a second transmit step 220, for a signal received by the RE 300 on a second antenna 332 a deviation of the signal from a determined relationship between signals on the first and second antennas is transmitted from the RE 300 to the REC 400 over the fronthaul 380, using a second number of bits, lower than the first number of bits.

And with reference to figures 1 and 2b, a method performed by an REC 400 for receiving data from an RE 300 by the REC 400 is as follows.

In a first receive step 240, receiving from the RE 300 over a fronthaul 380 a first signal received by the RE 300 on a first antenna 331 using a first number of bits.

In a second receive step 250, receiving from the RE 300, for a second signal received by the RE 300 on a second antenna 332, a deviation of the signal from a determined relationship between signals on the first antenna and the second antenna, using a second number of bits lower than the first number of bits.

In a reconstruct step 260, reconstructing the second signal based on the first signal, the relationship, and the deviation from the relationship.

With reference to figures 3, 4, 5 and 6, a first detailed example embodiment for a 2-antenna case is as follows.

After channel estimation, the signal received from each antenna is equalized. One of the received signals is used as reference and is quantized, the difference to the other signal(s) is calculated, this difference is quantized with fewer bits than the reference and transmitted together with the selected reference signal over the fronthaul from the RE to the REC. At the REC the reference signal together with the difference is used to reconstruct the other signal(s). The reconstructed signal(s) are decompensated (de-equalized) and then given as input to the used combining function, e.g. MRC or IRC. This can be described by the following steps:

In the RE 300:

In a transmit step 5010 the UE 310 transmits a signal.

In a receive step 5020 signals are received on the antennas. A signal reaches the first antenna 331 via the radio channel 321 between the UE 310 and the antenna 331. The signal received at the antenna 331 is (as described in the frequency domain) the signal transmitted by the UE 310 multiplied by the value of the channel 321 , hl, l . Likewise, the signal received at the second antenna 332 is in the frequency domain the signal from the UE310 multiplied by the value of the channel 322, h2, l .

In a conversion step 5030 the two received antenna signals are A/D (Analog- to-Digital) converted by A/D converters (not shown) in the OFDM

demodulators 341 and 342.

In a demodulation step 5040 sampling, OFDM-symbol and frame

synchronization is done, after this stage FFT is applied on both signals, in the OFDM demodulators 341 and 342.

In an equalization step 5050 a coarse frequency domain equalization is performed on the two signals in the equalizers 351 and 352. This typically means dividing the signal by a channel estimate.

In a reference select step 5060 a signal with the best or good enough quality is selected as the reference signal. For example, the strongest signal can be considered as the reference. In figure 3 the reference signal is the signal on the antenna 331.

In a quantization step 5070 the reference signal is re-quantized to desired number of bits in quantizer 365.

In a difference calculation step 5080 the difference between the reference and the other signal is calculated in adder 366. The difference would consist of non-ideal equalization mismatch and the receive noise (including receiver noise and interferences). The variance of the difference signal will be much smaller than the other signal which enables to use fewer bits to encode, if the channel estimate is good.

In a quantization step 5090 the difference signal is re-quantized to desired number of bits (fewer bits than the reference) in quantizer 367. In a frame assembly step 5100 the re-quantized difference signal and the reference signal are assembled into a frame by the frame assembler 370 and sent to the REC over the fronthaul 380 and also a message informing which signal is used as the reference.

In the REC 400:

In a receive step 6010 in the REC 400 a frame containing the reference signal and the difference signal are received over the fronthaul 380 by the frame deassembler 410.

In a frame deassembly step 6020 the frame is deassembled and the signals extracted.

In a reconstruct step 6030 the second signal is reconstructed in the adder 420 by adding the reference signal and the difference signal.

In a de-equlization step 6040 the two signals are de-equalized against the coarse equalization at RE in de-equalizers 431 and 432.

In a combine step 6050 the combiner 470 performs MRC, IRC or some other combining method on the de-equalized signals.

After combining the signals are normally passed on to further baseband processing, in the REC 400 or elsewhere.

In alternatives, the methods of the invention may be applied only if the first and second signals have sufficient quality, for example if the quality for both signals is higher than a predefined threshold.

