NETWORKING IN 5G
Unit - IV

NETWORKING IN 5G
• Coordinated multi-point transmission in 5G:
• Joint Transmission CoMP enablers
• Distributed cooperative transmission
• JT CoMP with advanced receivers
• Relaying and network coding in 5G:
• Multi-flow wireless backhauling
• Buffer-aided relaying.

NETWORKING IN 5G
• Coordinated multi-point transmission in 5G:
• Joint Transmission CoMP enablers
• Distributed cooperative transmission
• JT CoMP with advanced receivers
• Relaying and network coding in 5G:
• Multi-flow wireless backhauling
• Buffer-aided relaying.

Introduction
• The performance of a wireless network strongly depends on the user positions in a cell.
More precisely, the UEs (User Equipments) at the cell border typically experience much
lower throughput than those nearer to the transmitting Base Station (BS).
• This is mainly due to the presence of inter-cell interference, generated by concurrent
transmissions in other cells.
• Inter-cell interference is particularly relevant for modern wireless communication systems
like UMTS, LTE, and 5G, where the frequency reuse factor is one or very close to one.
• In such scenario the system is primarily interference limited, and the performance cannot be
improved by simply increasing the transmitted power. Hence, techniques are necessary in
order to target inter-cell interference and reduce the gap between the cell edge and average
throughput.
• Consequently, these alternative techniques allow a more even user experience throughout the
whole network.

What are the main benefits of Coordinated
Multipoint?
• In future cellular networks, the demand for very high capacity is expected to grow
at a quick pace, casting challenging requirements to the dimensioning and the
planning of the systems.
• Coordinated MultiPoint (CoMP) is based on transmission and/or reception at
multiple separated sites with dynamic coordination among them, to pro-actively
manage the interference for the users, with a particular emphasis on cell-edge
users.
• Some of the benefits of CoMP are:
• Makes better utilization of network: By providing connections to several base stations at once, using
CoMP, data can be passed through least loaded base stations for better resource utilization.
• Provides enhanced reception performance: Using several cell sites for each connection means that
overall reception will be improved.
• Interference reduction: By using specialized combining techniques it is possible to utilize the
interference constructively rather than destructively, thereby reducing interference levels.

The following techniques can be pursued to tackle inter-
cell interference:
 Interference can simply be treated as white noise.
 This is clearly suboptimal, as it ignores properties of the interfering signals that could be exploited in order to
improve signal reception quality.
 Interference can be avoided through statically leaving some transmit resources in
some cells muted (e.g., fractional frequency reuse), or otherwise constraining the
usage of resources, or through coordinated scheduling among cells.
 The impact of interference can be alleviated at the receiver side
 e.g., Interference Rejection Combining (IRC), where multiple receive antennas and subsequent receive filters
are used to attenuate the interference to a certain extent.
 Interference may be decoded and cancelled;
 a technique in 3GPP in the context of Network-Assisted Interference Cancelation (NAIC).

Cont.
 At the transmitter side, interference can also be partially avoided by performing
interference-aware precoding,
 i.e., applying precoding such that the interference caused toward adjacent cells is reduced
 Ultimately, signals from other cells can in fact be treated as a useful signal energy
instead of interference,
 if (downlink) multiple nodes jointly transmit signals that coherently overlap at the intended
receiver, and destructively overlap at interfered receivers.
 In the uplink (UL), multiple nodes can jointly receive and decode the signals from
multiple UEs, and in this form also exploit interference rather than seeing it as a
burden

• The latter two techniques are typically grouped under the term Coordinated Multi-Point (CoMP),
which in general refers to techniques where multiple nodes in the network coordinate or cooperate to
alleviate the impact of interference or exploit interference on physical layer.
• From an information-theoretic point of view, a joint transmission from multiple nodes (e.g., BSs) to
multiple UEs resembles a broadcast channel, and a joint detection of multiple UEs by multiple nodes
resembles a multiple access channel, where the capacity regions are well-known for Gaussian
channels.
• In the context of cellular systems, a form of joint detection was in fact already introduced in Code
Division Multiple Access (CDMA) systems through the so-called soft and softer handover.
• The joint detection between multiple cells was considered via a centralized unit. The centralized unit
acted as a receiver that exploited all UE signals collected by the BSs, treating the whole system as a
network-wide Multiple-Input Multiple-Output (MIMO) scheme.
• Joint transmission in the downlink (DL) was studied in and showed to provide more than 10-fold
increase in spectral efficiency if applied across a large number of cells. Motivated by these promising
results, CoMP has been widely studied in 3GPP as one of the features for LTE Advanced (LTE-A),
i.e., LTE Releases 11 and 12.
Cont.

 Joint Transmission (JT), where the data related to a UE is available at several transmitting nodes
and is transmitted simultaneously by each node over a frequency/time resource. Such transmission
can be coherent (and in this case it is sometimes referred to as Network MIMO) or non-coherent.
Coherency refers to the ability to precode in a way that exploits the phase and the amplitude
relations between channels associated with different transmission points.
 Dynamic Point Selection (DPS), where the data related to a UE is transmitted by a single
transmitting node for a frequency/time resource, while the other nodes can be either dedicated to
transmit data of other UEs or be muted exploiting Dynamic Point Blanking (DPB). However, the
data should still be available at all cooperating transmitters, since the selected point may change
dynamically from one transmission time interval to another.
 Coordinated Scheduler/Coordinated Beamforming (CS/CB), in this case, the data related to a
single UE is available and transmitted only by one node. Nonetheless, neighboring cells share
Channel State Information (CSI) in order to coordinate their scheduling, power control and
beamforming decisions, and reduce mutual interference
In 3GPP, CoMP techniques have been
classified into three groups:

• A further classification used in literature is to distinguish CoMP techniques that require both the
exchange of CSI and user data (sometimes referred to as cooperative approaches) and those that
involve only CSI exchange (coordination approaches). While JT and DPS fall in the first category,
CS/CB belongs to the latter. In the last few years, other novel approaches have arisen in this second
category.
• These approaches exploit the fact that it is possible to precode and transmit signals such that
interferences are always constrained to confined signal subspaces at each receiver. This allows the
receiver to reject efficiently the interference. This idea is known as Interference Alignment (IA)
and aims to manage the interference by combining an “align” and “suppression” strategy. Although
CoMP looks promising, there are several practical impairments that could prevent it from reaching
the full gain suggested by theoretical bounds.
• Backhauling limitations in terms of bandwidth and latency, imperfection on the CSI due to realistic
estimation procedures, quantization effects, signaling delays or limitations, and imperfect
frequency/time synchronization are all aspects that have an impact on potential CoMP gains and
should hence be carefully considered in the design of a CoMP scheme .
Cont.

• In this respect, it is more difficult to take all these aspects into consideration in a matured
wireless communications system such as LTE, which was never designed to support
CoMP in the first place, and which due to its standards matureness does not allow
introducing major changes on e.g., physical layer.
• In this chapter, hence the emphasis is on how a clean-slate 5G system could natively
better support CoMP techniques than any legacy system, by taking all above-stated
aspects already into account in its initial design.
Cont.