WO2014033669A1

WO2014033669A1 - A method of and system for providing diverse point to point communication links

Info

Publication number: WO2014033669A1
Application number: PCT/IB2013/058140
Authority: WO
Inventors: Stephanus Jacobus FOUCHÉ; Emile Van Rooyen
Original assignee: Mimotech (Pty) Ltd.
Priority date: 2012-08-30
Filing date: 2013-08-30
Publication date: 2014-03-06

Abstract

A method of providing diverse point to point communication links is operable in a MIMO network (100 - 500) including a plurality of transmitters (TX1, TX2) coupled to a plurality of transmit antennas (A1, A2) and a plurality of receive antennas (RX1, RX2) coupled to a plurality of receivers (Α1', Α2'), the transmit antennas (TX1, TX2) being directed towards the receive antennas (RX1, RX2) and each transmit/receive antenna pair defining a unique signal path (a11, a12, a21, a22) therebetween. The method is characterised in that it includes introducing a defined or definable delay (z11, z22) into a signal between a first transmitter/receiver pair (e.g. A1, A1') having a first signal path (e.g. a11), relative to another signal between a second transmitter/receiver pair (e.g. A2, A1') having a second signal path (e.g. a12) which is different from the first signal path (e.g. a11), the delay (z11, z22) being related to a difference (ΔL) between first and second signal path lengths.

Description

A method of and system for providing diverse point to point communication links FIELD OF INVENTION

The invention relates to wireless communication, and specifically to a method of and system for providing diversity for MIMO communication techniques in point to point radio links.

BACKGROUND OF INVENTION

To improve bandwidth efficiency, use of multiple transmit and receive antennas (or antennae) has become an accepted technique, commonly called Multi Input Multi Output (MIMO) as described in "Fundamentals of Wireless Communication", David Tse and Pramod Viswanath, Cambridge University Press, 2005. It has been incorporated into several standards like IEEE 802.1 1 n and LTE or U-TRAN (part of the 3GPP group of standards).

However, MIMO requires that receivers (coupled to the receive antennas) receive different mixtures of signals from the various transmitters (coupled to the transmit antennas). This is generally known as diversity and is provided by a scattering-rich environment. However, point to point microwave communication links, by design, provide no or very little scattering.

The Applicant thus desires a method and system for providing such diversity in point to point communication links not provided by a scattering-rich environment. The Applicant is further aware that it is possible to provide diversity for a communication link using antenna radiation patterns. However, this requires separation between the antennas in the order of half of the antenna beam width. Even with high gain antennas, the required spacing is generally not practicable for point to point links.

Using an array of antennas, it is possible to synthesise a very narrow beam with simple antennas. However, the beam width is still related to the angular dimension of the total array, as seen from the other side. Thus, the physical size of the antenna array becomes prohibitive for point to point communication links.

The use of cross polarised antennas is practical, and frequently seen, wherein two orthogonal radio wave polarisations are used to provide a diversity of two, allowing a factor two increase in channel capacity. It does not, however, scale to more antennas. (This method was developed before the MIMO techniques were well understood and has become known as XPIC or Cross Polarization Interference Cancellation.) The Applicant thus desires a method and system which overcomes or at least alleviates these drawbacks.

SUMMARY OF INVENTION

According to one aspect of the invention, there is disclosed a method of providing diverse point to point communication links, the method being operable in a MIMO network including: a plurality of transmitters coupled to a plurality of transmit antennas; and

a plurality of receive antennas coupled to a plurality of receivers, the transmit antennas being directed towards the receive antennas and each transmit/receive antenna pair defining a unique signal path therebetween,

characterised in that the method includes:

introducing a defined or definable delay into a signal between a first transmitter/receiver pair having a first signal path, relative to another signal between a second transmitter/receiver pair having a second signal path which is different from the first signal path, the delay being related to a difference between first and second signal path lengths.

It will be understood that the coupling between the plural signal paths combined with the introduced delay may create the desired diversity for MIMO communications.