With reference to figure 14, in a step 1410 a quality of the first and second signals is compared to a threshold. If the quality for both signals exceed the threshold, a method of the invention is applied, step 1420, else signals are transmitted normally, step 1430.

If only the SNR (Signal-to-Noise Ratio) of the second signal is too low, the second signal can be dropped and only the reference signal transported over fronthaul.

If the best signal also has too low SNR there is no point in performing a difference operation since the difference may require more bits than the reference. Then both branches may be transmitted in the regular way. In order for compression to be possible at the RE and decompression to be possible at the REC, the relationship between the signals on different antennas must in most cases be obtained by both these entities.

In one aspect, the RE makes channel estimates (channel gain and phase) based on received LTE reference signals, for example the Demodulation

Reference Signal (DMRS) or the Sounding Reference Signal (SRS), by dividing the received signal with the known transmitted signal.

The channels estimates may be forwarded to the REC, or alternatively, the received LTE reference signals may be forwarded to the REC so that the same channel estimates as made in the RE can also be determined in the REC. In another alternative the channel estimates are made by the REC based on the received LTE reference signals and sent to the RE.

The relationship need not have the form of a channel estimate. It may be a differential channel (i.e. the ratio of a channel for one antenna to the channel for another antenna) or a correlation. These may be determined at either the RE and transferred to the REC or at the REC and transferred to the RE or determined at both the RE and the REC.

Hence, the relationship may, regardless of whether it is a channel estimate, relative channel, correlation, etc, be determined in the RE and sent to the REC, determined in the REC and sent to the RE, or it may be determined in both the RE and the REC. It is also possible for the RE and/or the REC to obtain the relationship from some other unit where it has been determined.

With reference to figure 13, in a step 1310 signals are obtained, from which a relationship between them is determined in a step 1320. The so determined relationship may in alternatives be transmitted in a step 1330 and received in a step 1340. The steps 1310 together with 1320, the step 1330 and the step 1340 may be carried out in any of an RE, REC or other unit as explained above.

When a differential channel is used as the relationship, the deviation to be transferred may be the difference between the reference signal multiplied or divided by the relative channel and the other signal that the relative channel applies to, depending on how the relative channel is defined. In a variant the deviation is instead the difference between the reference signal and the other signal multiplied or divided by the relative channel.

Similar variants may be used when the relationship is a correlation, i.e. the deviation may be the difference between the reference signal multiplied or divided by the correlation and the other signal, or the difference between the reference signal and the other signal multiplied or divided by the correlation, depending on how the correlation is defined. In general, the deviation may be expressed as a linear combination of the signal received on the first antenna and the signal received on the second antenna where the weights of the linear combination are based on channels, a relative channel or a correlation.

In the case where several UEs are transmitting, the relationships will typically be different for different UEs. Hence, in this case it is typically needed to perform the methods of the invention on a per-UE basis, that is, determine and use relationships separately for each UE. I.e. determine a relationship separately for each UE and determine the deviation and reconstruct the signal for a UE based on the respective relationship. It may also be beneficial to do the selection of the antenna that is to be the reference antenna on a per-UE basis.

For example, for the methods of figures 5 and 6, channel estimation would be done for each UE, and equalization and de-equalization made for the signals from each UE based on the respective channel estimate.

Information on which radio resources that are assigned to each UE (e.g. OFDM symbols and subcarriers, LTE resource blocks, etc.) will typically be sent from the REC to the RE, so that the RE is able to apply the relationship relevant to a particular UE to the signals from that UE.

In general for all the methods of the invention, the more stable the relationship is over time, the less often is it necessary to determine the relationship. And the more stable the relationship is over frequency (e.g. changes less from one OFDM subcarrier to the next), the lower frequency resolution can be used when determining the relationship.

With reference to figures 3, 4, 5 and 6, performance of the methods may be assessed as follows.

For a 2-antenna case, the received signals for any subcarrier at antenna 331 and 332, respectively, can be modeled as

r₂ ri, r∑ denotes the received frequency domain symbols, xi is the UE transmitted frequency domain symbol, hi,i, denotes the channels and ni, ri2 denotes the receiver noise for each branch. When the combination method is MRC, the output signal y becomes

Here h^* denotes the conjugate of the channel. When the both channel functions have equal gain the MRC gives an increase of the SNR with 3dB.

