WO2014163027A1

WO2014163027A1 - A method implemented in a base station used in a wireless communications system

Info

Publication number: WO2014163027A1
Application number: PCT/JP2014/059382
Authority: WO
Inventors: Satha Sathananthan
Original assignee: Nec Corporation
Priority date: 2013-04-04
Filing date: 2014-03-24
Publication date: 2014-10-09

Abstract

A method implemented in a base station used in a wireless communications system is disclosed. The method comprising: transmitting, to a first user equipment (UE), a first demodulation reference signal (DM-RS) using a first DM-RS pattern; and transmitting, to a second UE, a second DM-RS using a second DM-RS pattern. The first DM-RS pattern uses less than a predetermined number of resource elements (REs) per physical resource block (PRB) per antenna port, and the second DM-RS pattern uses the predetermined number of REs per PRB per antenna port.

Description

DESCRIPTION

Title of Invention

A METHOD IMPLEMENTED IN A BASE STATION USED IN A WIRELESS COMMUNICATIONS SYSTEM

Technical Field

[0001]

The present invention relates to a method implemented in a base station used in a wireless communications system.

Background Art

[0002]

3 GPP has approved a small cell enhancement study item for LTE Rel-12 (Long Term Evolution Release 12). This study item is considered important for enhancing LTE

communication systems; in particular for supporting increased traffic demands and improving spectral efficiency.

[0003]

Referring to Fig. 1, each small cell 111 is considered a hotspot area (indoor or outdoor) under the coverage of a small base station 112. Small base stations 112 may also be referred to as small eNBs 112 (or SeNBs). Each small eNB will generally be a transmission node whose transmission power is lower than that of a macro base station 102; i.e. similar to Rel-11 Pico eNB and Home eNB base station classes. The macro base station 102 may also be referred to as a macro eNB 102 (or MeNB).

[0004]

Small cell deployments and enhancements may help to, for example, increase capacity by dense deployments of small cells, increase spectral efficiency, improve efficient traffic balancing and offloading, enable use of high frequency bands such as 3.5 GHz for small coverage, improve mobility performance, and increase energy efficiency.

[0005]

Two particular small cell deployment scenarios may be considered:

1. Small cells 111 (each with a small base station 112) operating with macro cell coverage 101 - this scenario is represented in Fig. 1 ; and

2. Small cells 113 (each with a small base station 114) operating without macro cell coverage - this scenario is represented in Fig. 2. [0006]

Referring to the scenario in Fig. 1, for small cells operating with macro cell coverage, the small eNBs 112 may use one carrier frequency Fl while the macro eNB 102 uses another carrier frequency F2. Alternatively, it is possible that both the small eNBs 112 and the macro eNB 102 may use the same carrier frequency (i.e. F1=F2). In this scenario, a cluster of small cells 112 could be connected to the LTE core network (the Enhanced Packet Core (EPC) 121) and a UE 151 may be connected to:

- a small eNB 112 (as shown in Fig. 1),

- the macro eNB 102 (this is not shown in Fig. 1), or

- both a small eNB 112 and the macro eNB 102 (this is not shown in Fig. 1). This last connection possibility, called "dual connectivity", is new and is being considered for Rel-12 and beyond LTE systems.

[0007]

Referring to the scenario in Fig. 2, for small cells operating without macro cell coverage, the small eNBs 114 in a cluster of small cells 113 may be connected to the LTE core network (the EPC) 121.

[0008]

In legacy LTE systems (i.e. LTE systems operating according to Rel-11, Rel-10, Rel-9, and earlier LTE specifications), downlink transmission is based on an OFDM (orthogonal frequency division multiplex) scheme, where data is transmitted in parallel sub-carriers. LTE systems support flexible bandwidth of 1.4, 3.0, 5, 10, 15 and 20 MHz with different numbers of sub-carriers.

[0009]

Fig. 3 illustrates that, in legacy LTE systems, one radio frame is made up of 10 sub- frames. The sub-frame duration is 1ms. Each sub-frame is divided into two slots (these are labelled slot #0 and slot #1 in Fig. 3). Each slot contains a number of OFDM symbols. The number of OFDM symbols per slot depends on cyclic prefix type. For normal cyclic prefix, each slot is divided into seven OFDM symbols. Data, control information and pilot signals are mapped to different sub-carriers, called "resources elements" (REs). A physical resources block (PRB) is defined as a unit of 12 REs in a slot. This is all illustrated in Fig. 3.

[0010]

[Note that in Fig. 3, and in also Figs. 8, 9 and 10, different letters (e.g. "Y", "G", "B", etc) appear on the boxes representing different REs. The letters themselves are meaningless. That is, the letters " Y", "G", "B", etc, themselves do not signify anything about the particular REs on which they appear. Rather, the purpose of labelling different REs with different letters is so that the different REs can be visually differentiated from one another, according to the information given on the right of the relevant Figure, when the Figures are represented in black and white. When the Figures are represented in black-and-white the different REs cannot be differentiated by their colour/shading.]

[0011]

PDSCH (physical downlink shared channel) data is transmitted on PRBs within a sub- frame. In certain transmission schemes, such as beam forming (TM8, TM9 and TM10 in Rel-11 LTE), UE specific pilot symbols called "UE specific reference signals" or "demodulation reference signals" (DM-RS) are inserted among the PDSCH data symbols and transmitted with the same precoding as used for the PDSCH. The DM-RS are used at the UE to estimate the channel fading coefficients, and then these channel fading coefficients are used to decode the PDSCH data. Summary of Invention

Technical Problem

[0012]

A legacy DM-RS pattern 003, which is mainly designed for macro cell deployment, is shown in Fig. 3. Legacy DM-RS antenna ports for PDSCH are denoted as ports #7, 8,..., 14. For EPDCCH they would be denoted as ports #107, 108, 109 and 110.

[0013]

The legacy DM-RS signals 003 shown in Fig. 3 occupy/use 12 REs per PRB-pair per antenna ports, resulting in a 7.14% overhead for normal cyclic prefix. (Note:

(12 REs/antenna ports) / ((12 REs/OFDM symbol)*(7 OFDM symbols/slot)*(2 slots/PRB)) = 0.0714 = 7.14%). These legacy DM-RS signals, which as just noted incur a considerable overhead, were originally designed to provide good channel estimation in highly frequency selective channels with high mobile speed, as is often the case in macro cell deployment for example. A new DM-RS design with reduced overhead may be desirable to support small cell operation as may be used in Rel-12 and beyond LTE FDD and LTE TDD systems. Also, legacy DM-RS symbols may not be well suited to improve channel estimates by concatenating adjacent PRBs (i.e. PRB bundling). It may also be desirable if a new DM-RS design could:

- improve spectral efficiency in small cell deployment scenarios;

- miriimize impact with legacy signals and channels (PSS/SSS/PBCH/CRS/CSI-

RS); and/or - support small cell deployment with new carrier type (discussed below).