The delay may be introduced by means of a delay network. The delay may not merely be related to the difference between the first and second signal path lengths, but specifically matched to the difference between the first and second path lengths. The first receiver/transmitter pair may have either a transmitter or a receiver (but not both) in common with the second receiver/transmitter pair.

In one embodiment, the difference between the first and second signal path lengths may be about a quarter wavelength. The method may thus include arranging the antennas (collectively referring to the transmit and receive antennas) such that the signal path lengths differ by about one quarter wavelength. This may allow for more compact antenna placement than is possible with prior art methods making use of antenna beam widths. In such case, the delay introduced may be a quarter wavelength delay. By matching the relative delay in the signals to the difference between the signal paths, the receive antennas (or at least the receivers) may receive a plurality of combined in-phase signals from one transmitter, resulting in receipt of a full or near full strength signal, but a plurality of out-of-phase signals from another transmitter, resulting in receipt of little or no signal from the other transmitter.

It will be noted that the method does not (necessarily) make use of polarisation and does not depend on a radiation pattern of the antennas used.

In an efficient implementation, there may be a symmetric arrangement of antennas. There may be an equal number (N) of transmit antennas and receive antennas. In such case, a link capacity may be increased by a factor of N in the same frequency band.

There may be a plurality of implementations to introduce the delay. The delay may be introduced on the transmitter side (e.g. between the transmitter and transmit antennas) only. In such case, introducing the delay may include feeding a first transmit antenna with at least one non-delayed signal (e.g. from a first transmitter) and one delayed signal (e.g. from a second transmitter). Simultaneously, the method may include feeding a second transmit antenna with a delayed signal (e.g. from the first transmitter) corresponding to the non- delayed signal of the first transmit antenna and a non-delayed signal (e.g. from the second transmitter) corresponding to the delayed signal of the first transmit antenna. The delays may be introduced in a symmetrical fashion.

Instead, the delay may be introduced on the receiver side only. In such case, the delay may be introduced in similar fashion to that of the transmitter side. More specifically, the method may include feeding a first receiver with at least one non-delayed signal (e.g. from a first receiver antenna) and one delayed signal (e.g. from a second receive antenna). Simultaneously, the method may include feeding a second receiver with a delayed signal (e.g. from the first receive antenna) corresponding to the non-delayed signal of the first receiver and a non-delayed signal (e.g. from the second receive antenna) corresponding to the delayed signal of the first receiver. Instead, in a hybrid approach, the delay may be introduced both at the transmitter and receiver sides. In such case, a delay may be introduced to at least one, but not all, transmit antennas, while a delay may also be introduced to at least one, but not all, receivers. Again, the delays may be introduced in a symmetrical fashion. The method may include calculating or measuring a phase difference between transmitters and compensating therefor. Compensating for the phase difference may include adjusting the relative delay in the delay networks accordingly. According to another aspect of the invention, there is disclosed a system for providing diverse point to point communication links, the system including:

a plurality of transmitters coupled to a plurality of transmit antennas; and

a plurality of receiver antennas coupled to a plurality of receivers, the transmit antennas being directed towards the receive antennas and each transmit/receive antenna pair defining a unique signal path therebetween,

characterised in that the system includes:

a delay network arranged between at least one of:

the transmitters and the transmit antennas (the transmitter side); or

the receive antennas and the receivers (the receiver side),

the delay network being operable to introduce a defined or definable delay into a signal between a first transmitter/receiver pair having a first signal path, relative to another signal between a second transmitter/receiver pair having a second signal path which is different from the first signal path, the delay being related to a difference between first and second signal path lengths.

The delay network may be arranged at the transmitter side only. The delay network may be operable to introduce a delay into a signal fed to each transmit antenna.

The delay network may be arranged at the receiver side only. The delay network may be operable to introduce a delay into a signal fed to each receiver.

The delay network may be arranged at both the transmitter and receiver sides. The delay network may be operable to introduce a delay into a signal fed to at least one, but not all, transmit antennas and receivers.