After the equalization using the channel estimation, the signals can be written as:

The second term represents the noise due to the non-equalization. In the ideal case with perfect channel estimation, h_1:1 = h_1:1 and h_{2 1} = h_2il, the second term disappears. In this case, if for example yi is used as reference the difference signal, d becomes:

It shows that only noise terms are left. The variance of the noise term is much smaller than the variance of yl. If the channel estimation is good enough, the noise from the non-ideal equalization is also small as « 1 That's why it is

^1,1

possible to use fewer bits to achieve the same signal quality.

In this way, the signal yi and d are encoded and sent over the fronthaul from the RE to the REC. At the REC the signal y∑ can be reconstructed as y₂ = y_± + d and then ri, r∑ are recalculated through reversing the FEQ (Frequency domain Equalization) operation r₁ = y₁ - h_1:1 and r₂ = y₂ ¹ h_2,i

In other example embodiments, the determined relationship is a correlation between signals on different antennas. The correlation between the two branches is calculated and after selecting a reference signal the correlation is used to create the difference signal. With reference to figures 7,8, 9 and 10 the methods work as follows.

In RE 700.

In a transmit step 9010, the UE 710 transmits a signal

In a receive step 9020 signals are received on the antennas. A signal reaches the first antenna 731 via the radio channel 721 between the UE 710 and the antenna 731. The signal received at the antenna 731 is the signal transmitted by the UE 710 multiplied by the value of the channel 721, hl, l . Likewise, the signal received at the second antenna 732 is the signal from the UE510 multiplied by the value of the channel 722, h2,2.

In a conversion step 9030 the two received antenna signals are A/D converted (converter not shown) in the OFDM demodulators 741 and 742.

In a demodulation step 9040, sampling, OFDM-symbol and frame synchronization is done, after this stage FFT is applied on both signals, in the OFDM demodulators 741 and 742.

In a correlation calculation step 9050, the correlation matrix between the antenna signals is calculated over a group of subcarriers in an OFDM symbol by the correlation calculator 768. A correlation is calculated by taking one of the off-diagonal elements from the covariance matrix and normalize it with one of the diagonal elements

In a reference select step 9060 a signal with the best or good enough quality is selected as the reference signal. For example, the stronger signal can be considered as the reference. In figure 7, the reference signal is the signal on antenna 731.

In a quantization step 9070 the reference signal is re-quantized to desired number of bits by quantizer 765.

In a difference calculation step 9080 a difference signal is created in adder 766 by taking the difference between the other signal and the reference signal scaled with the correlation. Scaling is done in scaler 769. The variance of the difference signal will be much smaller than the other signal which enables to use fewer bits to encode, if the correlation is large enough.

In a quantization step 9090 the difference signal is re-quantized to desired number of bits (fewer bits than the reference) in quantizer 767.

In a frame assembly step 9100 the re-quantized difference signal and the reference signal are assembled into a frame in frame assembler 770 and sent to the REC over the fronthaul 780 together with the correlation, and also a message informing which signal is used as the reference. If the channel is stable over a time period (e.g an LTE Transmission Time Interval, TTI, or similar) the same correlation can be used for all OFDM symbols in the time period. When the correlation is reused the sending of the correlation in step 7100 may be omitted until a new correlation is used. The correlation calculation can be redone after a few symbols and when needed the correlation value can be updated and a message about this sent to the REC.

In REC 800.

In a receive step 10010 in the REC 800 a frame containing the reference signal, the difference signal, the correlation and a message informing which signal is used as the reference are received over the fronthaul 780 by the frame deassembler 810.

In a frame deassembly step 10020 the fame is deassembled by the frame deassembler 810 and the signals extracted.

In a reconstruct step 10030 the reference signal scaled with the correlation in scaler 869 and the difference signal are added in adder 820 to reconstruct the second signal.

In a combine step 10040 combiner 870 performs MRC, IRC or some other combining method on the reconstructed signals.