[0014]

It is to be clearly understood that mere reference herein to previous or existing apparatus, products, systems, methods, practices, publications or other information, or to any associated problems or issues, does not constitute an acknowledgement or admission that any of those things individually or in any combination formed part of the common general knowledge of those skilled in the field, or that they are admissible prior art.

Solution to Problem

[0015]

In one form, the present invention relates broadly to a method implemented in a base station used in a wireless communications system, the method comprising:

transmitting, to a first user equipment (UE), a first demodulation reference signal (DM- RS) using a first DM-RS pattern; and

transmitting, to a second UE, a second DM-RS using a second DM-RS pattern, wherein the first DM-RS pattern uses less than a predetermined number of resource elements (REs) per physical resource block (PRB) per antenna port, and the second DM-RS pattern uses the predetermined number of REs per PRB per antenna port.

[0016]

The first UE is operable according to the first DM-RS pattern and the second DM-RS pattern.

[0017]

The base station comprises a small eNB.

[0018]

The one or more small eNBs may be provided within the coverage of a macro eNB, and like the small eNBs, the macro eNB may be operable to communicate with first UEs and second UEs within its coverage. Where one or more small eNBs are provided within the coverage of a macro eNB, the method may further comprise transmitting DM-RS using the second DM-RS pattern when a first UE and/or a second UE is communicating with the macro eNB.

[0019]

The first DM-RS pattern may support up to eight antenna ports. Also, in some embodiments of the invention, the predetermined number of REs may be 12.

[0020]

The first DM-RS pattern may comprise either one pattern (Pattern- 1) which uses 8 REs per PRB per antenna port or another pattern (Pattern-2) which uses 4 REs per PRB per antenna port, and normal cyclic prefix may be used. For Pattern- 1, two of the antenna ports may be multiplexed and located at sub-carriers 3 and 9 of OFDM symbols 2 and 3 of an even numbered slot of a PRB, and at sub-carriers 3 and 9 of OFDM symbols 5 and 6 of an odd numbered slot of the PRB and another two of the antenna ports may be multiplexed and located at sub-carriers 2 and 8 of OFDM symbols 2 and 3 of the even numbered slot and at sub-carriers 2 and 8 of OFDM symbols 5 and 6 of the odd numbered slot.

[0021]

In the situation described in the previous paragraph, ^r^^m^ may denote a legacy complex

m_₍ir 1 »rmax,DL _ i

valued DM-RS sequence where U,I, --,I^>RB _? and f_or antenna ports

/>e {207,208,209,210,211,212,213,214} _inaPRB *_PRB assigned for associated PDSCH transmission, the reference signal sequence ^r^ may be mapped to complex- valued modulation

a(p) lr l n

symbols at a ^K -th sub-carrier of an * -th OFDM symbol in a sub-frame ^s as follows:

XP!) _ =

+ 2 · »_PRB + where

w_p (z) (/72'+ra_PRB ) mod 2 = 0

w_p (3 - z) (m'+77_PRB ) mod 2 = 1 k = 6m'+N n_pi +k'

J3 pe {207,208,211,213}

'^~ i2 pe {209,210,212,214}

f/'mod2 + 2 if«_smod2 = 0

/ =

7'mod2-i-5 if«_smod2 = l

0,1 if«_smod2 = 0

/'=

2,3 if«_smod2 = l

rri= 0,1

and the sequence ^Wp is an orthogonal sequence given by the following table:

[0022]

As mentioned above, the first DM-RS pattern may comprise either one pattern (Pattern- 1) which uses 8 REs per PRB per antenna port or another pattern (Pattern-2) which uses 4 REs per PRB per antenna port, and normal cyclic prefix may be used. For Pattern-2, two of the antenna ports may be multiplexed and located at sub-carrier 6 of OFDM symbols 2 and 3 of an even numbered slot of a PRB and at sub-carrier 6 of OFDM symbols 5 and 6 of an odd numbered slot of the PRB, and another two of the antenna ports may be multiplexed and located at sub-carrier 5 of OFDM symbols 2 and 3 of the even numbered slot and at sub-carrier 5 of OFDM symbols 5 and 6 of the odd numbered slot.

[0023]

In the situation described in the previous paragraph, ^r^^m^ may denote a legacy complex valued DM-RS sequence where ^m ~

f_or antenna ports

p e {207,208,209,210,211,212,213,214} _{in a PRB} «_PRB a^gned to associated PDSCH transmission, the reference signal sequence ^r^ may be mapped to complex-valued modulation symbols at a ^K -th sub-carrier of an ' -th OFDM symbol in a sub-frame ^s as follows:

4? = w_p( ') - wsr'^DL+»_PRB)

where ^W _P( «_PRB mod2

w_p (3 - i) n_PRB mod 2

k N^mn + k'

6 /? e {207,208,21 1,213}

k'

5 > e {209,210,212,214}

/ = /'mod2 + 5

/'mod2 + 2 if «_s mod2 = 0

/'mod2 + 5 if «_s mod2 = l

0,1 if «_s mod2 = 0

2,3 if « mod2 = l and the sequence ^Wp is an orthogonal sequence given by the following table:

w_p(j)

[0024]

Methods for DM-RS transmission according to embodiments of the invention may further include selecting the first DM-RS pattern from among Pattern- 1 and Pattern-2 based on the number of layers and/or MCS (modulation and coding scheme) information. Embodiments of the invention may also include transmitting the selected first DM-RS pattern from the small eNB to a first UE on PDCCH or EPDCCH in downlink control information (DCI) format 2B, 2C and 2D.

[0025]

The first DM-RS pattern may be selected from among Pattern- 1 and Pattern-2 based on MCS information as follows:

for EPDCCH transmission, Pattern- 1 may always be used, and

for PDSCH, Pattern- 1 may be used if the modulation scheme is QPSK (quadrature phase shift keying), otherwise Pattern-2 may be used. [0026]

Alternatively, the first DM-RS pattern may be selected from among Pattern- 1 and Pattern-2 based on MCS information and the number of layers as follows:

for EPDCCH transmission, Pattern- 1 may always be used, and

for PDSCH, Pattern- 1 may be used when the number of layers is greater than 4, and when the number of layers is less than or equal to 4,

Pattern- 1 may be used if the modulation scheme is QPSK, or

Pattern-2 may be used if the modulation scheme is either 16QAM or 64QAM.

[0027]

Alternatively, one or more additional bits may be included in DCI (downlink control information) and the first DM-RS pattern may be selected from among Pattern- 1 and Pattern-2 based on the additional bits. In some embodiments, a single additional bit may be included in the DCI and the first DM-RS pattern may be selected from among Pattern- 1 and Pattern-2 based on the additional bit as follows:

for EPDCCH transmission, Pattern- 1 may always be used, and

for PDSCH, a bit value of "0" may indicate Pattern- 1 and a bit value of "1" may indicate Pattern-2.

