Patent Description:
Due to an explosive growth in the number of wireless user devices and the amount of wireless data that these devices can generate or consume, current wireless communication networks are fast running out of bandwidth to accommodate such a high growth in data traffic and provide high quality of service to users.

Various efforts are underway in the telecommunication industry to come up with next generation of wireless technologies that can keep up with the demand on performance of wireless devices and networks.

<CIT> describes using multiple transmit antenna transmission together with preferably pseudo-random, antenna-specific, scrambling (PRAS) to scramble incoming data differently on different antennas for those users requiring frequency diversity type transmissions. When PRAS is activated for a particular allocation, each transmit antenna applies a different scrambling sequence to the data symbols that are transmitted in the allocation.

<CIT> describes a control channel transmission method of a base station including acquiring a criterion for sorting control channels, sorting the controls channels into at least two control channel sets based on the criterion, configuring the control channels by allocating at least one antenna port to each control channel set, and transmitting the control channels as configured.

<CIT> discusses techniques for modulating and scrambling channel state information reference signals (CSI-RS) for more than eight antenna ports. Each of the plurality of CSI-RS signals can be associated with a distinct CSI-RS antenna port of a plurality of CSI-RS antenna ports, and the modulation sequence can be based at least in part on indices of the CSI-RS antenna ports or subcarriers associated with the plurality of CSI-RS signals.

<CIT> describes a method for generating a variable reference signal, comprising: initializing a scrambling sequence generator at the start of a <NUM> radio frame. The variable reference signal is generated for the radio frame based on different antenna ports, sequence length per antenna port and an initialization seed constructed with a specified equation. Additionally, the variable reference signal is initialized at the start of a <NUM> radio subframe based on a constructed initialization seed.

<CIT> describes method and device for generating and mapping a CSI-RS sequence, including: generating a pseudo-random sequence according to a pseudo-random sequence initial value, performing QPSK modulation on the pseudo-random sequence, and obtaining a first CSI-RS sequence according to maximum bandwidth of system; and cutting the first CSI-RS sequence according to the actual bandwidth of the system, obtaining a second CSI-RS sequence, and mapping the second CSI-RS sequence to a time frequency location of a CSI-RS antenna port. The CSI-RS reference signal sequence can be generated or obtained respectively at the UE terminal and eNB terminal in accordance with the stated methods for generating and mapping the reference sequence according to known parameters by the present invention, so that the calculated CSI-RS sequence can be utilized to measure the channel at the UE terminal.

This document discloses techniques for transmission of pilot signals from a transmitter having multiple antenna ports.

There is provided a method of wireless communication performed by a user equipment and a wireless communication device as defined in the independent claims, respectively.

Drawings described herein are used to provide a further understanding and constitute a part of this application. Example embodiments and illustrations thereof are used to explain the technology rather than limiting its scope.

To make the purposes, technical solutions and advantages of this disclosure more apparent, various embodiments are described in detail below with reference to the drawings. Unless otherwise noted, embodiments and features in embodiments of the present document may be combined with each other.

Section headings are used in the present document, including the appendices, to improve readability of the description and do not in any way limit the discussion to the respective sections only.

In present day orthogonal frequency division multiplexing (OFDM) systems such as the Long Term Evolution (LTE) system, multiple antenna ports may be used for transmission of signals, including data and pilot signals. Signals from the multiple antenna ports may be multiplexed together across the time domain. One limitation of prior art is that the performance of such a system degrades rapidly with high mobility user equipment (UE). The techniques disclosed in the present document overcome this limitation, among other advantageous aspects, by providing a different scrambling sequence for each antenna port for transmission of pilot signals. As demonstrated by simulation results discussed elsewhere in the present document, this technique provides superior performance.

<FIG> shows an example communication network <NUM> in which the disclosed technologies can be implemented. The network <NUM> may include a base station transmitter that transmits wireless signals s(t) (downlink signals) to one or more receivers <NUM>, the received signal being denoted as r(t), which may be located in a variety of locations, including inside or outside a building and in a moving vehicle. The receivers may transmit uplink transmissions to the base station, typically located near the wireless transmitter. The technology described herein may be implemented at a receiver <NUM> or at the transmitter (e.g., a base station).

The LTE system transmits data to each specific UE in the downstream in bursts of <NUM> msec length comprising of <NUM> OFDM symbols (for normal cyclic prefix length operation). The subcarrier allocation for that UE can be in blocks of <NUM> subcarriers.