After combining, the signals are normally passed on to further baseband processing, in the REC or elsewhere.

In a variant of the methods of figures 9 and 10, if the magnitude of the correlation is below a threshold, both signals are transmitted in full without applying compression as the reduction in data to be transfered in using differential coding may be too low or absent.

With reference to figures 7,8,9 and 10 the calculations may be done as follows.

The covariance matrix R12 for antenna signals rl and r2 is calculated as

Ri2 — E{r₂r } E{r₂r ]

Where r* is the conjugate as rl, r2 are complex.

The covariance matrix is calculated over the used subcarriers. The correlation COV is then calculated by taking the off-diagonal element and normalize it with diagonal element. For the case with rl as reference signal this gives gfar₂ ^*}

The reference signal dl2 is calculated as dl2 = r, - COF^■ r. dl2 is then quantized to dl2Q. with the desired number of bits. The RE transmits the rl, COV and dl2Q to the REC. At the REC the r2 signal is recreated through r₇ = dl2 + COV^■ r.

Simulations have been performed with the methods of figures 5 and 6 as well as the methods of figures 9 and 10. The results are shown in the following. A simulation of a simplified LTE uplink with only data symbols and

demodulation reference signals (DMRS), basically involving the blocks depicted in Figures 3 and 4, was performed. The constellation size was QAM-64 and the simulation was run over 1000 TTI with 14 OFDM-symbols in each TTL The channel estimation is simply done by taking the division between the received DMRS and the known DMRS. In the simulation, the channel coefficients of two antennas are set with the same amplitude \h_1:1\ = \h_2il \ and different phases. So the theoretical SNR gain is 3dB by MRC.

Embodiments of the invention utilize the fact that the differential signal is smaller than the original signal at the second antenna. In table 1 below, the ratio in dB between the differential signal (d) and the other signal (y2) is shown. The SNR values in the left column corresponds to the SNR per antenna set in the simulation that is used to generate the added Gaussian noise. The table shows that the differential signal is much smaller than the original signal when SNR is high. The difference decreases as the SNR decreases because the equalization gets worse due to the worse channel estimation for lower SNR and thus the noise term increases due to the equalization mismatch. It shows the possibility to reduce the number of bits.

SNR [dB] d/y2 [dB]

30 -25

Table 1. Ratio between the difference signal and the signal for some high, low and medium SNR values

Table 2 shows the resulting SNR in dB when the invention is not used. White noise was added to the antenna signal to get the desired SNR.

Achieved SNR in dB with and without MRC when the invention is not

Table 3 shows shows simulation results for the methods of figures 5 and 6 with MRC where both antenna signals after the FEQ are quantized to 6 bits and then the difference signal (d) is quantized with different number of bits, (lbit,2bit ,3bit and 4bit), it should be noted that all signals are complex thus 6 bits means (6 bits real and 6 bits imaginary).

SNR at 30 dB 20 dB 10 dB 5 dB

antennas:

1 bit 28.8 19.5 9.3 3.2

2 bit 30.0 21.5 1 1.3 4.6

3 bit 31.1 22.3 1 1.8 6.2

4 bit 31.5 22.7 12.6 7.3 Table 3. SNR achieved after MRC for the methods of figures 5 and 6 for different number of bits for the difference signal.

Table 4 shows results corresponding to table 3 for the methods of figures 9 and 10.

Table 4. SNR achieved after MRC for the methods of figures 9 and 10 for different number of bits for the difference signal.

The table illustrates that the SNR of the reconstructed signal is close to the SNR of the original signal and that at same time the objectives of reducing the number of bit with Rx diversity is achieved. For example, 3 bits with method 1 differential coding performs quite well, especially for higher SNR, and for lower SNR the method 2 perfoms well with 3 bits.. In this case, the total number of bits can be reduced by 25%.

In table 5 the achieved bit rate reduction is shown. The reduction is calculated as

⁽⁴-⁶-⁽²;^6+2d)) where d = 1,2,3,4

Number of Bit rate

bits reduction [%] 1 42

2 33

3 25

4 17

Table 5. Bit rate reduction when the difference signal is coded with 1 ,2,3 or 4 bits, given that the two signals are quantized to 6 bits.