[0028]

In yet another alternative, the first DM-RS pattern may be selected from among Pattern- 1 and Pattern-2 based on a SNR (signal to noise ratio) at a UE estimated by a small eNB. Suitably, Pattern- 1 may be selected if the estimated SNR is above a predetermined threshold, otherwise Pattern 2 may be selected.

[0029]

The method for DM-RS transmission according to embodiments of the invention may further include the following steps:

- determining the number of layers and MCS for DM-RS based data transmission based on channel state information (CSI) and traffic condition,

- selecting the DM-RS pattern in the manner described in paragraph [0025], [0026] or [0027],

- transmitting pre-coded DM-RS with the selected DM-RS pattern for PDSCH,

- transmitting PDCCH or EPDCCH, and

- transmitting PDSCH.

[0030]

In another form, the invention relates broadly to a method for reception at a UE of DM- RS which has been transmitted using a transmission method as described above. The method for reception of DM-RS may include:

- determining whether to monitor PDCCH or EPDCCH in a particular sub-frame, based on RRC configuration,

- if EPDCCH is to be monitored, performing DM-RS channel estimation based on

Pattern- 1 and decoding EPDCCH, otherwise decoding PDCCH,

- determining the DM-RS pattern, if DCI format 2B or 2C or 2D is detected, in the manner described in paragraph [0025], [0026] or [0027],

- de-multiplexing the received signal to generate the received DM-RS signal based on the detected DM-RS pattern,

- performing DM-RS channel estimation based the on the detected DM-RS pattern, and

- equalizing and decoding PDSCH data.

[0031]

In yet another form, the invention relates broadly to a UE operable to perform reception of DM-RS transmission according to the method described in the paragraph [0030].

[0032]

In a further form, the invention relates broadly to a small eNB operable to perform DM- RS transmission according to the method for doing so discussed above.

[0033]

Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention.

Advantageous Effects of Invention

[0034]

According to the present invention, it is possible to provide an UE specific reference signal transmission in LTE small cell wireless communication systems.

Brief Description of Drawings

[0035]

Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to understand and/or perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows:

[Fig- 1]

Fig. 1 is a schematic representation of a cluster of small cells operating with macro cell coverage.

[Fig. 2]

Fig. 2 is a schematic representation of a cluster of small cells operating without macro cell coverage.

[Fig- 3]

Fig. 3 is a graphical representation of the arrangement of, and relationship between, radio frames, sub-frames, slots, PRBs, subcarriers/REs and bandwidth configuration. Fig. 3 also illustrates a legacy DM-RS pattern.

[Fig. 4]

Fig. 4 is somewhat similar to Fig. 1 in that it illustrates a small cell deployment scenario with macro cell coverage.

[Fig. 5]

Fig. 5 is a schematic representation of a Rel-12 and beyond LTE wireless communication system with small cell operation.

[Fig. 6]

Fig. 6 illustrates the downlink DM-RS transmission process at a small eNB.

[Fig. 7]

Fig. 7 illustrates the downlink DM-RS reception process at a UE connected to a small eNB.

[Fig. 8]

Fig. 8 is a graphical representation of new downlink DM-RS Pattern- 1.

[Fig. 9]

Fig. 9 is a graphical representation of new downlink DM-RS Pattern-2.

[Fig. 10]

Fig. 10 is an illustration of PRB bundling to improve channel estimation with Pattern- 1 and Pattern-2.

[Fig. 11]

Fig. 11 illustrates a DM-RS pattern selector module.

[Fig. 12]

Fig. 12 is a flow chart for an adaptive DM-RS and data transmission procedure at a small eNB. [Fig. 13]

Fig. 13 is a flow chart for an adaptive DM-RS and data reception procedure at a UE.

Description of Embodiments

[0036]

As discussed above, it may be desirable if the signalling overhead associated with UE- specific reference signals (DM-RS) could be reduced. This may also assist to better match scheduling and feedback, in time and/or frequency, to the channel characteristics of small cells with low UE mobility. In this regard, it is envisaged that small cell design may be optimized to UE speeds of around 3 km/h (given the likely indoor and outdoor hotspot deployment with small eNBs providing small cell coverage). Thus, the propagation of mobile channel characteristics for small cells, in time and frequency, may be slow (i.e. channel characteristics may change slowly for small cells). This may enable a reduction in the number of pilot symbols required to estimate the channel characteristics in small cell deployment scenarios.

[0037]

In 3 GPP RANI, a design of a new carrier type (NCT) is considered. This NCT is another enhancement proposed for Rel-12 LTE. The NCT may eliminate control channels in the first to fourth OFDM symbols of a sub-frame, thus enabling PDSCH transmission from the first OFDM symbol of a sub-frame. The NCT may also be deployed for small cells. Therefore, it may be beneficial to design a new DM-RS pattern for small cell operation considering both legacy carrier type (LCT) and new carrier type (NCT) for Rel-12 and beyond LTE systems. Otherwise, multiple DM-RS patterns may be defined in the standard specifications, resulting in

implementation complexity for both UEs and eNBs.

[0038]

The new DM-RS pattern should perhaps be considered an additional DM-RS pattern to improve spectral efficiency for enhanced small cell operation. Rel-12 and beyond LTE eNBs may therefore configure a suitable DM-RS pattern for UEs depending on UE capability and the deployment scenario. In particular, it should perhaps be the case that:

- If a legacy (Rel-9/10/11) UE is connected to a Rel-12 small eNB, the small eNB should configure the legacy DM-RS pattern.

- If a Rel-12 UE is connected to Rel-12 macro eNB, the macro eNB should, again, configure the legacy DM-RS pattern.

- If a Rel-12 UE is connected to a Rel-12 small eNB, the small eNB should

configure a new DM-RS pattern. [0039]

Referring to Fig. 4, it can be appreciated that when small cells 111 are deployed with macro cell coverage 101, a UE 151 (which is within the coverage of the small eNB 112 and the macro eNB 102) should be able to receive data and control information from either the macro eNB 102 or the small eNB 112. In one possible deployment scenario:

- the macro eNB 102 is the same as in legacy LTE systems, thus operable to

transmit legacy signals and provide wider coverage 101;

- the small eNB 112 operates according to LTE Rel-12, thus being operable to transmit legacy signals and new signals optimized for smaller coverage 111; and

- the UE 151 is connected to the small eNB 112.

[0040]

Such a system is illustrated in Fig. 5, where 102 represents the legacy macro eNB and 112 represents the small eNB with an additional base band signal processing module 300 for generating the new adaptive DM-RS signal. The signal processing module 300 in the small eNB 112, and the corresponding processing unit 400 in the UE 151, are discussed further below.

