2015 Exam Flashcards
Merits of combining training sequence + decision directed methods to calc. error of equaliser o/p? (2)
- Decision directed adaptation between training seq. can refine co-effs + track a time variant channel
- If duration of training seq is short (relative to the time between transmissions), overheads will be low
Strongest tap in DFE
C0 - ensure that post cursor ISI is all cancelled by the feedback fiter which won’t amplify the noise
Benefits of G.I x2
Reduces ISI
- results in cyclic rather than linear convolution between transmitted signal + channel
- No ISI if GI is longer than the channel excess delay
Relaxes time synch. requirements
- any part of the extended OFDM symbol can be demodulated
- Any timing offset from the start of the unextended symbol simply manifest as a phase error on the o/p of the FFT
- easily cancelled in eq.
Parameters of propagation determing min + max # sub-carriers (2)
Delay Spread
- motivates a min # of sub-carriers to ensure the G.I can be longer w/o overhead
- determines # value of coherence b/w + dividing the total COFDM b/w by this value
Doppler Spread
- max # sub-carriers to ensure sufficient spread + not cause ISI
- carrier freq. + mobile speed determine the Doppler freq., assumng arrival rays from 360 degrees in Azimuth
- Doppler spread = 2x doppler shift
Signal / waveform processing in SS comms syst (3)
- Narrowband info is spread to a wider b/w signal by means of spreading code
- @ Rx, a synch despreading is applied to recover the narrowband message waveform
- Spreading process can either be a direct mapping from
- narrowband –> wideband (DS)
- FH pattern dictated by spreading code
Adaptive / Smart Performance Enhancement (3)
- Appl. of amplitude + phase weights to each part of smart antenna array
- Spatial filtering properties can be derived if the elements are appropriately spaced
- Spatial filtering can be used to enhance cell range
(through enhanced antenna gain directed @ user + cap. enhancement through interference rejection)
Freq response of ideal ZF linear eq.
MMSE Freq. Response
Motivation for using a larger N-point IFFT (3)
- LArger FFT size = longer symbol period
- Enables a larger GI w/ lower overhead
- Perferred in deployment w/ large delay spread
- e.g full freq reuse broadcast deployment
How does Rake Reciever improve performance in dynamically fading environment? (3)
- Wideband nature of a DS-SS exploits path diversity from multipath propagation
- Rake receiver can resolve + despread independent fading paths
- Comb. of these signals result in Diversity Gain
- reduces required energy per bit for a given bit error rate performance
Why is Pwr Control necessary in a multi-user DS-SS?
Impact on Network Cap. if pwr control errors occur?
- In DSS-SS, users separated by code domain
- If user transmit @ same pwr level, high path loss variation will result in a user near the BS saturating reception of a far user
- A 1dB stand. dev. in pwr control can result in a 33% reduction in network cap
Limits of Power Control (2)
- When mobile moves faster, pwr control loop will bnot be able to track the fading envelope w/ power error control
- Using a wider spreading b/w (More freq. diversity = less deep fades) and / or a faster update rate will help mitigate power control error
Spatial Multiplexing in MIMO system
SM - requirea fullrank channel + high eigen gains per spatial stream (Non LoS + good SNR)
Space-Time Block Codes in MIMO (3)
- uses Alamouti Tx diversity
- tx symbols are encoded + tx via 2 antennas in pairs
- Tx signals are truly orthogonal so depending on the channel the Rx will either receive the symbol, the conj. or the inverse
TDD interfaces best suited to Massive MIMO (2)
- CSI must continuously est. for every BS to Rx comb. (significant task)
- exploiting the reciprocal MS -> BS + BS -> MS in a TdD will greatly reduce processing required to derive the CSI