The REs and RECs presented in this disclosure may be implemented as separate physical entities such as network nodes, or in network nodes comprising also other functionality. REs and RECs may be realized by computer implementation.

Fig. 1 1 is a schematic diagram illustrating an example of a computer implementation suitable for a network node comprising an RE as disclosed herein. In this particular example, at least some of the steps, functions, procedures, modules and/ or blocks described herein are implemented in a computer program 1 1 10; 1 120, which is loaded into the memory 1 140 for execution by processing circuitry including one or more processors 1 150. The processor(s) 1 150 and memory 1 140 are interconnected to each other to enable normal software execution. An optional input/ output device 1 160 may also be interconnected to the processor(s) 1 150 and/or the memory 1 140 to enable input and/ or output of relevant data such as input parameter (s) and/ or resulting output parameter(s).

The term 'processor' should be interpreted in a general sense as any system or device capable of executing program code or computer program instructions to perform a particular processing, determining or computing task.

The processing circuitry including one or more processors 1 150 is thus configured to perform, when executing the computer program 1 120, well- defined processing tasks such as those described herein.

The processing circuitry does not have to be dedicated to only execute the above-described steps, functions, procedure and/ or blocks, but may also execute other tasks. In a particular embodiment, there is provided a computer program 1 120; 1 1 10 for enabling, when executed,

-transmitting to a REC a signal received on a first antenna, using a first number of bits

-transmitting to a REC, for a signal received on a second antenna, a deviation of the signal received on the second antenna from a determined relationship between signals on the first and second antenna, using a second number of bits lower than the first number of bits.

The computer program 1 120; 1 1 10 comprises instructions, which when executed by at least one processor 1 150, cause the at least one processor 1 150 to perform some or all of the steps described above and associated with an RE as described e.g. in figure 3 or 7.

The proposed technology also provides a carrier 1 130 comprising the computer program, wherein the carrier is one of an electronic signal, an optical signal, an electromagnetic signal, a magnetic signal, an electric signal, a radio signal, a microwave signal, or a computer-readable storage medium.

Fig. 12 is a schematic diagram illustrating an example of a computer

implementation suitable for a network node comprising an REC as disclosed herein. In this particular example, at least some of the steps, functions, procedures, modules and/or blocks described herein are implemented in a computer program 1210; 1220, which is loaded into the memory 1240 for execution by processing circuitry including one or more processors 1250. The processor(s) 1250 and memory 1240 are interconnected to each other to enable normal software execution. An optional input/output device 1260 may also be interconnected to the processor(s) 1250 and/or the memory 1240 to enable input and/ or output of relevant data such as input parameter(s) and/ or resulting output parameter(s).

The processing circuitry including one or more processors 1250 is thus configured to perform, when executing the computer program 1220, well- defined processing tasks such as those described herein.

The processing circuitry does not have to be dedicated to only execute the above-described steps, functions, procedure and/ or blocks, but may also execute other tasks. In a particular embodiment, there is provided a computer program 1220; 1210 for enabling, when executed,

-receiving a first signal received by an RE on a first antenna using a first number of bits,

-receiving, for a second signal received by the RE on a second antenna, a deviation of the signal from a determined relationship between signals on the first antenna and the second antenna, using a second number of bits lower than the first,

-reconstructing the second signal based on the first signal, the relationship, and the deviation from the relationship.

The computer program 1220; 1210 comprises instructions, which when executed by at least one processor 1250, cause the at least one processor 1250 to perform some or all of the steps described above and associated with an RE as described e.g. in figure 3 or 7.

The proposed technology also provides a carrier 1230 comprising the computer program, wherein the carrier is one of an electronic signal, an optical signal, an electromagnetic signal, a magnetic signal, an electric signal, a radio signal, a microwave signal, or a computer-readable storage medium.

The flow diagram or diagrams presented herein may be regarded as a

computer flow diagram or diagrams, when performed by one or more

processors. A corresponding apparatus may be defined as a group of function modules, where each step performed by the processor corresponds to a function module. In this case, the function modules are implemented as a computer program running on the processor. The computer program residing in memory may thus be organized as appropriate function modules configured to perform, when executed by the processor, at least part of the steps and/ or tasks described herein.