[0041]

Figs. 8 and 9 illustrate new DM-RS patterns for transmission of PDSCH and EPDCCH in Rel-12 and beyond LTE FDD and LTE TDD systems. In Fig. 8, the new DM-RS pattern (Pattern-1) for PDSCH transmission is identified generally by reference numeral 500. In Fig. 9, the new DM-RS pattern (Pattern-2) for PDSCH transmission is identified by reference numeral 600.

[0042]

Legacy LTE systems specify two cyclic prefix types to be used depending on cell coverage. The two cyclic prefix types are referred to as normal cyclic prefix and extended cyclic prefix. Small cells are deployed to provide smaller cell coverage. Therefore, it is thought that it should be sufficient for small eNBs to support normal cyclic prefix only. Thus, new DM-RS patterns, Pattern-1 (500) and Pattern-2 (600), are proposed for normal cyclic prefix, and there may be no need for a new DM-RS pattern for extended cyclic prefix.

[0043]

Legacy LTE systems define two frame structures, where Frame structure type 1 is for LTE FDD systems and Frame structure type 2 is for LTE TDD system. Frame structure type 2 contains normal and special sub-frames. The number of special sub-frames is either 1 or 2 per radio frame, depending on the uplink-downlink configuration. Therefore, there may not be much benefit in designing a new DM-RS pattern for special sub-frames since the DM-RS overhead reduction would likely be marginal. Therefore, new DM-RS patterns, namely Pattern- 1 500 (Fig. 8) and Pattern-2 600 (Fig. 9), are proposed for:

- LTE FDD normal and MBSFN sub-frame types

- LTE TDD normal and MBSFN sub-frame types

[0044]

Legacy LTE systems support up to 8 layer PDSCH transmission by using 8 antenna ports, #7, #8, .., #14. The proposed new DM-RS patterns, Pattern-1 (500) and Pattern-2 (600), may also support up to 8 layer PDSCH transmission.

[0045]

Proposed new DM-RS patterns

Referring to Fig. 8, it can be seen that Pattern-1 500 uses 8 REs per PRB per antenna ports to transmit DM-RS. This pattern supports up to 8 antenna ports, just like in the legacy DM- RS pattern. The antenna ports for Pattern-1 can be defined as ports #207, 208,..., 214, which could correspond to legacy antenna ports #7, 8,..., 14.

[0046]

In Pattern-1 500:

^■ Antenna ports #207 and #208 shall be code multiplexed and they are located at:

Sub-carriers 3 and 9 of OFDM symbol 2 and 3 of slot 0 (the even numbered slot in Fig. 8), and

Sub-carriers 3 and 9 of OFDM symbol 5 and 6 of slot 1 (the odd numbered slot in Fig. 8).

^■ Antenna ports #209 and #210 shall also be code multiplexed and they are located at:

Sub-carriers 2 and 8 of OFDM symbol 2 and 3 of slot 0, and

Sub-carriers 2 and 8 of OFDM symbol 5 and 6 of slot 1.

[0047]

Referring now to Fig. 9, it can be seen that Pattern-2 600 uses (i.e. occupies) 4 REs per PRB per antenna ports to transmit DM-RS. This pattern also supports up to 8 antenna ports, as in legacy DM-RS pattern. The antenna ports for this Pattern-2 can be defined as ports #207, 208,..., 214, which could correspond to legacy antenna ports #7, 8,..., 14.

[0048]

In Pattern-2 600:

^■ Antenna ports #207 and #208 shall be code multiplexed and they are located at: Sub-carrier 6 of OFDM symbol 2 and 3 of slot 0 (the even numbered slot in

Fig. 9).

Sub-carrier 6 of OFDM symbol 5 and 6 of slot 1 (the odd numbered slot in Fig. 9).

■ Antenna ports #209 and #210 shall also be code multiplexed and they are located at:

Sub-carrier 5 of OFDM symbol 2 and 3 of slot 0.

Sub-carrier 5 of OFDM symbol 5 and 6 of slot 1.

[0049]

For PDSCH transmission, antenna ports #207, #208,..., #214 shall be used, whereas antenna ports #207, #208, #209 and #210 shall be used for EPDCCH transmission.

[0050]

Pattern- 1 500 (Fig. 8) and Pattern-2 600 (Fig. 9) may provide or enable better channel estimation by PRB bundling, where DM-RS symbols from adjacent PRBs are combined together for channel estimation. Combined DM-RS patterns are represented in Fig. 10. For Pattern- 1, the combined DM-RS pattern provides constant frequency spacing 551 between DM-RS symbols.

[0051]

Transmission and reception processing of DM-RS signals

Fig. 6 illustrates a baseband signal processing module 300 in an eNB for DM-RS transmission. The new (i.e. Rel-12 and beyond) small eNB 112 shall use different time- frequency resources, as proposed for new DM-RS Pattern- 1 500 and new DM-RS Pattern-2 600 above, to transmit pilot symbols. In legacy LTE downlink systems, the following parameters are used to generate and transmit DM-RS symbols.

1. Cell ED or Virtual Cell ID

2. Number of layers

3. Scrambling ID (nscm)

4. Cyclic prefix type

5. Special sub-frame configuration if it is frame structure type 2 (i.e., TDD)

[0052]

It is proposed that DM-RS pattern ID (ND -RS-ID) 901 shall also be introduced to support adaptive DM-RS transmission, in addition to the above parameters. DM-RS pattern selector 431 (see Fig. 11) shall select the DM-RS pattern adaptively. The DM-RS pattern selection algorithm and signalling mechanism are described below.

[0053] Fig. 7 illustrates a baseband signal processing module 400 in a UE for DM-RS reception and processing to estimate fading channel coefficients. The functionality of the DM-RS generation module 411 in the UE is the same as that of the corresponding module 311 in the small eNB.

[0054]

The DM-RS generation module (311 in small eNB and 411 in UE) shall use the legacy DM-RS sequence generation methodology. The following parameters are used to generate the DM-RS sequence.

1. Cell ID or Virtual Cell ID

2. Scrambling ID (nscro)

x rmax,DL

3. The maximum number of resources blocks in the system bandwidth ( RB )

[0055]

Let the legacy complex valued DM-RS sequence be denoted ^r^^m^ where

m = 0,l,..,12N-^DL -l _{For antenna ports} p e {207,208,209,210,211,212,213,214} _{in a physical} resource block ^WpRB assigned for the associated PDSCH transmission, the reference signal sequence ^r^^m shall be mapped to complex-valued modulation symbols at the k -th sub- carrier of the ^-th OFDM symbol (i.e. at resources element (RE) (k,l)) in a sub-frame

s according to the following equations:

[0056]

For Pattern- 1 500:

«i? = wM') -r(2 -r-N -^DL + 2 - n_fm + m') where

|3 /? ε {207,208,211,213}

[2 p G {209,210,212,214}

^ _ l7'mod2 + 2 if «_s mod2 = 0

~ [/'mod2 + 5 - if «_s mod2 = l

m'= 0,1 [0057]

For Pattern-2 600:

where

/ = /'mod2 + 5

/'mod 2 + 2 if n_s mod 2 = 0

/'mod2 + 5 if «_s mod2 = l

0,1 if ra_s mod2 = 0

2,3 if «_s mod2 = l

[0058]

The sequence above is an orthogonal sequence, and like in legacy LTE systems it is given by.

