Method and apparatus for operating co-located transceivers on the same frequency band

CROSS-REFERENCE TO RELATED APPLICATIONS

BACKGROUND

0001A Long Term Evolution (LTE) wireless network includes an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) (also sometimes referred to simply as the "radio access network" or "RAN") and an Evolved Packet Core (EPC) network (also sometime referred to simply as the "core network").

0002The E-UTRAN comprises a set of base stations that wirelessly communicate with user equipment (such as smartphones) using licensed radio frequency spectrum. Each base station is also generally referred to as an "eNodeB" or "eNB."

0003One type of eNodeB is a "macro" eNodeB (or eNodeB macro cell), which is a higher-power base station that is typically used to provide base station capacity in a relatively large area that includes both outdoor areas and indoor areas. In general, each location within a service provider's network is notionally within the coverage area of at least one macro eNodeB. However, in practice, there are some locations (for example, within homes and office buildings) for which good coverage cannot be provided by any macro eNodeB in an operator's network. Also, there may be some locations (for example, within public venues such as office buildings, stadiums, airports, etc.) where a large number of users congregate during certain periods. During those periods, the associated macro eNodeBs may not be able provide sufficient base-station capacity to the congregated users, even if it is possible to provide sufficient wireless coverage.

0004One type of eNodeB is a "small cell" or "femtocell," which is a lower-power base station. A small cell can be used to provide improved wireless coverage and/or capacity in order to address the issues noted in the previous paragraph. This is done by deploying the small cell directly with the location that has a coverage and/or capacity issue.

0005Each eNodeB communicates with entities in the core network (such as, a Serving Gateway (S-GW) and a Mobility Management Entity (MME)) using the "S1 interface" defined by the 3rd Generation Partnership Project (3GPP). Each eNB also communicates with other eNBs using the "X2 interface" defined by the 3GPP. These protocols are Internet Protocol (IP) based and often use public networks such as the Internet. The communication link that couples an eNodeB to the core network is referred as the "back haul" link.

0006Such a back haul link can be implemented using a wired connection. The back haul link can also be implemented using a wireless connection. Indeed, in some small cell deployments, the wireless back haul for a small cell eNodeB is provided using a wireless communication link that is implemented with a macro eNodeB. That is, for such a small cell eNodeB, a wireless modem, or backhaul wireless modem, is provided with the small cell that functions as user equipment from the perspective of the macro eNodeB and that functions as the termination point of an IP connection from the perspective of the small cell eNodeB.

0007In some cases where this type of wireless backhaul is used with a small cell, the small cell and corresponding backhaul wireless modem are proximate and may operate in the same band using time division duplexing (TDD). However, transmissions by the transmitter of one transceiver in either the small cell or the backhaul wireless backhaul modem may desensitize the receiver of the other transceiver in either the backhaul wireless modem or the small cell, respectively, due to saturation of the receiver caused by the relatively high incident power from the nearby transmitter, and an increased noise floor due to noise, <i>e.g</i>. sideband noise, broadcast by the transmitter.

SUMMARY OF THE INVENTION

0008A communication system as defined by independent claim 1, comprising a base station serving a small cell and a backhaul wireless modem. And a method for operating the communication system of claim 1, as defined by independent claim 12. Advantageous embodiments are set out in the dependent claims.

DRAWINGS

0009Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail through the use of the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> illustrates a block diagram of an exemplary communications network with selective filtering;</li><li><figref idref="f0002">Figure 2</figref> illustrates an exemplary frequency band with a first sub-band and an adjacent second sub-band;</li><li><figref idref="f0003">Figure 3</figref> illustrates an exemplary small cell with selective filtering;</li><li><figref idref="f0004">Figure 4A</figref> illustrates one embodiment of a transmitter front end;</li><li><figref idref="f0004">Figure 4B</figref> illustrates one embodiment of a receiver front end;</li><li><figref idref="f0005">Figure 5</figref> illustrates an exemplary embodiment of a backhaul wireless modem with selective filtering;</li><li><figref idref="f0006">Figure 6</figref> illustrates an embodiment of a selective filter;</li><li><figref idref="f0007">Figure 7</figref> illustrates one embodiment of a three-port filter; and</li><li><figref idref="f0008">Figure 8</figref> illustrates one embodiment of operation of the exemplary communications network with selective filtering.</li></ul>

