each corner of the window,there are 1512 antennas in this Research direction mely large apert山reay MIMO [55 A M056 compact arrays illustrated in Fig.3:the antenna separation is at the order of meters,which is much larger than the wavelength lolographic Massive MIM (be ing in fron de s62 exa mple of an ELAA is when the antennas are distributed nt wa so that each user is essentially ce66 TL: alled Cell-fre eMIM0401-42. TABLE I:The proposed research directions 1 and 2 collect many other research topics as special cases concent has its roots in papers on distributed mimo from th early 00s [43].[44]andc oordinated multipoint from the early Impo tly,the spat an bt th so it is generally beneficial to beamforming at a particular point in space using we ons but are nhase-shifted to add con ution the 48 491 at the target point.Due to the different directivity.the signal ually decays when leaving target poin We use the flaa terminology to iointly des ibe a family gra of arch topics that have previously beer considered sepa- n a s d the tar with radius S 541.The rate owing c that features signal amplification is typically of this size in the BS an nas that are jointly and coherently serving many ear-field while it can be much larger in the far-field,implying distrbuted users. belia list cial cases s is provided in Table A.Vision quence of using elaas is that the radiative near The grand vision of ELAAs is to provide order -of field stretches many kil away magnitude hig oughput in wireless networks com y be in the ary spatial channel ies [391.[501.In the and the distribut ed ante field,the signal that reaches the array from a user s well and inc the appro by a superpos of pla ach in th don (UDN ith an Ao 661.167]that alsc resolve not only the Aoa of a wave but als but each antenna box is then an auton us BS that service has traveled (e.g.the spatial depth)by exploiting the spherical n exclus e set of users.It is known that the throughpu shape of t It is a that rences to visihle to a suhset of the in the this barrier as the number of antennas grows large [281.[29 blocked to the other antennas 51].Hence.channel mo deling [691-1711.at least in theory.The ultimate goal of ELAAs i is substantially o deploy so ma erently ope rating antennas that all the ore p when using EL MAs and ramel hard ilar to to a per- out since there are many antenna vith similar channel gain hannel without any propagation loss [53].[72].In additio exp I the rom ELA epa ate o enhancing mobile broadb services,which c the h grea in 53 which of an unprecedented number of machine-t is known favorable propagation in t ive MIMO devices [3].Anothe quenc rans The mas production of smartphones has tured adva om at dis atly the nid deve ent of Sim 5 each corner of the window, there are 1512 antennas in this example. Suppose the adjacent windows are 3 m apart, then the array spans an area of 24 m × 60 m. This is an Extremely Large Aperture Array (ELAA) [39] compared to the conventional compact arrays illustrated in Fig. 3; the antenna separation is at the order of meters, which is much larger than the wavelength (being in the range from one decimeter down to a few millimeters in the frequency ranges considered in 5G). Another example of an ELAA is when the antennas are distributed over a large geographical area so that each user is essentially surrounded by BS antennas, rather than the conventional case of each BS being surrounded by users. This has recently been called Cell-free Massive MIMO [40]–[42], but the basic concept has its roots in papers on distributed MIMO from the early 00s [43], [44] and coordinated multipoint from the early 10s [45]–[47]. Importantly, the spatial resolution of an array is not determined by the number of antennas but the array’s aperture, so it is generally beneficial to spread out antennas, even if this also gives rise to spatial aliasing phenomena where signals coming from widely different directions cannot be separated [3]. Non-uniform array geometries can provide better spatial resolution than uniform geometries [48], [49]. We use the ELAA terminology to jointly describe a family of research topics that have previously been considered sepa￾rately but all comply the following definition. Definition 1: An ELAA consists of hundreds of distributed BS antennas that are jointly and coherently serving many distributed users. A list of different special cases is provided in Table I. We believe that these special cases can, to a large extent, be jointly analyzed under the ELAA umbrella in the future. A consequence of using ELAAs is that the radiative near- field stretches many kilometers away from the array. Hence, the users will typically be in the near-field of the array, instead of the far-field as is traditionally the case, leading to non￾stationary spatial channel properties [39], [50]. In the far- field, the signal that reaches the array from a user is well approximated by a superposition of plane waves, each being determined by two parameters: a channel gain and an AoA. In contrast, an array with an extremely large aperture can resolve not only the AoA of a wave but also the distance it has traveled (e.g., the spatial depth) by exploiting the spherical shape of the wave and/or the channel gain differences to the antennas. It is also possible that some wave components are only visible to a subset of the antennas in the array and blocked to the other antennas [51]. Hence, channel modeling is substantially harder when using ELAAs and involves many more parameters. While conventional Massive MIMO benefits from channel hardening, where the small-scale fading average out since there are many antennas with similar channel gains, we cannot expect the same from ELAAs since well separated antennas have large gain differences [52]. On the other hand, the great spatial resolution will likely make the channels to different users nearly orthogonal [50], [52], [53], which is known as favorable propagation in the Massive MIMO literature [3]. Another consequence is that the transmission from the array cannot be illustrated as a beam, but rather as strong coherent signal amplification at distinct points in space, Research direction Special cases Extremely large aperture array Cell-free Massive MIMO [40] Coordinated multipoint [47] Very large aperture Massive MIMO [55] Distributed MIMO [56] Radio stripes [57] Network MIMO [58] Holographic Massive MIMO Holographic RF system [59] Holographic beamforming [60] Large intelligent surface [61] Reconfigurable reflectarrays [62] Intelligent walls [63] Software-controlled metasurfaces [64] Intelligent reflecting surface [65] TABLE I: The proposed research directions 1 and 2 collect many other research topics as special cases. as illustrated in Fig. 4(b), while the antennas’ signal compo￾nents add non-coherently at most other places. When aiming the beamforming at a particular point in space using well separated antennas, the signal components arrive from widely different directions but are phase-shifted to add constructively at the target point. Due to the different directivity, the signal amplification gradually decays when leaving the target point, but irrespective of the antenna configuration it is always strong in a sphere around the target with radius λ/8 [54]. The volume that features signal amplification is typically of this size in the near-field while it can be much larger in the far-field, implying that closely located users can be spatially multiplexed with less mutual interference when using an ELAA. A. Vision The grand vision of ELAAs is to provide orders-of￾magnitude higher area throughput in wireless networks com￾pared to what Massive MIMO with compact arrays can practi￾cally deliver. The keys to reaching this goal are the even larger number of antennas and the distributed antenna deployment that reduces the average propagation loss and increases the spatial resolution (particularly, in the horizontal domain). There is a competing concept of ultra-dense networks (UDN) [66], [67] that also relies on distributed antenna deployment, but each antenna box is then an autonomous BS that services its own exclusive set of users. It is known that the throughput of UDNs is fundamentally limited by inter-cell interference [68]. Cooperation between the distributed antennas can break this barrier as the number of antennas grows large [28], [29], [69]–[71], at least in theory. The ultimate goal of ELAAs is to deploy so many coherently operating antennas that all the users have mutually orthogonal channels, leading to a per-user throughput similar to that of an additive white Gaussian noise channel without any propagation loss [53], [72]. In addition to enhancing mobile broadband services, which constitute the majority of the wireless traffic [73], the great spatial resolution of an ELAA can also be exploited for spatial multiplexing of an unprecedented number of machine-type communication devices. The mass production of smartphones has turned advanced antennas and transceiver equipment into a commodity. Simi￾larly, the rapid development of integrated circuits (following