<FIG> shows pilot locations in case not claimed where two antenna ports can be multiplexed. <FIG> shows the allocation of pilot subcarriers for the first and second antenna ports. An antenna port can be a single antenna or some unique linear combination of multiple antennas. These particular ports have the names Port <NUM> and Port <NUM> in 3GPP-<NUM> release <NUM> (respectively represented by shaded squares labeled R<NUM> and R<NUM>). Notice that the two ports occupy the same Resource Elements (or subcarriers). An orthogonal Hadamard code of length two is used to separate the two ports. The code is applied to neighboring resource elements.

<FIG> shows an example of additional pilot locations where four Antenna ports (antenna ports <NUM> and <NUM> of <FIG> plus Antenna ports <NUM> and <NUM> of <FIG>) are multiplexed. <FIG> illustrates the pilot locations for an additional two antenna ports (R<NUM> and R<NUM>). Notice that the locations of <FIG> are shifted by one position with respect to the locations of <FIG> and thus avoid interference.

In an embodiment where more than four antenna ports are required, elements of <FIG> apply a code of length four is used in <FIG> across the time dimension (time and frequency), this way allowing the packing of pilots for four antenna ports. In the same manner four additional antenna ports can be accommodated in the pilot positions of <FIG>, allowing for a maximum of eight antenna ports. In the LTE specification they are enumerated as Antenna Port <NUM> - Antenna Port <NUM> and use the codes shown in <FIG>.

<FIG> shows an example of pilot multiplexing codes used for multiplexing pilot signals in a manner resulting in orthogonal pilot signals.

<FIG> shows an example of four pilot ports multiplexed with an Orthogonal Cover Code of Length <NUM>. <FIG> shows pictorially the way four pilot ports would be superimposed on the same resource elements by the use of an orthogonal code of length four. In this figure the vertical dimension denotes frequency and the horizontal dimension denotes time. As can be seen the orthogonal code is applied across time. In this figure, the same value appears to be transmitted across frequency for each of the pilot ports. In reality, each pilot port is scrambled across frequency by a pilot sequence.

<FIG> shows the block diagram of the pilot values generated by the modulation of the orthogonal codes by a scrambling sequence. As depicted in <FIG>, from left to right, data bits are first encoded using forward error correction coding (FEC encoding). The resulting encoded bits are modulated using a modulation scheme such as QAM. The modulated symbols are mapped to time-frequency resource elements along with pilot signals that are received from a pilot scrambling sequence generation module whose output is multiplied by orthogonal cover codes generated by an orthogonal cover code generation module. By applying the pilot scrambling sequence {a1, a2, a3,. } across frequency, the pilot values shown in <FIG> are changed to the pilot values of <FIG>.

The LTE specification uses orthogonal Hadamard codes to multiplex up to eight antenna ports in two groups of four ports each. However, the Hadamard code orthogonality is generally lost after the pilots have propagated through the wireless channel. This is addressed in prior art in two ways.

No code multiplexing is used across the frequency dimension where the channel frequency response can significantly impair orthogonality.

Code multiplexing is used across the time dimension, but is only used in situations where the channel is approximately time invariant within the <NUM> msec transmission time interval (low Doppler case). Then code orthogonality is preserved.

The limitation of prior art is that antenna port multiplexing across the time domain does not provide good performance for high mobility user equipment (UEs), especially when four pilot ports are multiplexed using an orthogonal cover code (Hadamard code) of length four.

The LTE design was inspired by a low Doppler use case (closed loop MIMO) and low complexity implementation. Indeed, in the low Doppler case, the cover codes across time are received perfectly orthogonal, and simple projection to each cover code perfectly separates each pilot port, even in the extreme case of four pilot ports for a four point cover code. Subsequently, channel interpolation via MMSE or other techniques can be performed on each pilot port separately.

However, under moderate Doppler effects, the orthogonality of the cover codes is compromised and the performance deteriorates.

In one advantageous aspect, the disclosed techniques can be used to improve the separability of the pilot waveforms for the different pilot ports by replacing the single pilot scrambling sequence present in the prior art with a scrambling sequence that is different for each pilot port. Two different ways are disclosed for designing the pilot sequences. This modification combined with joint MMSE pilot interpolation techniques results in significantly improved performance, as is discussed herein.