DETAILED DESCRIPTION

0010Embodiments described below provide a technique to enable a small cell and a backhaul wireless modem to operate when proximate. Advantageously, embodiments enable the small cell and the backhaul wireless modem to be co-located by using inexpensive components such as commercial off the shelf parts.

0011<figref idref="f0001">Figure 1</figref> illustrates a block diagram of an exemplary communications network with selective filtering 100. The communications network with selective filtering 100 includes one or more user equipment (UE) 102, a small cell with selective filtering 104, a backhaul wireless modem with selective filtering 106, and a macro cell 108. In one embodiment, the small cell with selective filtering 104 and the backhaul wireless modem with selective filtering 106 may be packaged together, forming an integrated small cell and backhaul wireless modem with selective filtering 105.

0012Both the small cell with selective filtering 104 and the macro cell 108 are base station entities (for example, eNodeBs in the case of a LTE network) and are coupled to the operator's core network 110. The macro cell 108 can be coupled to the core network 110 using any suitable wired or wireless backhaul communication link. The small cell with selective filtering 104 is coupled to the core network 110 using a wireless back haul link implemented with the macro cell 108. The backhaul wireless modem with selective filtering 106 is provided with the small cell with selective filtering 104, and functions as user equipment from the perspective of the macro cell 108 and functions as the termination point of an IP connection from the perspective of the small cell with selective filtering 104.

0013The small cell with selective filtering 104 and the backhaul wireless modem with selective filtering 106 may operate, or be used, on the same band (for example LTE band 41) using time division duplexing. When the small cell with selective filtering 104 and backhaul wireless modem with selective filtering 106 are proximate and operate simultaneously using TDD in the same band, the receiver of each may become de-sensitized. To remedy this problem and permit proximate, simultaneous operation, selective filtering is used. Selective filtering involves operating the transceivers of each of the small cell with selective filtering 104 and the backhaul wireless modem with selective filtering 106 on different, but adjacent sub-bands in the same operating band, and providing sufficient filtering in each transceiver to substantially reduce or even eliminate receiver desensitization. Adjacent means, with respect to the proximity of sub-bands, that the sub-bands are separated by a guard band whose bandwidth is equal to, greater than, or less than the guard band specified for frequency division duplexing using the same air interface.

0014Time division multiplexing is implemented as follows. In one embodiment, the one or more user equipment (UE) 102 and the small cell with selective filtering 104 transmit and receive wirelessly on a first sub-band of a frequency band when the backhaul wireless modem with selective filtering 106 and a macro cell 108 respectively receive and transmit on the second sub-band of the frequency band. <figref idref="f0002">Figure 2</figref> illustrates an exemplary frequency band 200 with a first sub-band 202 and an adjacent second sub-band 204. In one embodiment, the frequency band 200 is LTE band 41 which ranges from 2496 MHz to 2690MHz. In another embodiment, the first sub-band 202 is from 2496MHz to 2570MHz. In a further embodiment, the second sub-band 204 is from 2630 to 2496MHz. In yet another embodiment, the first sub-band 202 and the second sub-band are separated by a 60MHz guard band. In yet a further embodiment, the first sub-band 202 and the second sub-band 204 are respectively the uplink sub-band and the downlink sub-band for a frequency division duplexing band for another air interface, <i>e.g.</i> 3G UMTS. In another embodiment, the first sub-band 202 is approximately uplink sub-band for 3G UMTS band 7, and the second sub-band 204 is approximately downlink sub-band for 3G UMTS band 7. The uplink sub-band for 3G UMTS band 7 is 2500MHz to 2570MHz. The downlink sub-band for 3G UMTS band 7 is 2620MHz to 2690MHz.

