2015 Exam Flashcards
Why does Bluetooth MAC requires a silent period between frames? (2)
- Allows time for a low cost radio to turn from transmitter to reciever
- Freq synthesiser to settle to the new frequency of next hop
What do GPC + GPSS do?
Classes deal w/ how BS scheduler communicates UL resource allocations ‘grants’ to SSs
GPC Definition (3)
Grant Per Connection
- All scheduling is done by BS therefore SS is dumb
- Generates overheads since detailed scheduling info must be communicated from BS to SS
GPSS Definition (3)
Grant Per SS
- One grant for each SS irrespective of # services
- SS decides how to allocate resources, making SS complex
- Benefit is less resource requests + grants transmitted as additional overheads of channel
Duplexing options + challenges in 802.16 (4)
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Duplex Agnostic (standard incl. TDD + FDD)
- FDD SSs can be either full or half duplex
- FDD BS are full duplex
- An FDD BS must support a mixture of half + full duplex SS
- Scheduling difficult as not possible to schedule all half duplex SSs to receive before transmit in each MAC frame
MCS Meaning
Modulation + Coding Scheme
Why would a 1% PER be enforced + resultant QoS?
low prob of retransmission facilitating a combination of low delay jitter + low PLR
Methods in 802.11e EDCF MAC protocol to facilitate QoS provision (3)
- .11e nodes use AIFS rather than DIFS before contending
- AIFS < DIFS (both fixed) therefore .11e nodes have a deterministic priority over .11 nodes
- .11e different service classes have different starting CW range
- probabilistic adv to higher priority service classes
- .11de allows service classes to transmit multiple packets after winning contention (as many as TXOP duration)
- provides deterministic priority between service classes
QoS buffering implications? (3)
- Dst reads packet from buffer @ constant rate
- Reduces throughput + delay jitter
- Penalty is increased delay whilst buffer fills
How does fading give rise to multi-user diversity? (2)
- Gives rise to MUD since users experience uncorrelated fading
- DRA to users w/ good channel cond.
Why b/w is superior in Rayleigh fading than Rician fading?
Fading in Rician channel has less variance than a Rayleigh channel therefore capacity gain w/ increasing users is less
Why is there no gain in AWGN channel?
No fading
Why does ‘Theoretical B/W Eff’ graph have an exp. shape of the curves for fading channels? (2)
- Due to diminishing returns when there are more users, the best is more likely to have good conditions
- As # users increases, there is less likelihood that if one more is added, it will have a better channel
Do the b/w gains of MUD incr or decr when SNR increases? (3)
- Gains reduce for higher SNRs
- Multiuser diversity generates an increase in eff SNR
- B/W eff. only increases logarithmically w/ SNR
- e.g. if SNR is higher, making it higher achieves less of a result
QoS implications of using ARQ for error control? (2)
- ARQ will reduce PER + PLR @ expense of delay jitter (due to erroneous packet retransmission)
- Delay variance will be even worse if the round trip + access time of the link has a random element