<FIG> shows the block diagram of a pilot and data multiplexing system similar to the one in <FIG>, with the modification that a different pilot scrambling sequence is generated for each antenna port. In this system, data bits <NUM> are error correction coded using an FEC encoding module <NUM>. The error correction coded data bits are modulated to symbols (e.g., QAM) using a modulation module <NUM>. A pilot scrambling sequence generation module <NUM> generates pilot signals that are multiplexed (<NUM>) using orthogonal cover codes generated by a orthogonal cover code generation module <NUM>. The orthogonalized pilot signals are mapped, along with the output of the modulation module <NUM>, are mapped to time and frequency elements by a module <NUM>. For example, denoting the different pilot scrambling sequences by {a1, a2, a3,. }, {b1, b2, b3,. }, {c1, c2, c3,. }, {d1, d2, d3,. }, then the pilot values for each antenna port are shown in <FIG>.

In <FIG>, each resource element is shown with a corresponding scrambling sequence code, as above.

A computationally efficient way to generate those different scrambling sequences is to use the same basic scrambling sequence for all ports but use a different circular shift of the pilot waveform (in the time domain) or linear phase modulation (in the frequency domain) for each pilot port. For four ports we use a circular shift of <NUM>, N/<NUM>, N/<NUM> and 3N/<NUM> samples respectively for each port where N is the OFDM symbol length. This is actually implemented by modulating the PN sequence in the frequency domain by complex exponentials of frequency <NUM>, π/<NUM>, π, and <NUM>π/<NUM> respectively.

Given the pilot placement and pilot values above, the task of channel estimation is to separate the antenna ports and interpolate the channel response for each antenna port to the time-frequency grid points where no pilot signal is received.

If a receiver collects all received values for the time-frequency points where pilots are transmitted in a vector hp, and all values for the time-frequency point we wish to interpolate each antenna port by h(i), i = <NUM>,. ,<NUM> then the interpolation is achieved by an interpolation matrix, represented as follows: <MAT>.

The interpolation matrix C(i) is designed by minimizing the MSE criterion as is well understood to people versed in the art. In the case of antenna port dependent scrambling sequences, as disclosed here, a joint MMSE channel interpolation can be used. In this case, the above equation becomes: <MAT>.

In the above equation, the joint MMSE interpolation matrix is C and is designed according to the MMSE criterion as well.

<FIG> shows the channel estimation performance as a function of Doppler spread for the extreme case of multiplexing four pilot ports (namely ports <NUM>,<NUM>, <NUM>,<NUM>) on the four point cover code. The normalized channel estimation MS error is shown, averaged over the whole subframe, versus max Doppler spread. The curve <NUM> indicates the performance using the LTE pilot sequence, which is identical for all pilot ports. Three SNR points are depicted, <NUM>, <NUM> and <NUM> dB with curves <NUM>, <NUM> and <NUM> respectively. Notice that the performance deteriorates even for moderate Doppler, especially for the high SNR case; for example, for <NUM> Doppler a loss of more than <NUM> dB is observed compared to <NUM> Doppler.

The Doppler performance can be improved with a better design of the pilot sequences which exploits the potential of more advanced signal processing, i.e. joint pilot port MMSE interpolation. In that case the system benefits from pilot sequences that are not identical across pilot ports but afford some separability.

The curve <NUM> in <FIG> shows the performance when different pilot sequences are used for different antenna ports. The different sequences are generated by utilizing different initial conditions for the feedback shift register of the PN generator. Notice a remarkable improvement even for moderate mobility. For example, for <NUM> Doppler, the performance improvement is more than <NUM> dB.

When the second method of generating different sequences by modulating a base sequence is used, the performance obtained is shown in <FIG> in curve <NUM>. The performance is similar and slightly better than the PN sequence randomization approach depicted in curve <NUM>.

In the next example, we multiplex four ports on an OCC-<NUM> orthogonal code spanning four time and frequency points of the pilot grid of <FIG>. In this example the <NUM>-point orthogonal code spans both time (<NUM> points) and frequency (<NUM> points). Similar to the previous case, one value of the pilot scrambling sequence is applied to (multiplies) each <NUM>-point OCC code for each pilot port.

<FIG> shows the results for the TDLC-<NUM> channel with a <NUM> input <NUM> output configuration, which appear similar to those for the LTE pilots of <FIG>. A similar significant gain for both the random initialization pilot sequence case and the circularly shifted pilot sequence case.

<FIG> is an example of a Pilot and Data Mapping on the LTE Resource Grid. The time slots <NUM> and <NUM> shows time slots on in which the pilot signals (dark shaded REs) are transmitted.