0015The first sub-band 202 and the second sub-band 204 are used respectively by the small cell with selective filtering 104 and the backhaul wireless modem with selective filtering 106, or vice versa. The small cell with selective filtering 104 and the backhaul wireless modem with selective filtering 106 operate in TDD each transmitting when the other is receiving. To avoid desensitizing the receiver in each, selective filtering is used in each of the small cell and the backhaul wireless modem as will now be described. The selective filtering in a transceiver, operating in a sub-band, provides enhanced filtering and suppression of undesired signals in the adjacent sub-band used by the other transceiver.

0016<figref idref="f0003">Figure 3</figref> illustrates an exemplary small cell with selective filtering 300. The small cell with selective filtering 300 employs multiple-input multiple-output (MIMO) technology having two transmission channels and two reception channels. However, in alternative embodiments, the small cell with selective filtering 300 may have more than two transmission channels and may have more than two reception channels.

0017One transmission channel and one reception channel are coupled to a common antenna, through a transmit-receive switch, and to the first transceiver 305. The first transceiver 305 is also coupled to the transmit receive-switch, and is configured to select whether the antenna is connected to either the receive channel or the transmit channel depending upon whether the first transceiver 305 is transmitting or receiving. The first transceiver is also coupled to the backhaul wireless modem with selective filtering 106.

0018The transmit-receive switches enable time domain duplexing operation by the small cell with selective filtering 300, and the subsequently described backhaul wireless modem with selective filtering 106. The transmit-receive switches must have sufficient insertion loss, isolation, and power compression to attain the design parameters for the transceiver in which the transmit-receive switches are used. The illustrated transmit-receive switches are single pole double throw (SPDT) switches. However, N pole double throw (NPDT) switches, where N > 2, can be used to couple two or more pairs of reception and transmission channels to corresponding antennas.

0019In one embodiment, each transmit channel is comprised of a transmitter front end with selective filtering, and an upconverter and a baseband processor of the first transceiver 305. In another embodiment, each receive channel is comprised of a receiver front end with selective filtering, and a downconverter and the baseband processor of the first transceiver 305.

0020The exemplary small cell with selective filtering 300 comprises a first transmitter front end 302a and a second transmitter front end 302a, and a first receiver front end 304a and a second receiver front end 304a. The first transmitter front end 302a and the first receiver front end 304a are alternately coupled to a first antenna 316a. The second transmitter front end 302b and the second receiver front end 304b are alternately coupled to a second antenna 316b. The first transmitter front end 302a, the second transmitter front end 302b, the first receiver front end 304a, and the second receiver front end 304b are coupled to a first transceiver 305. In one embodiment, the first transceiver 305 includes at least one baseband processor, at least one upconverter, and at least one downconverter. In another embodiment, <i>e.g</i>. for a first transceiver 305 implementing two MIMO transmit and two MIMO receive channels, the first transceiver 305 includes at least one baseband processor, two upconverters, and two downconverters. In another embodiment, the first transceiver 305 includes at least one baseband processor, two upconverters, and two downconverters.

0021The output of the first transmitter front end 302a and the input of the first receiver front end 304a are respectively coupled to the first terminal and the second terminal of a first transmit-receive (TR) switch 312a. The output of the second transmitter front end 302b and the input of the second receiver front end 304b are respectively coupled to the first terminal and the second terminal of a second transmit-receive switch 312b. In one embodiment, the first transmit-receive switch 312a and the second transmit-receive switch 312b are single pole double throw (SPDT) switches. The common terminals of the first transmit-receive switch 312a and second transmit-receive switch 312b are respectively coupled to a first antenna 316a and a second antenna 316b. The first transmit-receive switch 312a and the second transmit-receive switch 312b are respectively coupled to the first transceiver 305 by the first TR switch control line 330a and the second TR switch control line 330b. Control signals, from the first transceiver 305 are communicated over the first TR switch control line 330a and the second TR switch control line 330b, and respectively control the position of the first transmit-receive switch 312a and the second transmit-receive switch 312b to permit the small cell with selective filtering 300 to switch between transmitting and receiving to properly operate in time division duplexing mode.

