1 Introduction
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outdoors on street furniture (e.g. lamp posts, bus shelters and buildings sides) to provide service to the surrounding streets and the lower floors of buildings; or
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indoors in public spaces and highly demanding areas as well as in the middle floors of high buildings to provide service to its middle and high floors and those of neighbouring buildings.
2 Small cell backhaul
2.1 Backhaul architecture
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Macrocell refers to the coverage area provided by a high transmit power BS. The macrocell radius is around 0.25–10 km with antenna heights over 25 metres.×
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Small cell refers to the coverage area provided by a low transmit power BS. The small cell radius is around 10–200 m with antenna heights under 25 metres.
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Point of Presence (PoP) refers to a central access point where the traffic from different cells is aggregated. Rooftop macrocell BSs can act as PoPs to underlay small cell BSs, with a PoP density of around 9 sites per square kilometre assuming an ISD of 500 m.
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Small cell (aggregation) gateway can be used to provide connectivity for a number of small cells to the backhaul network, acting as an aggregation point and a PoP. The small cell aggregation gateway improves scalability, reduces the number of required S1 interfaces and provides control and user plane functionalities to lower the signalling load on the core network components [3]. However, small cell connectivity to the small cell aggregation gateway may not always be available.
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Line-of-Sight (LOS) refers to a scenario where the small cell BS accesses the PoP via a direct non-blocked link, while Non-Line-of-Sight (NLOS) refers to a situation where the radio transmission across the direct path between the small cell BS and the PoP is obstructed, usually by a physical object. In case of NLOS, the main communication occurs through reflection, diffraction and/or diffusion.
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Point-to-Point (PtP) refers to a one-to-one communication between the PoP and a small cell BS.
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Point-to-Multipoint (PtMP) refers to a one-to-many communication between the PoP and multiple small cell BSs. PtMP communications are very much related to NLOS conditions and low-frequency bands (e.g., sub-6 GHz) and are able to overcome signal obstructions. In this case, the PoP acts as a unique data sink and can be equipped with either an omnidirectional antenna or a number of directional antennas pointing in different directions, e.g., antenna arrays with static beams, large scale antenna systems (LSAS) [7]. The latter solution with directional antennas enables the use of higher frequency bands and thus larger bandwidths due to the higher antenna gains, provided that LOS exists. However, the use of an omnidirectional antenna at the PoP eases the built-in installation and coordination requirements imposed due to beamforming.
2.2 Macro and small cell backhaul differences
3 Technical challenges for small cell backhauling
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In terms of wired solutions, due to the high costs associated with the installation of new wired connections, the existing infrastructure may highly dominate the deployment of small cell BSs and PoPs. For example, small cell BSs and PoPs can be deployed to leverage current fibre infrastructure. However, this may result in sub-optimal small cell BS and PoP placement from an off-loading or radio propagation perspective.
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The wireless solutions have to consider whether LOS or NLOS links and PtP or PtMP communications are available/used between the small cell BSs and the PoPs. Considering the provision of wireless backhaul coverage through LOS wireless links, the main challenge is the availability of a clear link between the small cell BS and the PoP. Figure 2 shows the probability of LOS versus distance in urban environments based on the WINNER II channel model, which drops to less than 0.5 for distances beyond 75 m. Therefore, in co-channel deployments, where small cells are not deployed close to macrosites to avoid interference, LOS wireless backhaul may not be seen as a feasible solution in a large number of cases. Moreover, the wireless backhaul coverage is also impacted by atmospheric attenuation. Certain frequencies suffer from higher attenuation than others, due to the mechanical resonance of gas molecules [8, 9]. Since atmospheric loss has its most significant impact on links of over 1 km, this may not be a bottleneck to small cells if they are located within distances of a hundred metres from the PoP. As a result, LOS links for small cell BSs may not always be feasible, only at short distances, and NLOS links may have to be used in dense urban areas, as they enable more small cell deployment locations. PtMP communications may also facilitate backhaul deployment with respect to PtP communications since the PoP covers a wider area and does not require antenna alignment. However, NLOS and PtMP solutions both suffer from low capacity because of the constrained spectrum availability at lower frequency bands, usually associated to them, and due to the multiplexing of several small cell flows at the PoP.×
Type of cost | Value | Microwave backhaul | Fibre optic backhaul |
---|---|---|---|
Capacity cost ($) |
A
0,1
| 5000 | 5000 |
A
1,2
| 9000 | 5000 | |
Infrastructure cost ($) |
B
0,1
| 10,000 | 10,000 |
B
1,2
| 20,000 | 100,000 | |
Equipment cost ($) |
C
1
| 50,000 | 50,000 |
C
2
| 100,000 | 100,000 |
4 Solutions for small cell backhaul
Backhaul type | Backhaul technology | Latency | Throughput |
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Ideal backhaul | Optical fibre | <2.5 μs | Up to 10 Gbps |
Non-ideal backhaul | Deficient fibre access | 5–10 ms | 100–1000 Mbps |
DSL | 15–60 ms | 10–100 Mbps | |
Wireless | 5–35 ms | 10–100 Mbps up to Gbps |
4.1 Wired backhaul from small cell BS to PoP
4.2 Wireless backhaul from small cell BS to PoP
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Sub-6 GHz: This range of frequencies is suitable for NLOS links. This solution also works well with omnidirectional antennas, and in this case, there is no need for antenna alignments [3]. The coverage is reliable as long as there is sufficient scattering, and the penetration losses do not significantly attenuate the signal. Spectrum bandwidth is the main constraint to the capacity. Interference coordination may be essential, particularly when using the license-exempt bands, since Wi-Fi and Bluetooth transmissions can cause significant interference and reduce the signal quality.×
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Microwave (6–56 GHz): As a result of the short wavelength, diffraction and penetration through obstacles incur high losses, and thus, LOS connection dominates the propagation at 6–56 GHz, though near LOS (nLOS) is also possible at the lower frequencies. Due to the short wavelength, compact directional antennas with high gain and narrow beamwidth are possible, which require antenna alignment to achieve optimal performance [3]. Due to its LOS operation and high gains, microwave backhaul is suitable for long range fixed links and interference is highly mitigated. For frequencies above 10 GHz, the absorption and scattering of electromagnetic waves by rain cause significant attenuation (see Fig. 3), and this is a phenomena to consider when performing planning. Microwave solutions can be divided into PtP and PtMP ones. In a microwave PtMP, as the network becomes denser, it is likely that the peak traffic of each small cell decreases and the total traffic is shared among neighbouring ones, which should boost backhaul performance due to multiplexing [6].
