In a traditional cellular telecommunications system, the coverage of a cell is defined by the geographical area where Radio Frequency (RF) signals transmitted from a base station to a mobile wireless device, and vice versa, can be successfully received and decoded. For ease of discussion, a mobile wireless device may be interchangeably referred to as a User Equipment (UE). Also, the “RF signals” may be more simply referred to herein as “radio signals.” The base station may be equipped with an antenna or a two-dimensional (2D) antenna array that transmits and receives radio signals according to an antenna beam pattern that typically spans a quite large angle in azimuth and/or elevation. The wider the angle is, the lower the antenna gain becomes. Hence, there is a tradeoff between angular coverage and coverage range for a given antenna pattern. In order to have a large angular coverage in combination with high antenna gain, a steerable antenna array can be used to form and steer beams in desirable directions.
In the coverage-related discussion herein, a “cell” and its associated base station such as, for example, an eNodeB, or a base station and its antenna array, may be referred to in an interchangeable manner and identified using the same reference numeral for ease of discussion. For example, a UE may be interchangeably referred to as receiving radio signals from a cell or an eNB, or the UE may be interchangeably referred to as receiving signals from a base station or the base station's antenna/antenna array.
Beamforming
The base stations in modern cellular systems may also employ beamforming (or beam steering). Beamforming or spatial filtering is a signal-processing technique used in antenna arrays for directional signal transmission and/or reception. It is understood that digital content may be transmitted using analog radio signals. In beamforming, the analog radio signals may be processed/shaped such that signals at particular angles experience constructive interference, while others experience destructive interference. Such analog beamforming can be used at both the transmitting and receiving ends to achieve spatial selectivity, such as, for example, rejection of unwanted signals from specific directions. The spatial selectivity may provide improved reception/transmission of signals in the system. Thus, beamforming can help improve wireless bandwidth utilization, and it can also increase a wireless network's range. This, in turn, can improve video streaming, voice quality, and other bandwidth- and latency-sensitive transmissions.
Beamforming can be achieved by controlling the phase and amplitude of different signals transmitted from and/or received from spatially separated antenna elements. This can be done, for example, using an antenna array with multiple ports or an active antenna with multiple sub-elements. Each sub-element may have a polarization direction, which potentially can be orthogonal to another sub-element's polarization.
For a beamforming system that only supports a set of fixed beams, all signals may be beamformed, although the desired direction of transmission may be unknown or only known to some extent. Furthermore, some beamforming systems, such as, for example, analog beamforming systems, can only transmit in one or a few beams simultaneously. In such systems, multiple beams may have to be scanned through in time domain to provide coverage to all the UEs attached to the base station.
Some form of information related to the radio channel between a transmitter, such as an eNB, and a receiver, such as a UE, is typically needed in order to perform efficient beamforming. Channel State Information (CSI) is given either in explicit or implicit form. Explicit CSI contains gain and phase difference between all pairs of transmit and receive antennas. On the other hand, implicit CSI is typically given by spatial precoder selections from the UE.
Beamforming is commonly performed so as to maximize the received power at a UE. Certain beamforming techniques have other objectives in addition to boosting the received signal power, for example, to remove or reduce the interference. Two examples of such objectives are the zero-forcing criterion, and the signal-to-leakage-plus-noise objective function.
Beamforming using implicit CSI is often more limiting in interference suppression capabilities since the complete channel is not known at the eNodeB. In this case, the straightforward beamformer (precoder) would be to use the one recommended by the UE, although eNB-based adjustments of the beamformer to reduce the interference is conceivable.
A special type of implicit CSI is beam selection feedback. In this case, the eNodeB transmits a plethora of spatially-distinct probing signals that are beamformed in specific directions. A UE is then instructed to select the most preferred beam, for example, in terms of received signal power, and report this to the eNodeB. An advantage of this type of feedback is that the number of antenna elements can be de-coupled from the CSI feedback; the UE need not estimate the full channel matrix. It is noted here that even if beam selection feedback may be appropriate for a dynamic beam selection system, other kinds of implicit or explicit CSI also can be used in a dynamic beam selection system. For example, a dynamic beamforming system may very well be based on implicit CSI reports, where, instead of the eNodeB transmitting a distinct set of beamformed reference signals, it transmits non-precoded reference signals from each antenna element.
Beamforming systems may have a calibration mismatch between the transmit (Tx) and receive (Rx) sides of an antenna array. On the other hand, some beamforming systems may even have separate 2D antenna arrays for transmission and reception, such that beamforming-related directional information regarding a beam received in the Uplink (UL) may not be applied to a beam transmitting in the Downlink (DL). These antenna arrays, however, may form part of a base station's antenna system. It is noted here that the terms “uplink” and “downlink” are used in their conventional sense: a transmission in the UL refers to a UE's transmission to a base station, whereas a transmission in the DL refers to a base station's transmission to a UE.
Elevation Beamforming
As previously stated, one way to perform beamforming is to use active antennas. An active antenna consists of a number of sub-elements that jointly form the antenna. The sub-elements can be virtualized. For example, pairs of physical sub-elements could be fed the same signal and, hence, share the same virtualized sub-element antenna port. Furthermore, in the case where an active antenna is mounted in such a way that the sub-elements are spread out on a vertical axis, a beamforming technique known as “elevation beamforming” may be possible. In the elevation beamforming, the transmitted and/or received signal may be directed in the elevation domain. This may be done by using different phases and amplitudes for the different sub-elements of the active antenna such that at certain angles—relative to the active antenna's vertical axis—the different signals experience constructive interference, whereas at other angles they experience destructive interference. In the discussion herein, the term “elevation beamforming” may be exclusively used as a dynamic beam-selection technique in the sense that an eNodeB using elevation beamforming may use different elevation beams to serve different UEs. Elevation beamforming may be a component of the more general case of joint elevation-azimuth beamforming from a two-dimensional antenna array.
