2016 Exam Flashcards
Advof Iterative alg. LMS + RLS in comp to direct calc. ZF + MMSE (3)
- Iterative alg. enable computational load of calculating the co-effs to be spread over multiple symbol periods (each symbol of training seq)
- Dir calc. can only be implemented @ end of seq
- Needs al computation completed in a very short time, incr. peak computational load
Traceback Length for compr. between compl. + eq. performance
Traceback over 5L,
5x longer than the constraint length (L+1)
Freq. Hopping Definition
instantaneous carrier freq. is changed periodically to a new value by means of a hopping sequence.
Carrier is hopped over a much larger b/w than info signal
(appl - BT)
Direct Sequence Definition
narrowband info signal is spread to a wider b/w waveform by directly modulating w/ a wideband SS
Spreading Code Generation
Mathematical property of gnerator polynomial to ensure good op. cond.?
Needs to be a primitive polynomial
Range Extension Definition
incr. gain offered by the array over a single element antenna yields an incr. in the cell radius / coverage area
Spatial Filtering for Interference Reduction (SFIR) (2)
- smart antenna focuses energy at an addr. volume, thereby reducing both interference ‘caused by’ as well as ‘seen by’ the wireless channel
- co-channel interference is reduced
- re-use distances can be reduced
- incr. network cap.
- re-use distances can be reduced
SDMA Definition
supports time, freq + code re-use through the spatial separation of users within the same cell
Why are TDD air based interfaces suited to Massive MIMO?
Massive MIMO can exploit channel reciprocity in order to derive the transmit vector (pre-coding) directly from the UL
Why are values w-2 + w-1 intuitively obvious?
Both are zero because zero forcing causes all terms to be estimated except for the kth symbol
Why is ZF only approx + change required for accuracy?
ZF only approx due to only 2x eq. taps used to cancel 2x post cursor taps
–> Significantly more taps required in order to approach exact ZF
Merits of incr/decr GI (3)
- incr GI allow more tolerance for timing, but reduces eff.
- Good choice if timing synch. is an issue + large mod. order (64-QAM)
- decr. GI results in some ISI but incr. eff. - good for low order mod + robust to ISI
How can narrow-band + wide-band interference be suppressed?
Extended anaylsis for a multi-user CDMA DS-SS
How is Power Control implemented in 3G cellular for FDD? (4)
- Freq. selective fading on UL + DL is decorrelated, so both UL & DL pwr control is necessary
- Av path loss + shadowing is estimated by OL controller based on received signal strength @ mobile
- BS corrects for the freq. selective, uses CL info channel, instructing mobile to adjust transmit power
- 1-bit Delta Sigma control can be employed for CL pwr control to save b/w
- incr. the update rate by using multi-bit power control or a wider spreading b/w (more freq. diversity = less power errors)
Which paramters have the greatest impact on the channel capacity
nT scales the transmit pwr, so total energy is the same when making comparisons
HH* describes the orthogonality of teh spatial channels
SNR has biggest impact on channel capacity
What can be inferred regarding spatial channel structure + offered information capacity (2)
- individual Eigen channel gains + info bearing capacity of the spatial streams can be observed
- Checking to see if system is full rank
- i.e. all channels convey data