0022A first transmit signal 320a flows in the first transmitter front end 302a from the first transceiver 305 towards the first antenna 316a. A second transmit signal 320b flows respectively in the second transmitter front end 302b from the first transceiver 305 towards the second antenna 316b. A first receive signal 322a flows in the first receiver front end 304a to the first transceiver 305 away from the first antenna 316a. A second receive signal 322b flows respectively in the second receiver front end 304b to the first transceiver 305 away from the second antenna 316b.

0023<figref idref="f0004">Figure 4A</figref> illustrates one embodiment of a transmitter front end 400. In another embodiment, each transmitter front end includes a power amplifier 408, a low pass filter (LPF) 410, and a first selective filter (SF) 416a. An input of the power amplifier 408 is coupled to an output of a transceiver, <i>e.g</i>. an output of an exciter in the transceiver. An output of the power amplifier 408 is coupled to an input of the low pass filter 410. An output of the low pass filter 410 is coupled to an input of the first selective filter 416a. The output of the first selective filter 416a is coupled to a transmission-receive switch.

0024In one embodiment, a buffer amplifier (BA) 414 is inserted between the transceiver and the power amplifier 408. The input of the buffer amplifier 414 is coupled to an output of the transceiver. The output of the buffer amplifier 414 is coupled to an input of the power amplifier 408.

0025In one embodiment, a second selective filter 416b is inserted between the transceiver and the power amplifier 408 to provide additional filtering and suppression of out of sub-band signals broadcast from the transceiver. The input of the second selective filter 416b is coupled to the output of either the buffer amplifier 414, if used, or alternatively to the output of the transceiver. The output of the second selective filter 416b is coupled to the input of the power amplifier 408. The second selective filter 416b can be employed to further suppress out-of-band noise, <i>e.g</i>. sideband noise, generated by the transmitter of a transceiver. The second selective filter 416b may be required if the transmitter front end has high gain which would amplify such noise. In one embodiment, the one or more selective filters in the transmitter front end attenuate signals, broadcast by the transmitter of the transceiver and in the sub-band of the other transceiver, by at least fifty-five decibels.

0026<figref idref="f0004">Figure 4B</figref> illustrates one embodiment of a receiver front end 430. In another embodiment, each receiver front end includes a third selective filter 416c and a low noise amplifier 436. An output of the third selective filter 416c is coupled to an input of a low noise amplifier 436. An input of the third selective filter 416c is coupled to a transmit-receive switch. An output of the low noise amplifier 436 is coupled to an input of the transceiver, <i>e.g</i>. the input of a downconverter. In the embodiment illustrated in <figref idref="f0004">Figure 4B</figref>, a fourth selective filter 416d is coupled between the input of a transceiver and the output of the low noise amplifier 436 to provide enhanced filtering and suppression of out-of-band signals, <i>e.g.</i> from the adjacent transmitter. In one embodiment, the selective filter(s) in the receiver front end attenuates signals, broadcast by and in the sub-band of the other transceiver, by at least fifty-five decibels.