Table 1 : The sequence

[0059]

The UE detects DCI (downlink control information) formats transmitted on either

PDCCH or EPDCCH. If DCI formats linked with DM-RS based transmission are detected, such as format 2B, 2C or 2D, then DM-RS pattern selector 431 (see Fig. 11) determines the DM-RS pattern ID (NDM-RS-ID) 901 from the detected DCI 421 information such as MCS 912 and the number of layers 911. The pattern selection algorithm shall be the same as used in the small eNB, which will be specified in the 3 GPP specification. The algorithms described below are proposed to support adaptive DM-RS transmission.

[0060]

DM-RS channel estimation module 412 (See Fig. 7) is implementation specific, and proposed Pattern- 1 500 and Pattern-2 600 are designed to improve the performance of this module.

[0061]

Adaptive DM-RS transmission

It is envisaged that embodiments of the present invention may utilise transmission of adaptive DM-RS symbols. In one example, adaptive DM-RS symbols shall be selected from among Pattern- 1 500 and Pattern-2 600. Alternatively, adaptive DM-RS symbols shall be selected from among the legacy pattern 003, Pattern- 1 500 and Pattern-2 600. To define this selection, a parameter namely the DM-RS pattern ID (NDM-RS-ID) 901 is defined, and the DM-RS pattern selector module 431 (Fig. 11) selects the DM-RS pattern based on the following information available in legacy LTE system.

1. Number of layers (911)

2. Modulation and coding scheme (MCS) information (912)

[0062]

The above information (911 and 912) is transmitted to the UE either on PDCCH or EPDCCH as legacy DCI formats 2B, 2C and 2D. DM-RS pattern selector module 431 in small eNB 112 could use the above information in different ways to select the DM-RS pattern. The UE 151 shall interpret the selected pattern from the received DCI 421 by the DM-RS pattern selector module 431.

[0063]

The following algorithms show how a small eNB may select the DM-RS pattern adaptively, based on the DCI information 421. One algorithm should be defined in the specification, enabling a UE to uniquely identify the correct pattern.

[0064]

Algorithm- 1 :

DM-RS pattern selection algorithm using MCS information 912:

For EPDCCH transmission, Pattern- 1 500 shall be always used.

- For PDSCH, Pattern- 1 500 shall be used if the modulation scheme is QPSK (quadrature phase shift keying). Otherwise, Pattern-2 600 shall be used.

[0065] Algorithm-2:

DM-RS pattern selection algorithm using MCS information 912 and number of layers

911:

For EPDCCH transmission, Pattern- 1 500 shall be always used.

- For PDSCH, Pattern-1 500 shall be used if the number of layers is greater than 4, and if the number of layers is less than or equal to 4,

o Pattern-1 500 shall be used if the modulation scheme is QPSK, or o Pattern-2 600 shall be used if the modulation scheme is either 16QAM or 64QAM (QAM refers to quadrature amplitude modulation).

[0066]

Algorithm-3:

This approach introduces an additional bit(s) in the DCI format to indicate the DM-RS pattern.

For EPDCCH transmission, Pattern-1 500 shall be always used.

- For PDSCH, new DCI bits indicate which pattern is to be used. If one bit is allocated for new DCI, then bit value "0" shall indicate Pattern-1 500 and bit value "1" shall indicate Pattern-2 600.

[0067]

The small eNB may be able to approximately estimate SNR (signal to noise ratio) condition experienced by the UE, based on UE RSRP/RSRQ reporting or UL received signal measurements. If the estimated SNR at UE by small eNB is greater than a threshold, then Pattern-2 could be used. Otherwise, Pattern-1 could be used.

[0068]

Adaptive DM-RS and data transmission procedure for small eNB:

The steps involved in transmitting adaptive DM-RS symbols to maximize spectral efficiency and performance are illustrated in Fig. 12, and summarised below.

- Step 1 (701):

o Determine the number of layers and MCS for DM-RS based data

transmission such as legacy TM8, TM9 and TM10 schemes (or new transmission mode to be defined in Rel-12 LTE), based on channel state information (CSI) and traffic condition. (This could be a scheduling function in the legacy LTE system.)

- Step 2 (702):

o Select the DM-RS pattern based on Algorithm- 1 or Algorithm-2 or Algorithm-3 as performed by DM-RS pattern selector module (311).

- Step 3 (703):

o Transmit pre-coded DM-RS with selected DM-RS pattern for PDSCH.

- Step 4 (704):

o Transmit PDCCH or EPDCCH with a DCI format such as legacy 2B, 2C and 2D (or a new DCI format to be defined in Rel-12 LTE for DM- RS based transmission scheme). (This could be a control channel transmission function in the legacy LTE system.)

- Step 5 (705):

o Transmit PDSCH with a beam forming transmission scheme such as legacy TM8, TM9 and TM10 schemes. (This step may support any new transmission scheme based on DM-RS transmission.)

[0069]

The above steps shall be repeated for each sub-frame if there is UE data for transmission.

[0070]

Adaptive DM-RS and data reception procedure for UE connected with small eNB:

The steps involved in receiving adaptive DM-RS symbols by the UE are illustrated in Fig., and summarised below.

- Step 1 (801):

o Determine whether to monitor either PDCCH or EPDCCH in a

particular sub-frame, based on RRC configuration. (This is Rel-11 LTE legacy mechanism.)

- Step 2 (802/803):

o (803) If EPDCCH is to be monitored:

^■ Perform DM-RS channel estimation based on Pattern- 1 and decode EPDCCH,

o (802) Otherwise

^■ Decode PDCCH.

- Step 3 (804):

o Determine DM-RS pattern if DCI format 2B or 2C or 2D is detected based on Algorithm- 1 or Algorithm-2 or Algorithm-3. (This step shall support any new DCI formats that support DM-RS based data transmission.)

- Step 4 (805): o De-multiplex the received signal to generate the received DM-RS signal based on the detected DM-RS pattern.

- Step 5 (806):

o Perform DM-RS channel estimation based the on detected DM-RS

pattern.

- Step 6 (807):

o Equalize and decode PDSCH data, and continue.