The system may include at least one processor operable to control or coordinate the delay network. It is to be understood that the processor may be one or more microprocessors, controllers, digital signal processors (DSPs), or any other suitable computing device, resource, hardware, software, or embedded logic.

The invention extends further to a non-transitory computer-readable medium having stored thereon a computer program which, when executed by a computer, is operable to calculate and introduce a delay or delays as per the method defined above. BRIEF DESCRIPTION OF DRAWINGS

The invention will now be further described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings:

FIGURE 1 shows a schematic view of a first embodiment of a system for providing diverse point to point communication links, in accordance with the invention; FIGURE 2 shows a schematic view of a second embodiment of a system for providing diverse point to point communication links, in accordance with the invention; FIGURE 3 shows a schematic view of a third embodiment of a system for providing diverse point to point communication links, in accordance with the invention; FIGURE 4 shows a schematic view of the system of FIGURE 1 , with an extra delay mechanism;

FIGURE S shows a schematic view of a fourth embodiment of a system for providing diverse point to point communication links, in accordance with the invention; and

FIGURE 6 shows a schematic view of a delay network which may be used in the systems of FIGURES 1 - 5. DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The following description of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that many changes can be made to the embodiment described, while still attaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be attained by selecting some of the features of the present invention without utilising other features. Accordingly, those skilled in the art will recognise that many modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances, and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not a limitation thereof.

The example embodiments will be described first from a more practical (albeit ideal) perspective. Referring first to FIGURE 1 , reference numeral 100 generally indicates a first embodiment of a system for providing diverse point to point communication links, in accordance with the invention. For brevity of explanation, a 2x2 network is illustrated, but it will be appreciated that the invention may be scaled upwardly to larger networks of NxN antennas. More specifically, the system 100 has two transmitters TX1 , TX2 which are coupled via a delay network (further described below and in FIGURE 5) to two transmit antennas A1 , A2. A distance between the transmit antennas A1 , A2 is d . The system 100 also has two receive antennas A1 ', A2' directly coupled to respective receivers RX1 , RX2. The receive antennas are also separated by a distance d .

Corresponding transmit and receive antennas (in other words, A1 and A1 ', and A2 and A2') are separated by a distance L which corresponds to a signal path length of the signal paths a1 1 , a22 between the corresponding transmit and receive antennas. Ideally, L » d . For a point to point link and with the same antennas, the difference in signal strength from antennas A1 to A1 ' (corresponding) and from A1 to A2' (cross-over) is small.

The signal path length of the oblique or cross-over signals paths a12, a21 is greater than L . The difference in signal path length between a corresponding signal path length (of a1 1 or a22) and a cross-over signal path length (of a12 or a21 ) is given by:

Al ¾ .

2 L λ

Al in this example is a quarter wavelength (— ) of the frequency band being transmitted by

4

the system 100.

The delay network is represented by elements z1 1 , z22. In this embodiment, both transmitters TX1 , TX2 feed both transmit antennas A1 , A2. The signal from each transmitter TX1 , TX2 is equally divided between the two paths to the two transmit antennas A1 , A2. While all paths may delay the signals, only the relative delays are important. The signal from TX1 to A1 is delayed by z1 1 and similarly the signal from TX2 to A2 is delayed by z22. The signals from TX1 to A2 and from TX2 to A1 do not have additional delays introduced. Thus the delay network z1 1 adds an additional delay to the path from TX1 to A1 (over and above any inherent delay from TX1 to A2). In an ideal implementation of the 2x2 system 100, it is arranged so that the difference in path length, Al , and the delays z1 1 and z22 are matched and indeed correspond to a quarter λ

wavelength delay (— ). From transmitter TX1 to receiving antenna A1 ', two signal paths

4

exist: TX1 -> z1 1 -> A1 -> A1 ' (signal path a1 1 ) and TX1 -> A2 -> A1 ' (signal path a12) and each of these paths delays the signal equally. More specifically, the signal from the signal λ

path a1 1 is delayed by a quarter wavelength delay (— ) by means of the delay network z1 1 ,

4

λ while the signal from the signal path a12 is delayed also by a quarter wavelength delay (— )

4 by the relatively longer path L + Al . Thus, receiver RX1 receives two in-phase signals which yields a full strength signal from TX1 .