Claims

1. A method for transmitting signals from a radio equipment, RE, serving a plurality of antennas, to a radio equipment controller, REC, the method being performed by the RE and comprising

-transmitting to the REC a signal received on a first antenna, using a first number of bits

-transmitting to the REC, for a signal received on a second antenna, a deviation of the signal received on the second antenna from a determined relationship between signals on the first and second antenna, using a second number of bits lower than the first number of bits.

2. A method for receiving signals from a radio equipment, RE, serving a plurality of antennas, by a radio equipment controller, REC, the method being performed by the REC and comprising

-receiving a first signal received by the RE on a first antenna using a first number of bits,

3. A method according to claim 1 or 2 comprising the steps of

-obtaining signals received on the first and second antenna,

-determining the relationship based on the received signals.

4. A method according to claim 1 or claim 3 when dependent on claim 1 comprising the step of

-transmitting the determined relationship to the REC.

5. A method according to claim 1 comprising the step of

- receiving the determined relationship from the REC.

6. A method according to claim 2 or claim 3 when dependent on claim 2 comprising the step of

-transmitting the determined relationship to the RE.

7. A method according to claim 2 comprising the step of

- receiving the determined relationship from the RE

8. The method of any of the claims 1-7 wherein the relationship is based on channel estimates for the first and second antennas.

9. The method of any of the claims 1-7 wherein the relationship is based on an estimate of a relative channel between the first and second antenna.

10. The method of any of the claims 1-7 wherein the relationship is based on a correlation between signals on the first and the second antenna.

1 1. A method according to any of the claims 1 -7 wherein the signal received on the first antenna is transmitted to the REC or received from the RE in equalized form.

12. A method according to claim 8 or 1 1 wherein the deviation is a difference between an equalized form of the signal received on the first antenna and an equalized form of the signal received on the second antenna.

13. A method according to claim 9 wherein the deviation is a linear

combination of the signal received o the first antenna and the signal received on the second antenna, the linear combination having weights based on the relative channel between the first and second antenna.

14. A method according to claim 10 wherein the deviation is a linear

combination of the signal received on the first antenna and the signal received on the second antenna, the linear combination having weights based on a correlation between signals received on the first and second antennas.

15. A method according to any preceding claim wherein the antenna of the plurality having a best signal quality is selected to be the first antenna.

16. A method according to any preceding claim wherein the method is triggered by a determination that a signal quality on the first antenna and a signal quality on the second antenna are better than a predetermined threshold.

17. A method according to any preceding claim wherein where multiple user equipments, UEs, are active, the relationship is obtained and used on a per- UE basis.

18. A radio equipment, RE, for serving a plurality of antennas and transmitting signals to a radio equipment controller, REC, the RE having

-means for transmitting to the REC a signal received on a first antenna, using a first number of bits

-means for transmitting to the REC, for a signal received on the second antenna, a deviation of the signal from a determined relationship between signals on the first and second antenna, using a second number of bits lower than the first.

19. A radio equipment, RE, having a processor and memory configured to perform the method steps according to claim 1.

20. A radio equipment controller, REC, for receiving signals from a radio equipment, RE, serving a plurality of antennas, the REC having

-means for receiving a first signal received by the RE on a first antenna using a first number of bits,

-means for receiving, for a second signal received by the RE on a second antenna, a deviation of the signal from a determined relationship between signals on the first antenna and the second antenna, using a second number of bits lower than the first,

-means for reconstructing the second signal based on the first signal, the relationship, and the deviation from the relationship.

21. A radio equipment controller, REC, having a processor and memory configured to perform the method steps according to claim 2.

22. A computer program for controlling transmission of signals from a radio equipment, RE, serving a plurality of antennas, to a radio equipment controller, REC, the program comprising instructions which when executed by the RE causes the RE to perform the steps of

-transmitting to the REC, for a signal received on the second antenna, a deviation of the signal from a determined relationship between signals on the first and second antenna, using a second number of bits lower than the first.

23. A computer program product comprising a program according to claim 22 and computer readable media on which the program is stored.