0027<figref idref="f0005">Figure 5</figref> illustrates an exemplary embodiment of a backhaul wireless modem with selective filtering 500. The exemplary backhaul wireless modem with selective filtering 500 employs multiple-input multiple-output (MIMO) technology having two transmission channels and four reception channels. However, in another embodiment, the backhaul wireless modem with selective filtering 500 may have more than two transmission channels, and may have two or more than four reception channels. In a further embodiment, a multielement, <i>e.g.</i> eight elements, smart antenna having beam steering is used in lieu of the individual antennas in the backhaul wireless modem with selective filtering 500. Such beam steering increases equivalent isotropic receiver sensitivity and radiated power of the backhaul wireless modem with selective filtering 500

0028Two pairs of transmission and reception channels are coupled to common antennas through transmit-receive switches, and to the second transceiver 505 in a manner similar to the illustration of <figref idref="f0003">Figure 3</figref>. Two additional receiver front ends with selective filtering are used to provide extra link margin, and are also coupled to the second transceiver 505 and individual antennas. The implementations of the transmitter front ends and receiver front ends of the backhaul wireless modem with selective filtering 500 may be implemented as described above in <figref idref="f0004">Figures 4A and 4B</figref>. In one embodiment, the second transceiver 505 includes at least one baseband processor, at least one upconverter, and at least one downconverter. In another embodiment, the second transceiver 505 includes at least one baseband processor, two upconverters, and four downconverters. The second transceiver 505 is also coupled to the small cell with selective filtering 104.

0029A third transmitter front end with selective filtering 502a has an input coupled to an output of the second transceiver 505, and an output coupled to a terminal of a third transmit-receive switch 512a. A third receiver front end with selective filtering 504a has an input coupled to another terminal of the third transmit-receive switch 512a, and an output coupled to an input of the second transceiver 505. The output of the third transmit-receive switch 512a is coupled to a third antenna 516a. A third TR switch control line 530a couples the second transceiver 505 to the third TR switch 512a to ensure proper time division duplexing operation.

0030A fourth transmitter front end with selective filtering 502b has an input coupled to an output of the second transceiver 505, and an output coupled to a terminal of a fourth transmit-receive switch 512b. A fourth receiver front end with selective filtering 504b has an input coupled to another terminal of the fourth transmit-receive switch 512b, and an output coupled to an input of the second transceiver 505. The output of the fourth transmit-receive switch 512b is coupled to a fourth antenna 516b. A fourth TR switch control line 530b couples the second transceiver to the third TR switch 512a to ensure proper time division duplexing operation.

0031A fifth receiver front end with selective filtering 504c has an input coupled to a fifth antenna 516c, and an output coupled to an input of the second transceiver 505. A sixth receiver front end with selective filtering 504d has an input coupled to a sixth antenna 516d, and an output coupled to an input of the second transceiver 505.

0032A third transmit signal 520a flows in the third transmitter front end 502a from the second transceiver 505 towards the third antenna 516a. A fourth transmit signal 520b flows respectively in the fourth transmitter front end 502b from the second transceiver 505 towards the fourth antenna 516b. A third receive signal 522a flows in the third receiver front end 504a to the second transceiver 505 away from the third antenna 516a. A fourth receive signal 522b flows respectively in the fourth receiver front end 504b to the second transceiver 505 away from the fourth antenna 516b. A fifth receive signal 522c flows in the fifth receiver front end 504c to the second transceiver 505 away from the fifth antenna 516c. A sixth receive signal 522d flows in the sixth receiver front end 504d to the second transceiver 505 away from the sixth antenna 516d.

0033Returning to <figref idref="f0004">Figure 4</figref>, each selective filter can be implemented with one filter, <i>e.g.</i> a band pass filter having a center frequency and corner frequencies, corresponding to the sub-band being used by the corresponding transceiver system in which the selective filtering is used. <figref idref="f0006">Figure 6</figref>, however, illustrates another embodiment of a selective filter 600 which utilizes low cost, commercial off the shelf technology. The selective filter 600 includes a single pole double throw (SPDT) switch 604 coupled to a three-port filter 602. The SPDT switch 604 has two terminals 610a, 610b respectively coupled to two ports 608a, 608b, of the three-port filter 602, corresponding to different sub-bands. The SPDT switch 604 also has a common terminal 606b. The three-port filter 602 has a common port 606a. The common terminal 606b of the SPDT switch 604, and the common port 606a of the three-port filter 602, are the input /output terminals of the selective filter. In one embodiment, because the selective filter 600 is a reciprocal network the common terminal 606b and common port 606a can be used respectively as the input and output of the selective filter 600, or vice versa.

