Fixed Mobile Convergence

Question 1

How do you increase mobile capacity by 1000 times?

Answer 1

To increase capacity, you can:

1. Effiency (MIMO, smart scheduling, enhanced-CoMP) -> x3
2. Spectrum (carrier aggregation, new bands, authorised shared access) -> x2
3. density (advanced macros, hetnet management, flexible small cells)

Question 2

Why is high cell density a well
known 5G challenge?
A. There will be too many radio
emissions that could damage
our health
B. The dense copper network
interconnecting the base
stations will create too much
interference
C. There will be too much
concentration of energy to
feed the base stations
D. The fibre backhaul network
for the cells will be more
expensive than the radio
network itself

Question 3

Explain the cell density issue

Answer 3

With higher cell density, each small cell only serves a few users at a time

• Revenue of a small cell is much less than a macro cell
• Cannot justify the cost of an individual fibre connection

Question 4

‘Shared PON for mobile backhauling.’ Explain this statement

Question 5

What mobile backhauling?

Answer 5

Mobile backhaul refers to the transport network that connects the core network and the RAN of the mobile network.

Question 6

What is the RAN?

Answer 6

Radio Access Network; wireless telco system that connects individual devices to other parts of a network through a radio link

Made up of three elements:

1. Antennas -> convert electrical signals into radio waves
2. Radios -> transform digital information into signals that can be sent wireleslly
3. Baseband units -> provide set of signal processing functions that make wireless communication possible.

Question 7

What can a well architected optical access network provide?

Answer 7

Allow service multiplexing: any access point (home, macro cell, small cell) can request assured capacity from low Mb/s to multiple 100s Gb/s

Question 8

Explain the evolution from a coaxial cable to a fibre cable in distributed RAN

Answer 8

1. BBU connected to the antenna using coax cable.

• Problem as large coax (to decrease transmission loss) was needed to connect the bottom of the mast to the top -> bulkiness, wind resistance and cost

2. Replace coax cable with fibre cable -> one single fibre has enougb capacity to replace all copper wires and takes up a very small space.

Question 9

What is the protocol for the connection between BBU and RRU

Answer 9

Common Public Radio Interface (CPRI)

• Idea is that all digital processing is done at the BBU
• RF signal is sampled, digitised and sent to the RRU over fibre

Question 10

What is the BBU?

Answer 10

The baseband unit is a unit that processes baseband in telecomm systems.

Question 11

Explain BBU hoteling

Answer 11

Fibre link between BBU and RRU -> increased transmission link and capacity

• Store the BBU inside a building -> more secure, less prone to weather conditions, and not constrained to the location of the mast.
• Same building and add more BBU, each connected through fibre to its own antenna mast.

Question 12

Explain BBU pooling

Answer 12

Use statistical multiplexing of the data and use a smaller number of BBUs

• e.g. two BBUs for three antennas masts

Question 13

Give an application of BBU pooling

Answer 13

Implemented through Network Function Virtualisation

• BBUs get implemented through software running on top of off-the-shelf servers

Question 14

Explain Cloud-RAN

Answer 14

BBU software running on a server -> evolve to cloud computing

• Large data centre and store everything there

Question 15

What is the difference between fronthaul and backhaul

Answer 15

CPRI signal transmitted is part of the front end of the base station

Question 16

What does the fronthaul interface do?

Answer 16

It samples the wireless signal and then transmits I/Q samples over fibre.

Transmits a digitised version of the radio wave

Question 17

Compare the transmission capacity of the fronthaul in comparison to the backhaul rate

Answer 17

The fronthaul requires a transmission capacity 16 times higher than the equivalent backhaul rate

• Is independent of usage .. a sustained rate

Question 18

Explain fronthaul’s issue with latency

Answer 18

The HARQ system requires that messages between UI and BBU are acknowledged within 3 ms.

Several processing units that take up time: 200-400 microseconds -> 20-40km RTT in fibre

Question 19

What is the solution for fronthaul latency issue

Answer 19

Fronthaul over PON:

1. Virtual ptp works similarly to ptp fibre

• uses less fibre than ptp but more expensive transceivers

Question 20

What is the solution for fronthaul capacity issue

Answer 20

Split PHY processing or midhauling:

• Doing a bit more processing at the RRU can reduce the optical transmission rate considerably.
• Restores proportionality with user traffic -> possibility of carrying out statistical multiplexing