Similarly, but oppositely, another two paths couple TX1 to A2': TX1 -> z1 1 -> A1 -> A2' (signal path a21 ) and TX1 -> A2 -> A2' signal path a22). However, the first signal path (a21 ) λ

contains two quarter wavelength a delays ( 2 x— ), one from the delay network z1 1 and

4

another from the relatively longer path L + l , which equals a half wavelength. Thus, the two signals are entirely out of phase resulting in no signal reception. By symmetry, TX2 also couples with RX2 completely and not at all with RX1 , respectively due to constructive and destructive interference. Referring now to FIGURE 2, reference numeral 200 generally indicates a second embodiment of a system for providing diverse point to point communication links, in accordance with the invention. In the system 200, the delay elements z21 , z12 are switched from the horizontal paths from the cross-coupling paths. As consequence, the paths from TX1 to A2' add, the receiver RX2 thus receiving a full signal from the transmitter TX1 . The λ

paths from TX1 to AV are half a wave length (— ) out of phase and therefore cancel. Similarly, TX2 couples with AV (and thus RX1 ) completely and not at all with A2'.

Referring to Figure 3, reference numeral 300 generally indicates a third embodiment of a system for providing diverse point to point communication links, in accordance with the invention. The system 300 includes a delay network z21 , z2V placed at both the transmitter and receiver sides.

The delay networks of Figures 1 and 2 could be arranged at the receiver side (i.e. between the receive antennas AV and A2' and the receivers RX1 and RX2).

Without wishing to be bound by theory, the Applicant puts forward the following mathematical description. Initially, the focus is only phase shifts and large variations in signal strength. For this purpose, the signal flow can be modelled with complex numbers and matrices: r = Ht . A transmit coupling network Z_t , the antenna coupling network ^ , and the receive coupling network Z_r can be identified. The complete signal flow from transmitters to receivers is therefore: H = Z_rAZ_t .

With reference to the system 100 in Figure 1 , the coupling between the transmitters and receivers becomes:

π .

with z, ^J22 0.5e 1^J

π .,

and a,,

The variation in signal strength is not relevant to the first order, j is not used as an index, but as a complex number. In the ideal implementation, H will have non-zero elements only on its diagonal (or some permutation thereof).

For the system 200 of Figure 2, the coupling matrix is:

For the system 300 of Figure 3, the coupling matrix is:

π .

with z 21 1^J

In general terms, an array of N transmit and N receive antennas may be used, spaced d apart. With the basic differential path length, Al₀ , equal to some fraction of the wavelength: ΔΙ, o

m 2 L

Then the phase delay between transmit antenna / and receive antenna k can be written

2π

-j(k - l)

m

aki ^{= e}

Ideally, H should have zero elements except on its diagonal (or same permutation thereof). More generally, it is advantageous to minimise condition numbers of H which reduces the burden on the processing needed at the receiver.

The design procedure can be summarised as:

Select some elements of Z_t and Z_r to be zero from engineering considerations

(zero implies no delay element in that branch); and

Find the other coefficients optimising the configuration.

The Applicant notes that an interesting option when N is even is to choose m equal to 2N, then the elements of A become:

which corresponds to Zadoff Chu sequences. As a consequence,

H

A A

that is to say, the conjugate transpose of A is its inverse.

This provides many ways to construct Z_t and Z_r . Some are:

Select Z_t (or Z_r ) as identity and the other as A^H ; and

Construct Z, and Z„ from the LU factorization of A^H . λ

In a 4x4 example, the differential path delay, ΔΙ , is equal to— . Z_r is then selected as the

8

identity matrix and Z_t as A^H . The angles representing the delays are then: arctan(Z_t) -

This is non-causal, hence all the entries must uniformly adjusted to be physically realisable.

