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AMRT > 2017 Exam > Flashcards

2017 Exam Flashcards

1
Q

How a DFE is able to remove noise from ‘post cursor’ symbols (2)

A
  • DFE hardlimits the eq o/p to create noise free est of transmitted symbols
  • Passed into feedback filter section which cancels post cursor ISI
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2
Q

Adv/disadv of DFE over LTE (2,2)

A

Adv

  • hardlimited symbols introduce (if correct) no noise into eq o/p symbols
  • # taps feedback section = # post cursor taps
    • not true for feedforward filter

Disadv

  • hardlimited symbols may not be correct, impair eq. when used in the feedback filter to cancel ISI
  • Errors can propagate
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3
Q

Adv/disadv of training seq w/ decision directed adaptation (2,2)

A

Adv

  • training seq allows exact calc. of o/p error by providing perfect nowledge of the transmitted symbol
  • decision directed calc use imperfect est

Disadv

  • No info transmitted, represents an overhead
  • allow error calc when actual info is being transmitted
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4
Q

How are training seq and decision directed adaptation best combined? (3)

A
  • Best combo is to use periodic training symbol / seq
  • Keep data symbols > training symbols
  • Keep loss in useful throughput fairly minimal
    • provides accurate error calc to initilalise the eq to a good state before switching to decision directed calc
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5
Q

Benefits of MMSE over ZF (3)

A
  • MMSE considerst the noise power in determining eq. co-effs in order to min. overall error by noise + channel disp.
  • ZF only considers the disp + attempts to cancel it completely
  • In deep fades, ZF can cause noise ampl.
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6
Q

When do ZF + MMSE perform similarly? When does MMSE work best?

A
  • @ high SNR, (absence of noise), MMSE = ZF
  • @ low SNR, noise ampl. of ZF can be severe (MMSE performs better)
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7
Q

Assumptions used in LMS alg. to approx Wiener-Hopf? (2)

A
  • Treats gradients separatly for each dimension (tap) rather than calculate for all dimensions
  • LMS uses instantaneous values as approx. of long term av.
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8
Q

Index of Discrimination definition

A

difference between peak auto and cross-correlation function

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9
Q

How to maximise Index of Discrimination

A

can be maximised by selection of preferred pairs of spreading codes (or m-sequences)

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10
Q

Near Far Effect definition (2)

A

For a DS-SS UL w/ all users transmitting @ same pwr level, users near the BS will suppress/saturate far users

Only separated in the code domain

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11
Q

Prevention of Near Far Effect

A

UL pwr control can be adjusted such that all users are receieved w/ same pwr @ BS receiver

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12
Q

Additional process required for FDD air interface?

A

As UL + DL have uncorrelated fading in FDD network, closed loop pwr control us used to adjust for freq selective multipath fading

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13
Q

Difference between SDMA w/ BF and Spatial Multiplexing using MIMO? (2)

A

SDMA BF

applied to separate users provided that they are sufficiently separatd in space

Spatial Multiplexing

exploits the spatial temporal characteristics of the multi-user channel, allowing channel separation even when users are in close proximity

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14
Q

Multiple isolated spatial channels can be associated with …

A

… a single user thereby incr. the spectrum eff. of the link

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15
Q

Difference between Massive MIMO + sub-6GHz mmWave freq? (3)

A
  • Massive MIMO has an excess of antennas at BS serving fewer users (10-100)
  • Sub 6GHz Massive MIMO full digital signal processing is associated w/ each antenna @ BS therefore full spatial multiplex is possible
  • mmWave, analogue BF is applied to the wider b/w channel in order to compensate for higher path loss at these freq.
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16
Q

Block Diagram of COFDM Modulator

A
17
Q

Effects of Delay Spread, GI duration, modulation scheme and coding rate onthe error rate of the system (4)

A
  • Low order modulation + coding rates would allow the system to work reliably even w/ same ISI
  • Necessary for min FFT size + overall system b/w eff. is limited
  • Higher FFT size use longer GI, retain good GI eff.,
  • use high order modulation + coding rates –> achieve much higher b/w eff.
18
Q

Suitable arrangement of pilots to facilitate channel est. (3)

A
  • Burst transmission w/ low Doppler + low mobility
  • Channel can be est. w/ a pilot on every sub-carier (training seq.) @ beginning of the burst
  • No need to repeat pilot for remainder of the burst
19
Q

Effects of interleaving

A

Interleaving will exploit diversity + prevent bursts of errors in the bit stream i/p to the FEC decoder

20
Q

Benefits of leaving some sub-carriers un-modulated and which sub-carriers (3)

A
  • System Baud Rate = channel b/w, difficult to suppress adjacent channel interference w/o applying some very tight bandlimiting filtering
  • Leaving top + bottom 5% of sub-carriers unmodulated will make filtering much simler to implement @ modest b/w eff. cost
  • DC sub-carrier should NOT be modulated
21
Q

Why is OFDM preferable to using single carrier mod?

A

OFDM allows equalisation @ equivalent of 1 tap per symbol

22
Q

Method where single carrier modulation can be used

A

Used in freq. domain equalisation

23
Q

Assumption for Mutual Interference in DS-SS

A

Mutual Interference (NI) from other CDMA users can be modelled as AWGn and dominates over thermal noise

24
Q

A receiver that maximises M when in a mobile radio fading environment (6)

rake, left, message, time, despreaders, bit

A

Rake Receiver

  • r(t) - received waveform before despreading
  • local spreading code in receiver c(t) is aligned w/ first multipath component
  • message imprinted on this path is despread
  • Multiple time delays (TC) are equal to the spreading chip interval (min time resolution)
  • Multiple de-spreaders extract messages imprinted on the resolvable multipath component w/phase alignment in the combinator
  • Bit detection uses the integrator
    • Multiple replicas of the signal must be co-phased for additive combining.*
    • Combining can be equal weight w/ A0 = A1 = AL-1, or AL is made proportional to branch SNR offering MRC*
    • Provided that the time delay between multipath components is greater than chip period of the spreading code, multipath components can be resolved*
25
Q

Process of soft-handover in a multi-BS network (2)

A
  • A mobile can be connected to more than 1 BS simultaneously
  • As connections will suffer independent fading, this offers macro diversity + improves link quality
26
Q

Benefits of Soft vs hard Handover

A

Soft - “make before break” vs hard “break then make” so reduces prob. of dropped call

27
Q

Need for switching thresholds + timers in soft-handover? (4)

A
  • # mobiles in soft handover, otherwise resources (capacity) will be taken from network
  • Shadowing in the network results in temporary handover as users move
  • Swtuching threshold + timers are used to add/drop users from soft handover pool
  • Ensures that when a new BS is identified, it is a stable connection + vice versa when dropping a BS
28
Q

Why is lambda/2 element spacing necessary in a BF array? (2)

A
  • Grating lobes will occur if element spacing > lambda/2
  • Result in secondary or phantom main beam response + highly undesirable
29
Q

What are Grating Lobes

A

added / unanted main beams

30
Q

What is Antenna Boresight?

A

Axis of max gain of a directional antenna

AMRT (8 decks)

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