5g / mmWave networks Flashcards

1
Q

What are mmWaves?

A

mmWaves are radio waves with a wavelength of the order of millimeters.

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2
Q

How do you convert the wavelength into frequency?

A

You use the equation: c = f \lambda, where c is the speed of light (3 * 10^8 ms/s^-1) and \lambda is the wavelength.

So with a wavelength of 3mm, you get a frequency of 100GHz and a wavelength of 10mm gives 30GHz.
So shorter waves, gives higher frequency.

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3
Q

What does the Shannon- Hartley theorem state?

A

That the channel capacity C, meaning the tightest upper bound on the bit rate of data that can be communicated at an arbitrarily low bit rate error using an average received signal power S through an analog communication channel subject to additive white Gaussian noise of power N.

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4
Q

What is the formula in the Shannon- Hartley Theorem?

A

C = B log_2 ( 1 + S/N)

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5
Q

What happens when signal power S increases according to Shannon- Hartley Theorem?

A

The Channel Bandwidth B(Hz) will be multiplied by a higher number and the Channel Capacity increases.

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6
Q

In terms of Shannon- Hartley Theorem, why does beam forming give higher rates?

A

With beam forming, S/N (Signal power to noise ratio) is higher which gives higher rates.

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7
Q

In terms of Shannon- Hartley Theorem, why does higher carrier rates also give channel capacity?

A

Because B(Hz) increases so channel bandwidth increases. (You could say because the waves are smller?)

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8
Q

What is a phased antenna array?

A

An array of small light bulb like antennas placed very close to each other. They constructively and destructively interfere with each other to get a very narrow but strong beam in certain directions.

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9
Q

What are the advantages of beam forming and steering?

A
  • Saves energy: You don’t wast power by sending all directions, gives better battery life.
  • Improve S/N which gives a higher channel capacity. (higher bps)
  • It gives better frequency reuse, enabling more concurrent connections
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10
Q

What are some operational issues with beam steering?

A

mmWave mobility events like tracking, steering and handover strongly depend on pedestrian walk nature, so blockages can happen often. Affects TCP throughput and delay.
Delayed beam training and stale connection data makes it even worse.

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11
Q

How does the User Equipment(UE) choose gNB to connect to.

A

It chooses based on best signal- to- noise- ratio (SNR)

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12
Q

What is a beam Tracking mobility event?

A

The gNB uses the same physical Line of sight or Not- line of sight link with a minor adjustment of the gNB or UE antenna alignment.

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13
Q

What is a beam switching mobility event?

A

Another secondary link from the same serving gNB is assigned.

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14
Q

What is a beam handover mobility event?

A

A link from another gNB is assigned.

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15
Q

What is good about mmWaves?

A
  • High capacity
  • Small antenna (allows arrays)
  • Can do directional beam forming and steering (potentially lower power usage)
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16
Q

What is bad about mmWaves?

A
  • Signal path loss (as frequency f goes up, signal power S goes down)
  • High loss, so only works short distances
  • Fast accurate low power beam forming is still a work in progress.
17
Q

What is both good and bad about mmWaves?

A
  • It bounces less and therefore needs LoS, which means it’s sensitive to blockages and capacity can change abtruptly by a factor of 100+
  • Potential frequency reuse without interference(since it bounces less?)
18
Q

How does normal Cubic with a default aggressiveness parameter C=0.4 cope with mmWaves?

A

Since mmWaves vary between 0 and 1000mbps in a matter of seconds, cubic can be to slow to use any of the bandwidth.

19
Q

What is way to make Cubic work better with mmWaves?

A

Make it much more aggressive, e.g. set aggressiveness C=400.

20
Q

What is the problem with using aggressive Cubic with mmWaves?

A

It works very well while in 5G- land but it will kill the internet.

21
Q

What is the solution for making TCP work both with mmWaves and in the normal internet?

A

Use normal TCP in the internet, then use a “domain splitter” when entering mmWave territory and use aggressive TCP there. The domain splitter could be some sort of proxy.

22
Q

What is needed to make TCP work in mmWave 5G cellular networks?

A
  • Larger packet sizes.(since TCP is more aggressive?)
  • Need for short control loops
  • Need for multi- connectivity
  • Buffer need to be big enough. (Since TCP is more aggressive?)
23
Q

What are some mmWave proxies from the literature?

A
  • mmPEP
  • milliProxy
  • LwPEP
24
Q

What is mmPeP and general characteristics?

A

It is a special proxy for mmWave, it tries to take advantage of the relatively short time from NLOS to LOS betting that LOS will come soon and not slowing down the TCP sender.

Characteristics:

  • splitting proxy
  • Buffer, ack early (typical PEP)
  • Opportunistic Rtx (Don’t know what it means)
25
Q

What is milliProxy and general characteristics?

A

A proxy for mmWave that splits the TCP control and manipulates the MSS(max segment size)

characteristics:

  • splitting proxy
  • buffer and acks
  • packet aggregation: (wireless side handles larger packets)
26
Q

What is LwPEP?

A

A proxy for mmWave that buffers but does not split, which allows QUIC. Uses some tricks for proxy ack, like encrypted payload fragment and a magic number.(???)