With regard to the coupling matrices H described above, ideally it is a permutation of a diagonal matrix (i.e. all the off-diagonal elements in the matrix being zero). This allows the receiver to recover the transmitted data as simple division operations. However, practical imperfections will result in these terms being non-zero, but still small in comparison with the diagonal elements. So, the receiver can employ simple iterative schemes to solve the resultant linear equations. As an example of such a procedure, the Jacobi method can be used (see, for example, https://en.wikipedia.org/wiki/Jacobi method). As noted in the references, this procedure can be implemented efficiently in parallel.

The Applicant has noted another practical difference between the ideal implementation described above. The ideal description assumes all transmitters work perfectly in phase. In practice, there will always be differences some differences in the phases of the individual transmitters. The delay networks can be used to compensate for this.

FIGURE 4 shows a system 400 with slightly different transmitter phases z_t1 and z_t2. Using the same coefficients as for FIGURE 1 , with the additional constraints,

zn^zti^ai2 + ^zt2 ^a22 - 0 and z₂₂z_t2a₂₁ + z_tla_n

Re-arranging yields

22 and ^J22

^Zti^ai2 ^Zt2 ^a21

This results in a correction to the values of the z_n and z₂₂ corresponding to the relatively phase difference between the two transmitters TX1 and TX2. In the event that the delay network z1 1 and z22 has adjustable gain, differences in the two transmitters TX1 and TX2 powers can also be accommodated. The method thus includes calculating phase differences between transmitters TX1 and Tx2 and compensating therefor. Similarly, equivalent conditions can be formulated and the corresponding corrections solved for larger systems. Further, in a practical system, it may be desirable to update the delay network parameters to compensate for the drift of environmental conditions, such as to optimise the resulting coupling matrix H . Typically, a calibration procedure is periodically executed to achieve this. Such a calibration procedure will typically transmit known data patterns, from which the receivers RX1 and RX2 can characterise the complete coupling from transmitters TX1 and TX2 to the receivers RX1 and RX2. This is then communicated back to the transmitters TX1 and TX2 to allow update of the delay networks (for example z1 1 and z22). Because it is a linear system, conventional linear solutions can be employed.

FIGURE 5 shows a system 500 for providing diverse point to point communication links, in accordance with the invention. The system 500 has a 4x4 antenna array and illustrates that the present disclosure is not limited to 2x2 systems (which have been illustrated in FIGURES 1 - 4 for ease of explanation).

FIGURE 6 shows a schematic view of a delay network 600 which may be used in one or more of the systems 100 - 500. The delay network 600 includes a signal processor 602 operable to introduce a delay to a received input signal 604 thereby to create a delayed but otherwise identical output signal 606. The signal processor 602 calculates the delay based on path length criteria 608 and optionally also phase difference criteria 610. The delay network 600 typically also includes a computer-readable medium having stored thereon a computer program which, when executed, directs the operation of the signal processor 602.

The Applicant believes that the systems 100 - 500 in accordance with the invention as exemplified have a number of advantages:

Many topologies are possible, each identified with different networks Z_t and Z_r ;

Many ways exist to implement the phase delays: different cable lengths, electronic phase shifting networks, etc. The exact technology used is an engineering choice and is not germane to the invention;

The examples do not make use of polarisation, so the invention equally applies to horizontal and vertical (or left and right) polarisation. All three examples can be converted into a 4x4 MIMO systems by combining them with polarisation;

The invention does not depend on the modulation technique used or any special timing; it may be used with analogue modulation (e.g. AM or FM);

The method does not concern itself with beam width or side-lobe suppression, typical figures of merit for phased array and beam forming design; For other frequencies, the differential path-length between the antennas will not exactly be a quarter wavelength. This results in residual, non-ideal, coupling, but can be removed by further processing on the receiving side (MIMO or processes similar to XPIC). Exact processing required is application dependent; and

Materials or filters with frequently-dependent delay can be used in the delay networks to broaden the bandwidth of the system.

