2017 Exam Flashcards
How a DFE is able to remove noise from ‘post cursor’ symbols (2)
- DFE hardlimits the eq o/p to create noise free est of transmitted symbols
- Passed into feedback filter section which cancels post cursor ISI
Adv/disadv of DFE over LTE (2,2)
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
Adv/disadv of training seq w/ decision directed adaptation (2,2)
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
How are training seq and decision directed adaptation best combined? (3)
- 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
Benefits of MMSE over ZF (3)
- 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.
When do ZF + MMSE perform similarly? When does MMSE work best?
- @ high SNR, (absence of noise), MMSE = ZF
- @ low SNR, noise ampl. of ZF can be severe (MMSE performs better)
Assumptions used in LMS alg. to approx Wiener-Hopf? (2)
- Treats gradients separatly for each dimension (tap) rather than calculate for all dimensions
- LMS uses instantaneous values as approx. of long term av.
Index of Discrimination definition
difference between peak auto and cross-correlation function
How to maximise Index of Discrimination
can be maximised by selection of preferred pairs of spreading codes (or m-sequences)
Near Far Effect definition (2)
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
Prevention of Near Far Effect
UL pwr control can be adjusted such that all users are receieved w/ same pwr @ BS receiver
Additional process required for FDD air interface?
As UL + DL have uncorrelated fading in FDD network, closed loop pwr control us used to adjust for freq selective multipath fading
Difference between SDMA w/ BF and Spatial Multiplexing using MIMO? (2)
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
Multiple isolated spatial channels can be associated with …
… a single user thereby incr. the spectrum eff. of the link
Difference between Massive MIMO + sub-6GHz mmWave freq? (3)
- 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.
Block Diagram of COFDM Modulator
Effects of Delay Spread, GI duration, modulation scheme and coding rate onthe error rate of the system (4)
- 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.
Suitable arrangement of pilots to facilitate channel est. (3)
- 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
Effects of interleaving
Interleaving will exploit diversity + prevent bursts of errors in the bit stream i/p to the FEC decoder
Benefits of leaving some sub-carriers un-modulated and which sub-carriers (3)
- 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
Why is OFDM preferable to using single carrier mod?
OFDM allows equalisation @ equivalent of 1 tap per symbol
Method where single carrier modulation can be used
Used in freq. domain equalisation
Assumption for Mutual Interference in DS-SS
Mutual Interference (NI) from other CDMA users can be modelled as AWGn and dominates over thermal noise
A receiver that maximises M when in a mobile radio fading environment (6)
rake, left, message, time, despreaders, bit
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*
Process of soft-handover in a multi-BS network (2)
- A mobile can be connected to more than 1 BS simultaneously
- As connections will suffer independent fading, this offers macro diversity + improves link quality
Benefits of Soft vs hard Handover
Soft - “make before break” vs hard “break then make” so reduces prob. of dropped call
Need for switching thresholds + timers in soft-handover? (4)
- # 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
Why is lambda/2 element spacing necessary in a BF array? (2)
- Grating lobes will occur if element spacing > lambda/2
- Result in secondary or phantom main beam response + highly undesirable
What are Grating Lobes
added / unanted main beams
What is Antenna Boresight?
Axis of max gain of a directional antenna