It is noted here that the discussion herein primarily focuses on elevation beamforming as an example only. For the sake of brevity, all different types of beamforming techniques—such as, for example, azimuth beamforming, or joint azimuth and elevation beamforming using a 2D antenna array—are not discussed in appreciable detail.
FIG. 1 illustrates an example of a dynamic elevation beam selection. A base station or eNodeB 10 is shown to dynamically perform beam selection among three different elevation beams 12-14 (also identified as beams A-C, respectively). One UE 16 is shown—by way of an example—as being physically present and operating (or registered) within the cell (not shown) associated with the base station 10. For the sake of discussion herein, a UE, such as the UE 16, may be considered “attached to” or under the operative control of a base station or eNB, such as the base station 10. When a UE is attached to an eNB, a bi-directional communication session is established between the UE and the eNB for data transfer to/from the UE, thereby enabling the UE's user to carry out voice calls, data sessions, web browsing, and the like, using the cellular network of the eNB. In the case of a static beamformer, the eNodeB 10 would need to use one beam for all transmissions and, hence, would not be able to focus the transmitted power in the direction towards its UE of interest—here, the UE 16. On the other hand, in the directional transmissions using dynamic elevation beamforming, the eNB 10 can dynamically select the most appropriate elevation beam for the UE 16. For example, in the context of FIG. 1, the eNB 10 may select the elevation beam 13 (beam B) to transmit to the UE 16. The choice of beam B has the advantage that the transmitted energy will be directed in the same direction as the propagation path 18 between the eNodeB 10 and the UE 16. This leads to the UE 16 receiving a stronger signal from the eNB 10.
It is noted here that three selection beams 12-14 are shown in FIG. 1 just as an example. In a more general setting, there can be any number of selection beams. Furthermore, the used transmission beams can even be created dynamically, pointing in an arbitrary elevation direction and with an arbitrary shape (for example, beam width). Hence, an infinite number of elevation beams may be possible.
Cell Selection for an Elevation Beamforming System
In order for a UE to be served by an eNB, the UE first needs to connect (or attach) to the eNB in some way. This is typically done using some kind of control signaling such that a UE can compare different control signals, transmitted from different eNodeBs, and then attach to the eNodeB that corresponds to the strongest signal. The control signal may be typically beamformed and such a beamformed control signal may be referred to as a “cell selection beam” in the discussion herein.
FIG. 2 depicts cell selection beams 20-21 along with data transmission beams 23-26 and 28-31 in an elevation beamforming system. In FIG. 2, by way of an example, two eNodeBs 33-34 in the system are shown as performing elevation beamforming—using four elevation beams each 23-26 and 28-31, which are shown by dotted lines—when transmitting data. On the other hand, each eNB 33-34 provides one cell selection beam 20-21 (shown using solid lines), respectively. The different UEs in the system may then attach to the eNodeB that corresponds to the strongest cell selection beam. Thus, for example, the UEs 36-38 may attach to the eNB 33, whereas the UEs 40-42 may attach to the eNB 34. As before, the number of UEs, the number of eNBs, and the number of elevation beams in FIG. 2 are exemplary only.
Once a UE has attached to a certain eNodeB, it can have its data transmitted with one of the data transmission beams (or elevation beams) from the eNodeB. Three exemplary data transmission beams per eNB in FIG. 2 are shown by dotted lines. It is pointed out here that there may be many other signals also transmitted by an eNodeB. One such example is a cell selection beam—like the beam 20 or 21—that may also serve legacy UEs which are not able to utilize UE-specific elevation beamforming. In the same manner, there also may exist other data channels.
It is noted here that the term “cell” is used quite generally. In addition to referring to a typical cell in a cellular telecommunication network, it may also refer to a node, a point, a transmission point, a group of transmission points, and the like, in a wireless communication network.
Importance of Interference Avoidance
The above-described dynamic elevation beam selection is a powerful tool for directing the transmitted energy towards the UE of interest, thereby increasing the received signal level at the UE. However, interference is another aspect of this approach that needs to be taken into consideration to maximize the system performance. FIG. 3 is an exemplary illustration of how dynamic elevation beamforming may cause interference. In FIG. 3, two base stations (eNBs) 45-46 are shown to provide three elevation beams each 48-50 and 52-54. Each elevation beam in the configuration of FIG. 3 serves one of the three respective mobile wireless devices (or UEs) 56-58 and 60-62, as shown. It is observed here that when an eNB directs its transmitted power towards a specific UE, it may at the same time also direct the transmitted energy towards another UE that may be currently receiving its serving signal from another eNB. This situation is illustrated in FIG. 3 by the arrow 64, which shows that the directional signal transmission to the UE 57 served by the eNB 45 may also reach the UE 61 served by the other eNodeB 46. Thus, eNodeBs may cause interference to their neighboring cells when performing dynamic elevation beamforming, and this interference may be very harmful for the overall performance of the system or cellular network. In fact, it is possible that employing dynamic elevation beamforming in a communication system may not lead to a system-level gain such as, for example, when the increase in the received signal level by dynamic beam selection may be less than the simultaneous increase in the interference level.
As before, it is emphasized that elevation beamforming is used as an example only to illustrate how interference may arise in beamformed systems—whether utilizing elevation beamforming, azimuth beamforming, or joint azimuth and elevation beamforming. Furthermore, various elevation beam patterns shown in FIGS. 1-3 (and later in FIGS. 5-6) are also by way of examples only. In practice, a single base station may not provide all of such beam patterns, and different base stations may provide different types of elevation beam patterns having different angular power profile.