0034In one embodiment, the isolation between an unconnected terminal <i>(i.e.</i> not selected terminal) and the common terminal of the SPDT switch 604 in the adjacent sub-band is equal to or greater than the corresponding attenuation across the adjacent sub-band of the selective filter. This prevents the undesired out-of-band signals from leaking through the unselected terminal of the SPDT switch 604. Also, the insertion loss and power compression of the SPDT switch 604 should be sufficiently low and high to achieve the design of parameters the transceiver in which the SPDT switch 604 is used.

0035In one embodiment, the three-port filter 602 is a duplexer, such as a commercial off the shelf duplexer for 3G UMTS band 7. In another embodiment, the three-port filter 602 is a diplexer.

0036<figref idref="f0007">Figure 7</figref> illustrates one embodiment of a three-port filter 700. The three-port filter 700 comprises filter 1 702 and filter 2 704 having a common port 710, and unique ports 706, 708 for each filter. In one embodiment, both filter 1 702 and filter 2 704 are bandpass filters, <i>e.g.</i> each having a center frequency and corner frequencies corresponding approximately to each sub-band. In another embodiment, the filter corresponding to the higher frequency sub-band can be implemented with a high pass filter. In a further embodiment, the filter corresponding to the lower frequency sub-band can be implemented with a low pass filter.

0037Returning to <figref idref="f0006">Figure 6</figref>, the SPDT switch 604 has a control terminal 612 which can be biased, <i>e.g</i>. by a voltage, to select which terminal 610a, 610b is coupled to the common terminal. This can be used select whether the selective filter 600 uses the filter corresponding to the upper or lower sub-band. Thus, one selective filter 600 can be manufactured for use in both the small cell with selective filtering 104 and the backhaul wireless modem with selective filtering 106. When implemented in the small cell with selective filtering 104 and the backhaul wireless modem with selective filtering 106, the control terminals 612 of each selective filter 600 is biased to select the one filter in one sub-band for the small cell with selective filtering 104 and the other filter in the second sub-band for the backhaul wireless modem with selective filtering 106. For example, if the small cell with selective filtering 104 operates in the higher frequency sub-band, then the control terminal 612 for the selective filters 600 in the small cell with selective filtering 104 are biased so that they use the filter having a center and corner frequencies about that sub-band.

0038<figref idref="f0008">Figure 8</figref> illustrates one embodiment of operation of the exemplary communications network with selective filtering 100. In block 802, in one embodiment, select filters, <i>e.g.</i> by biasing the control terminals 612 of the selective filters 600 used in the small cell with selective filtering 104 and the backhaul wireless modem with selective filtering 106. In block 804, transmit in a first sub-band of a frequency band in which time division duplexing is required, while receiving in a second sub-band of the frequency band. In one embodiment, transmit a first signal in the first sub-band from the small cell with selective filtering 104, <i>e.g.</i> to user equipment 102; receive a second signal in the second sub-band at the backhaul wireless modem with selective filtering 106, <i>e.g.</i> from the macro cell 108. In block 806, transmit in the second sub-band, while receiving in the first sub-band. In one embodiment, receive a third signal in the first sub-band at the small cell with selective filtering 104, <i>e.g.</i> from user equipment 102; transmit a fourth signal in the second sub-band from the backhaul wireless modem with selective filtering 106, <i>e.g.</i> to the macro cell 108.

0039A number of embodiments of the invention defined by the following claims have been described. Nevertheless, it will be understood that various modifications to the described embodiments may be made without departing from the scope of the claimed invention. Accordingly, other embodiments are within the scope of the following claims. For example, a signal may be a voltage signal or a current signal.