Claims

1. A method of providing diverse point to point communication links, the method being operable in a MIMO network including:

a plurality of transmitters coupled to a plurality of transmit antennas; and a plurality of receive antennas coupled to a plurality of receivers, the transmit antennas being directed towards the receive antennas and each transmit/receive antenna pair defining a unique signal path therebetween, characterised in that the method includes:

2. The method as claimed in claim 1 , in which the delay is introduced by a delay network.

3. The method as claimed in claim 2, in which the delay is specifically matched to the difference between the first and second path lengths.

4. The method as claimed in any of the preceding claims, which includes arranging the antennas such that the signal path lengths differ by a quarter wavelength and in which the delay introduced is a quarter wavelength delay.

5. The method as claimed in any of the preceding claims, in which, by matching the relative delay in the signals to the difference between the signal paths, the receivers receive a plurality of combined in-phase signals from one transmitter, resulting in receipt of a full or near full strength signal, but a plurality of out-of-phase signals from another transmitter, resulting in receipt of little or no signal from the other transmitter.

6. The method as claimed in any of the preceding claims, in which:

there are a symmetric arrangement of antennas; and

there are an equal number (N) of transmit antennas and receive antennas.

7. The method as claimed in any of the preceding claims, in which the delay is introduced on the transmitter side only.

The method as claimed in claim 7, in which introducing the delay includes:

feeding a first transmit antenna with at least one non-delayed signal from a first transmitter and one delayed signal from a second transmitter; and feeding a second transmit antenna with a delayed signal from the first transmitter corresponding to the non-delayed signal of the first transmit antenna and a non-delayed signal from the second transmitter corresponding to the delayed signal of the first transmit antenna.

The method as claimed in any of claims 1 to 6, in which the delay is introduced on the receiver side only.

The method as claimed in claim 9, in which introducing the delay includes:

feeding a first receiver with at least one non-delayed signal from a first receiver antenna and one delayed signal from a second receive antenna; and feeding a second receiver with a delayed signal from the first receive antenna corresponding to the non-delayed signal of the first receiver and a non- delayed signal from the second receive antenna corresponding to the delayed signal of the first receiver.

The method as claimed in any of claims 1 to 6, in which the delay is introduced on both the transmitter and receiver sides.

The method as claimed in claim 1 1 , in which, a delay is introduced to at least one, but not all, transmit antennas, while a delay may also be introduced to at least one, but not all, receivers.

The method as claimed in any of the preceding claims, which includes:

calculating or measuring a phase difference between transmitters; and compensating for the calculated or measured phase difference by adjusting the relative delay in the delay networks accordingly.

A system for providing diverse point to point communication links, the system including:

a plurality of transmitters coupled to a plurality of transmit antennas; and a plurality of receiver antennas coupled to a plurality of receivers, the transmit antennas being directed towards the receive antennas and each transmit/receive antenna pair defining a unique signal path therebetween, characterised in that the system includes:

a delay network arranged between at least one of:

the transmitters and the transmit antennas (the transmitter side); or the receive antennas and the receivers (the receiver side),

15. The system as claimed in claim 14, in which the delay network is arranged at the transmitter side only.

16. The system as claimed in claim 15, in which the delay network is operable to introduce a delay into a signal fed to each transmit antenna.

17. The system as claimed in claim 14, in which the delay network is arranged at the receiver side only.

18. The system as claimed in claim 17, in which the delay network is operable to introduce a delay into a signal fed to each receiver.

19. The system as claimed in claim 14, in which the delay network is arranged at both the transmitter and receiver sides.

20. The system as claimed in claim 19, in which the delay network is operable to introduce a delay into a signal fed to at least one, but not all, transmit antennas and receivers.

21. The system as claimed in any of claims 14 to 20, which includes at least one processor operable to control or coordinate the delay network.

22. A non-transitory computer-readable medium having stored thereon a computer program which, when executed by a computer, is operable to calculate and introduce a delay or delays as per the method as claimed in any of claims 1 to 13.