[0071]

It should be noted that advantages provided by the present invention include the following:

The proposed downlink DM-RS patterns (i.e. Pattern-1 500 and Pattern-2 600 above) improve spectral efficiency for small cell deployment scenarios for LTE FDD and LTE TDD systems.

The proposed two new DM-RS patterns reduce overhead by 33% and 66% (respectively) compared with the legacy DM-RS pattern, and they provide overall theoretical gains of 2.4% and 4.8% (respectively).

- The adaptive DM-RS transmission mechanism helps to maximize spectral efficiency.

[0072]

In the present specification and claims, the word 'comprising' and its derivatives including 'comprises' and 'comprise' include each of the stated integers but does not exclude the inclusion of one or more further integers.

[0073]

Reference throughout this specification to 'one embodiment' or 'an embodiment' means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases 'in one embodiment' or 'in an embodiment' in various places throughout this

specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[0074]

In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted by those skilled in the art.

[0075]

The above-mentioned processing may be executed by a computer. Also, it is possible to provide a computer program which causes a programmable computer device to execute the above - mentioned processing. The program can be stored and provided to a computer using any type of non-transitory computer readable media. Non-transitory computer readable media include any type of tangible storage media. Examples of non-transitory computer readable media include magnetic storage media (such as floppy disks, magnetic tapes, hard disk drives, etc.), optical magnetic storage media (e.g. magneto-optical disks), CD-ROM, CD-R, CD-R/W, and semiconductor memories (such as mask ROM, PROM (Programmable ROM), EPROM

(Erasable PROM), flash ROM, RAM (Random Access Memory), etc.). The software modules may be provided to a computer using any type of transitory computer readable media. Examples of transitory computer readable media include electric signals, optical signals, and

electromagnetic waves. Transitory computer readable media can provide the software modules to a computer via a wired communication line (e.g. electric wires, and optical fibers) or a wireless communication line.

[0076]

The whole or part of the exemplary embodiments disclosed above can be described as, but not limited to, the following supplementary notes.

(Supplementary note 1)

A method for DM-RS (demodulation reference signals) transmission in a wireless communication system in which one or more small eNBs (small base stations) are provided, each small eNB being operable to communicate with first UEs and second UEs within its coverage, wherein

first UEs are operable according to either a first DM-RS pattern which uses less than a predetermined number of REs (resource elements) per PRB (physical resource block) per antenna port or a second DM-RS pattern which uses the predetermined number of REs per PRB per antenna port, and

second UEs are operable according to the second DM-RS pattern,

the method comprising:

transmitting DM-RS using the first DM-RS pattern when a first UE is communicating with a small eNB, and transmitting DM-RS using the second DM-RS pattern when a second UE is communicating with a small eNB.

(Supplementary note 2)

A method for DM-RS transmission as in Supplementary note 1, wherein the one or more small eNBs are provided within the coverage of a macro eNB (a macro base station), the macro eNB being operable to communicate with first UEs and second UEs within its coverage, the method further comprising:

transmitting DM-RS using the second DM-RS pattern when a first UE and/or a second UE is communicating with the macro eNB.

(Supplementary note 3)

A method for DM-RS transmission as in Supplementary note 1, wherein the

predetermined number of REs is 12.

(Supplementary note 4)

A method for DM-RS transmission as in Supplementary note 1, wherein the first DM-RS pattern supports up to eight antenna ports.

(Supplementary note 5)

A method for DM-RS transmission as in Supplementary note 3, wherein the first DM-RS pattern comprises either one pattern (Pattern- 1) which uses 8 REs per PRB per antenna port or another pattern (Pattern-2) which uses 4 REs per PRB per antenna port, and normal cyclic prefix is used.

(Supplementary note 6)

A method for DM-RS transmission as in Supplementary note 5, wherein for Pattern- 1, two of the antenna ports are multiplexed and located at sub-carriers 3 and 9 of OFDM symbols 2 and 3 of an even numbered slot, and at sub-carriers 3 and 9 of OFDM symbols 5 and 6 of an odd numbered slot, and

another two of the antenna ports are multiplexed and located at sub-carriers 2 and 8 of OFDM symbols 2 and 3 of the even numbered slot, and at sub-carriers 2 and 8 of OFDM symbols 5 and 6 of the odd numbered slot.

(Supplementary note 7)

A method for DM-RS transmission as in Supplementary note 5, wherein ^r^^m^ denotes a

7 max,

legacy complex valued DM-RS sequence where ^_ U,I, ...,I JV _RB ι _^ ^_QR ^Q^Q_^ p_0rts p e {207,208,209,210,211,212,213,214} «_{PRB for} p_DSCH transmission, the reference signal sequence ^r(jn^ is mapped to complex-valued modulation

i _j n symbols at a ^K -th sub-carrier of an ¹ -th OFDM symbol in a sub-frame ^s as follows: where

^W _P( (m'+«_PRB)mod2

it

3 /? e {207,208,211,213}

2 p e {209,210,212,214}

0,1

and the sequence ^p is an orthogonal sequence given by the following table:

(Supplementary note 8)

A method for DM-RS transmission as in Supplementary note 5, wherein for Pattern-2, two of the antenna ports are multiplexed and located at sub-carrier 6 of OFDM symbols 2 and 3 of an even numbered slot, and at sub-carrier 6 of OFDM symbols 5 and 6 of an odd numbered slot, and

another two of the antenna ports are multiplexed and located at sub-carrier 5 of OFDM symbols 2 and 3 of the even numbered slot, and at sub-carrier 5 of OFDM symbols 5 and 6 of the odd numbered slot.

(Supplementary note 9)

A method for DM-RS transmission as in Supplementary note 5, wherein ^{r m}^ denotes a legacy complex valued DM-RS sequence where ^{m ~ ~ 1} _s and for antenna ports p e {207,208,209,210,211,212,213,214} _{in a PRB} «_PRB assigned for associated PDSCH transmission, the reference signal sequence ^r^ is mapped to complex-valued modulation symbols ^kJ at a * -th sub-carrier of an * -th OFDM s mbol in a sub-frame ^s as follows:

where «_PRB mod2

«_PRB mod2

Vsc PRB ^{+ K}

/ = /'mod2 + 5

/'mod2 + 2 if «_s mod2 = 0

/'mod2 + 5 if n mod2 = l

and the sequence ^p is an orthogonal sequence given by the following table:

(Supplementary note 10)

A method for DM-RS transmission as in Supplementary note 5 further comprising selecting the first DM-RS pattern from among Pattern- 1 and Pattern-2 based on the number of layers and/or MCS (modulation and coding scheme) information.

(Supplementary note 11)

A method for DM-RS transmission as claimed in Supplementary note 10, further comprising transmitting the selected first DM-RS pattern from a small eNB to a first UE on PDCCH or EPDCCH in downlink control information (DCI) format 2B, 2C and 2D.