<FIG> shows a flowchart representation of an example method <NUM> of wireless communication. The method <NUM> may be implemented by the receiver <NUM> (on transmit side) or by the base station (on its transmit side).

The method <NUM> includes, at <NUM>, receiving data symbols for transmission over a wireless communication channel using multiple antenna ports. The data symbols may be locally generated by the transmitting device or may be received from a user interface or a network interface of the device (not shown in the drawings). The receiving operation may include, for example, receiving the data from user applications running on the transmitting device. In some cases, the data may be received at a peripheral or second network interface from other users for transmission over the wireless channel.

The method <NUM> includes, at <NUM>, generating a plurality of scrambling sequences, each corresponding to one of the multiple antenna ports. Alternatively, the plurality of scrambling sequences may be pre-generated and stored in a memory such as a look-up table, and may be read in some pre-determined manner for use. The scrambling sequences may be generated to follow a certain mathematical property such as spreading the spectral use uniformly, and so on. For example, in some cases, the scrambling sequences may be circularly shifted versions of each other. The scrambling sequence generation may be performed by the module <NUM> described herein.

The method <NUM> includes, at <NUM>, mapping, for each antenna port, a corresponding pilot signal to time and frequency transmission resources using a corresponding scrambling sequence. The mapping may be performed by the module <NUM>.

The method <NUM> includes, at <NUM>, multiplexing a first input from the data symbols (e.g., output of module <NUM>) and a second input from the mapping of the corresponding pilot signal (e.g., output of the stage <NUM>) to generate an output signal.

The method <NUM> includes, at <NUM>, transmitting the output signal over a wireless communication channel. When the method <NUM> is implemented by a user device (e.g., receiver <NUM>), each user device may generate its own pilot signals using a scrambling sequence that is generated by the user device. Each user device may thus apply a different scrambling sequence to pilot signals corresponding to different antenna ports of the user device. The randomization of the scrambling sequences thus ensures that, at the base station, the pilot signals corresponding to different antenna ports of a same UE and pilot signals from different UEs can all be separated by the base station, even when these pilot signals are assigned to the same resources.

In some embodiments, a wireless communication apparatus includes a memory and a processor. The processor reads instructions stored in the memory and implements the method <NUM>.

<FIG> shows an example of a wireless transceiver apparatus <NUM>. The apparatus <NUM> may be used to implement method <NUM>. The apparatus <NUM> includes a processor <NUM>, a memory <NUM> that stores processor-executable instructions and data during computations performed by the processor. The apparatus <NUM> includes reception and/or transmission circuitry <NUM>, e.g., including radio frequency operations for receiving or transmitting signal and/or receiving data or information bits for transmission over a wireless network.

It will be appreciated that the disclosed techniques are useful in improving channel estimation performance of a wireless communication system by providing more robust channel estimation even under high Doppler conditions.

The embodiments, modules and the functional operations described in this document can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this document and their structural equivalents, or in combinations of one or more of them. The embodiments can be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a computer readable medium for execution by, or to control the operation of, data processing apparatus.

While this description contains many specifics, these should not be construed as limitations on the scope of an invention that is claimed or of what may be claimed, but rather as descriptions of features specific to particular embodiments. Certain features that are described in this description in the context of separate embodiments can also be implemented in combination in a single embodiment.

Claim 1:
A method of wireless communication, performed by a first user equipment, comprising:
receiving data symbols for transmission over a wireless communication channel, the transmission using four antenna ports; the method being characterized in that it further comprises:
generating a plurality of scrambling sequences, such that for any two of the four antenna ports the scrambling sequence differs;
mapping, for each of the four antenna ports, a corresponding scrambled pilot signal to time and frequency transmission resources, the corresponding scrambled pilot signal generated by scrambling a corresponding pilot signal and a corresponding scrambling sequence;
multiplexing, for each of the four antenna ports, respective data symbols and a respective scrambled pilot signal to generate an output signal; and
transmitting the output signal over a wireless communication channel,
wherein the generating the plurality of scrambling sequences comprises:
using a pseudorandom number generator to generate a pseudorandom sequence,
generating a plurality of circular shifts such that a linear phase modulation in a frequency domain is used for each of the four antenna ports, wherein the plurality of circular shifts are <NUM>, N/<NUM>, N/<NUM>, and 3N/<NUM> samples, and
generating each of the plurality of scrambling sequences by performing a circular shift operation on the pseudorandom sequence using each of the plurality of circular shifts for each of the four antenna ports.