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V-band (57–66 GHz) and E-band (70–80 GHz): As a result of the very short wavelength, diffraction and penetration through obstacles are now hardly possible and thus only LOS links are feasible. In addition, the range is confined by high atmospheric absorption (see Fig. 3). Due to the very short wavelength, very compact directional antennas with very high gains and narrow beamwidth are possible, which require a very precise antenna alignment to achieve optimal performance [3]. High capacity short links of over 1 km can be achieved due to several GHz-wide bandwidths. Interference is much reduced due to high antenna gains and the significant penetration losses. Attenuation in V-band is mostly dominated by oxygen, whereas attenuation in E-band is mainly due to rain, which may limit the link distance to less than a few kilometres in some geographical areas [28]. In [28], it is suggested that V-band is an appropriate choice for street-to-street and street-to-roof connection, while E-band is a more effective solution for roof-to-roof links.
Frequency band | Main advantages | Main disadvantages | Backhaul licensing | Backhaul network |
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topology | ||||
Sub-6 GHz | LOS and NLOS are both possible, trafficaggregation is supported, faster installation and lower deployment cost | Spectrum limitations resulting in lower capacities, interferencesensitive, lack of carrier grade,higher cost for licensed spectrum | Licensed (3.5 GHz) | PtMP, PtP |
Microwave (6–56 GHz) | Available large spectrum, high capacity upto 1 Gbps, high-gain antennas with small footprint, long distance connection, PtMPsupports traffic aggregation | LOS required, node alignment mayreduce the deployment scalability | Licensed | PtMP, PtP |
V-band (57–66 GHz) | Available large spectrum, extremely highcapacity up to several Gbps, unlicensed,high-frequency reuse factor | Very short links, LOS required, narrow beamwidth | Unlicensed | PtP |
E-band(70–80 GHz) | Available large spectrum, extremely high capacity up to several Gbps, light license,higher reuse factor | Short links, LOS required, verynarrow beamwidth | Light license | PtP |
4.3 Synergy of wireless solutions
4.4 Self-organising wireless backhaul networks
5 Small cell backhaul case study
5.1 Scenario
5.2 Small cell backhaul optimisation model
Cost parameter | Sub-6 GHz PtMP | Microwave PtMP | E-band PtP |
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Link capacity t (Mbps) | 2000 | 1000 | 250 |
Hub equip. ($) | 12,000 | 6000 | 4000 |
Hub install ($) | 12,000 | 2000 | 1000 |
Remote equip. ($) | 12,000 | 3000 | 2000 |
Remote install ($) | 12,000 | 1000 | 500 |
5.3 Performance analysis
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In the sub-6 GHz scenario, all the small cells could directly connect to the macrocell PoP without any need to add new hopping nodes.
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In the microwave PtMP scenario, 10 small cells could directly connect to the macrocell PoPs but other intermediate hopping nodes were needed to backhaul the traffic generated by the remaining small cells.
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In the E-band PtP scenario, eight small cells could directly connect to the macrocell PoPs. Due to the short range LOS links of less than <1 km in the E-band, more intermediate hopping nodes were needed in this case. There were solely two LOS connections between small cells.
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Hybrid I/II scenarios refer to two novel solutions, which exploit sub-6 GHz for NLOS backhaul and microwave PtMP and E-band PtP for LOS backhaul. Extra nodes can be used to mitigate the interference. Hybrid I refers to the combination of PtP LOS and PtMP NLOS, and hybrid II refers to the case where PtMP topology is used for LOS and NLOS.
Backhaul | Sub-6 GHz PtMP | Microwave PtMP | E-band PtP | Hybrid I/II |
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Small cells | 23 | 23 | 23 | 23 |
New nodes | 0 | 9 | 10 | 3 |
Max agg. nodes | 14 | 14 | 4 | 12/6 |
Total antennas/gain | 25 | 45 | 66 | 34/43 |
TCO (K$)/overhead | 67.5 (0×) | 232 (3.44×) | 396 (5.87×) | 126 (1.87×)/ 206 (3.05×) |
Min peak rate (Mbps)/gain | 89 (0×) | 357 (4.0×) | 2000 (22.5×) | 250 (2.8×)/ 250 (2.8×) |
Average peak rate (Mbps)/gain | 108 (0×) | 434 (4.0×) | 2000 (18.5×) | 330 (3.0×)/ 486 (4.5×) |
Average cost efficiency/gain | 0.62 (0×) | 0.53 (1.17×) | 0.19 (3.26×) | 0.38 (1.63×)/ 0.42 (1.47×) |
Peak cost efficiency/gain | 0.62 (0×) | 1.16 (0.53×) | 1.98 (0.31×) | 0.63 (1.0×)/ 1.03 (0.60×) |