(Supplementary note 12)

A method for DM-RS transmission as in Supplementary note 10, wherein the first DM- RS pattern is selected from among Pattern- 1 and Pattern-2 based on MCS information as follows:

for EPDCCH transmission, Pattern- 1 shall be always used, and

for PDSCH, Pattern- 1 shall be used if the modulation scheme is QPSK (quadrature phase shift keying), otherwise Pattern-2 shall be used.

(Supplementary note 13)

A method for DM-RS transmission as in Supplementary note 10, wherein the first DM- RS pattern is selected from among Pattern- 1 and Pattern-2 based on MCS information and the number of layers as follows:

for EPDCCH transmission, Pattern- 1 shall be always used, and

for PDSCH, Pattern- 1 shall be used when the number of layers is greater than 4, and when the number of layers is less than or equal to 4,

Pattern- 1 shall be used if the modulation scheme is QPSK, or

Pattern-2 shall be used if the modulation scheme is either 16QAM or 64QAM. (Supplementary note 14)

A method for DM-RS transmission as in Supplementary note 5, wherein one or more additional bits are included in DCI (downlink control information) and the first DM-RS pattern is selected from among Pattern- 1 and Pattern-2 based on the additional bits.

(Supplementary note 15)

A method for DM-RS transmission as in Supplementary note 14, wherein a single additional bit is included in the DCI and the first DM-RS pattern is selected from among Pattern- 1 and Pattern-2 based on the additional bit as follows:

for EPDCCH transmission, Pattern- 1 shall be always used, and

for PDSCH, bit value "0" shall indicate Pattern-1 and bit value "1" shall indicate Pattern-

2.

(Supplementary note 16)

A method for DM-RS transmission as in Supplementary note 5, wherein the first DM-RS pattern is selected from among Pattern-1 and Pattern-2 based on a SNR (signal to noise ratio) at a UE estimated by a small eNB.

(Supplementary note 17) A method for DM-RS transmission as in Supplementary note 16, wherein Pattern- 1 is selected if the estimated SNR is above a predetermined threshold, otherwise Pattern 2 is selected. (Supplementary note 18)

A method for DM-RS transmission as in Supplementary note 10, comprising the following steps:

determining the number of layers and MCS for DM-RS based data transmission based on channel state information (CSI) and traffic condition, selecting the DM-RS pattern based on Algorithm- 1 or Algorithm-2 or Algorithm-3,

- transmitting pre-coded DM-RS with the selected DM-RS pattern for

PDSCH,

- transmitting PDCCH or EPDCCH, and

- transmitting PDSCH.

(Supplementary note 19)

A method for reception at a UE of DM-RS transmitted using the method for transmission in Supplementary note 10, the method for reception including:

determining whether to monitor PDCCH or EPDCCH in a particular sub- frame, based on RRC configuration,

- if EPDCCH is to be monitored, performing DM-RS channel estimation

based on Pattern- 1 and decoding EPDCCH, otherwise decoding PDCCH,

- determining the DM-RS pattern, if DCI format 2B or 2C or 2D is detected, based on Algorithm- 1 or Algorithm-2 or Algorithm-3, de-multiplexing the received signal to generate the received DM-RS signal based on the detected DM-RS pattern,

equalizing and decoding PDSCH data.

(Supplementary note 20)

A small eNB (small base station) operable to perform DM-RS (demodulation reference signals) transmission in a wireless communication system in which the small eNB is operable to communicate with first UEs and second UEs within its coverage, wherein

second UEs are operable according to the second DM-RS pattern,

the small eNB being operable to:

transmit DM-RS using the first DM-RS pattern when a first UE is communicating with the small eNB, and

transmit DM-RS using the second DM-RS pattern when a second UE is communicating with the small eNB.

(Supplementary note 21)

A UE operable to perform reception of DM-RS transmission by:

- determining whether to monitor PDCCH or EPDCCH in a particular sub- frame, based on RRC configuration,

if EPDCCH is to be monitored, performing DM-RS channel estimation based on a Pattern- 1 and decoding EPDCCH, otherwise decoding PDCCH,

- determining the DM-RS pattern, if DCI format 2B or 2C or 2D is detected, based on Algorithm- 1 or Algorithm-2 or Algorithm-3 , de-multiplexing the received signal to generate the received DM-RS signal based on the detected DM-RS pattern,

- equalizing and decoding PDSCH data.

(Supplementary note 22)

A wireless communication system in which one or more small eNBs (small base stations) are provided, each small eNB being operable to communicate with first UEs and second UEs within its coverage, wherein

first UEs are operable according to either a first DM-RS (demodulation reference signals) pattern which uses less than a predetermined number of REs (resource elements) per PRB

(physical resource block) per antenna port or a second DM-RS pattern which uses the

predetermined number of REs per PRB per antenna port, and

second UEs are operable according to the second DM-RS pattern,

wherein:

DM-RS is transmitted using the first DM-RS pattern when a first UE is communicating with a small eNB, and

DM-RS is transmitted using the second DM-RS pattern when a second UE is

communicating with a small eNB. [0077]

This application is based upon and claims the benefit of priority from Australian provisional patent application No.2013901158, filed on April 4, 2013, the disclosure of which incorporated herein in its entirely by reference.

Reference Signs List

[0078]

101 MACRO CELL COVERAGE

102 MACRO BASE STATION

111, 113 SMALL CELL

112, 114 SMALL BASE STATION

121 ENHANCED PACKET CORE (EPC)

151 USER EQUIPMENT (UE)

300 SIGNAL PEOCESSING MODULE

311 DM-RS GENERATION MODULE

312 PRECODING MODULE

313, 314 RESOURCE ELEMENT MAPPER FOR DM-RS

400 SIGNAL PEOCESSING MODULE

411 DM-RS GENERATION MODULE

412 DM-RS CHANNEL ESTIMATION MODULE

413, 414 RESOURCE ELEMENT DE-MAPPER

421 DCI

431 DM-RS PATTERN SELECTOR

Claims

[Claim 1]

A method implemented in a base station used in a wireless communications system, the method comprising:

[Claim 2]

The method as in claim 1, wherein the first UE is operable according to the first DM-RS pattern and the second DM-RS pattern.

[Claim 3]

The method as in claim 1, wherein the base station comprises a small eNB.

[Claim 4]

The method as in claim 3,

wherein the first UE and the second UE communicate with the small eNB and a macro eNB, and

wherein the macro eNB transmits, to the first UE and the second UE, a third DM-RS using the second DM-RS pattern.

[Claim 5]

The method as in claim 1 , wherein the predetermined number of REs is 12.

[Claim 6]

The method as in claim 1 , wherein the first DM-RS pattern supports up to eight antenna ports.

[Claim 7]

The method as in claim 1 ,

wherein the first DM-RS pattern comprises either pattern 1 which uses 8 REs per PRB per antenna port or pattern 2 which uses 4 REs per PRB per antenna port, and

wherein a normal cyclic prefix is used.

[Claim 8]

The method as in claim 7, wherein, for pattern 1,

two of the antenna ports are multiplexed and located at sub-carriers 3 and 9 of OFDM symbols 2 and 3 of an even numbered slot, and at sub-carriers 3 and 9 of OFDM symbols 5 and 6 of an odd numbered slot, and

[Claim 9]

The method as in claim 7, wherein ^^m^ denotes a legacy complex valued DM-RS

™ _ Q I 12N™^X'^DL— 1

sequence where ^{> >} ~^{>l J Y} RB , and for antenna ports

^ {207,208,209,210,211,212,213,214} _{in a PRB}„_{PRB for associated physical} downlink shared channel (PDSCH) transmission, the reference signal sequence ^r^ is mapped to complex-valued modulation symbols at a ^K -th sub-carrier of an ' -th OFDM symbol in a sub-frame ^s as follows:

? = w_p(0 · r(2 · l'-N -^DL + 2 · _½ + m') where , \ _p W (/w^*+"_PRB) mod 2 = 0

w (z ) =

" [w_p(3 - i) (m^*+«_PRB)mod2 = l

i3 {207,208,211,213}

" 2 e {209,210,212,214}

f/^,mod2 + 2 if w_s mod2 = 0

{/'mod2 + 5 if n_s mod2 = l

m'= 0,1 the sequence ^Wp is an orthogonal sequence given by the following table:

[Claim 10]

The method as in claim 7, wherein for pattern 2,

two of the antenna ports are multiplexed and located at sub-carrier 6 of OFDM symbols 2 and 3 of an even numbered slot, and at sub-carrier 6 of OFDM symbols 5 and 6 of an odd numbered slot, and

[Claim 11]

The method as in claim 7, wherein ^r^^m^ denotes a legacy complex valued DM-RS sequence where m = 0 1 12 1 _{^ m<}^ f_or antenna _0rt_s p e {207,208,209,210,211,212,213,214} _{in a pRB} «_{PRB for assoc}i_ated physical downlink shared channel (PDSCH) transmission, the reference signal sequence ^r^ is mapped to complex- valued modulation symbols ^akJ at a ^ -th sub-carrier of an ^ -th OFDM symbol in a sub-frame ^s as follows:

where

^ ~ ^sc ^WPRB ⁺

/ = /'mod2 + 5

/'mod2 + 2 if «_s mod2 = 0

/

/'mod2 + 5 if j mod2 = l

W [ if

and the sequence ^p is an orthogonal sequence given by the following table:

w„(

[Claim 12]

The method as in claim 7, further comprising:

selecting the first DM-RS pattern from among pattern 1 and pattern 2 based on the number of layers and/or MCS (modulation and coding scheme) information.

[Claim 13]

The method for as in claim 12, further comprising:

transmitting the selected first DM-RS pattern from a small eNB to a first UE on physical downlink control channel (PDCCH) or enhanced physical downlink control channel (EPDCCH) in downlink control information (DCI) format 2B, 2C and 2D.

[Claim 14]

The method as in claim 12, wherein the first DM-RS pattern is selected from among pattern 1 and pattern 2 based on MCS information as follows:

for enhanced physical downlink control channel (EPDCCH) transmission, pattern 1 is used, and

for physical downlink shared channel (PDSCH) transmission, pattern 1 is used if the modulation scheme is QPSK (quadrature phase shift keying) and pattern 2 is used if the modulation scheme is other than QPSK.

[Claim 15]

The method as in claim 12, wherein the first DM-RS pattern is selected from among pattern 1 and pattern 2 based on MCS information and the number of layers as follows:

for physical downlink shared channel (PDSCH) transmission, pattern 1 is used when the number of layers is greater than 4, and when the number of layers is less than or equal to 4, pattern 1 is used if the modulation scheme is QPSK and pattern 2 is used if the modulation scheme is either 16QAM or 64QAM.

[Claim 16]

The method as in claim 7, wherein one or more additional bits are included in DCI (downlink control information) and the first DM-RS pattern is selected from among pattern 1 and pattern 2 based on said one or more additional bits.

[Claim 17]

The method as in claim 16, wherein a single additional bit is included in the DCI and the first DM-RS pattern is selected from among pattern 1 and pattern 2 based on the single additional bit as follows:

for physical downlink shared channel (PDSCH) transmission, bit value "0" indicates pattern 1 and bit value "1" indicates pattern 2.

[Claim 18]

The method as in claim 7, wherein the first DM-RS pattern is selected from among pattern 1 and pattern 2 based on a SNR (signal to noise ratio) at the first UE estimated by a small eNB.

[Claim 19]

The method as in claim 18, wherein pattern 1 is selected if the estimated SNR is above a predetermined threshold and pattern 2 is selected if the estimated SNR is not above the predetermined threshold.

[Claim 20]

A method implemented in a user equipment (UE) used in a wireless communications system, the method comprising:

receiving, from a base station, a first demodulation reference signal (DM-RS) using a first DM-RS pattern,

wherein the base station transmits, to a second UE, a second DM-RS using a second DM- RS pattern, and

wherein the first DM-RS pattern uses less than a predetermined number of resource elements (REs) per physical resource block (PRB) per antenna port, and the second DM-RS pattern uses the predetermined number of REs per PRB per antenna port.

[Claim 21]

A base station used in a wireless communications system, the base station comprising: a first transmitter configured to transmit, to a first user equipment (UE), a first demodulation reference signal (DM-RS) using a first DM-RS pattern; and

a second transmitter configured to transmit, to a second UE, a second DM-RS using a second DM-RS pattern,

[Claim 22]

A user equipment (UE) used in a wireless communications system, the UE comprising: a receiver configured to receive, from a base station, a first demodulation reference signal (DM-RS) using a first DM-RS pattern,

[Claim 23]

A method implemented in a wireless communications system, the method comprising: transmitting, from a base station to a first user equipment (UE), a first demodulation reference signal (DM-RS) using a first DM-RS pattern; and '

transmitting, from the base station to a second UE, a second DM-RS using a second DM- RS pattern,

[Claim 24]

A wireless communications system comprising:

a base station configured to transmit a first demodulation reference signal (DM-RS) using a first DM-RS pattern and a second DM-RS using a second DM-RS pattern;

a first user equipment (UE) configured to receive the first DM-RS; and

a second user equipment (UE) configured to receive the second DM-RS,

wherein the first DM-RS pattern uses less than a predetermined number of resource elements (REs) per physical resource block (PRB) ^"per antenna port, and the second DM-RS pattern uses the predetermined number of REs per PRB per antenna port.