Simulation Results Presentation Questions
6/6/00
1. Check the radio channel model ITU-R M-1225 to verify that the one that we use
is actually within this standard
Answer: Still under investigation. However the parameters and assumptions that
we used for simulations are the ones stated in the contribution.
2. (From page 7): How were the Eb/No values calculated? (i.e. Is it energy per bit
or per symbol?)
Answer: For the SNR calculations, we divided the bit energy (Eb) by the spectral
density value of the AWGN rather than the symbol energy over No. With 16-QAM,
the symbol energy is 4 times larger than the bit energy. Compared with the
theoretical curves, our simulation result are very close to the ideal curves and the
differences are not huge especially in high SNR region.
3. SNR: QPSK and 16 QAM modulation: Compare QPSK (indoor case) and 16
QAM modulation (currently about a 4.5 dB difference). What are the Eb/No
definitions used to calculate these BER and FER values? Qualcomm believes that
4.5 dB difference is too much.
Answer: For further information on the SNR difference between QPSK and 16QAM, please refer to page 281 of "Digital Communications" written by John Proakis,
where it is documented that such a difference does exist.
4. Verify the retransmission delay (i.e. from when the first transmission is sent to the
time the first retransmission occurs.)
Answer: Still under investigation.
5. What is conventional ARQ that LinkAir claims to use?
Answer: Please refer to contribution 3GPP2-C20000425-LAS 003 for an ARQ
model. More details will be provided at a later time.
6. Explain why in Figure 4 the 16 QAM (circular) and 64 QAM graph curves cross.
Answer: Still under investigation.
7. Explain why in (fig 4) rectangular 16 QAM and circular 16 QAM graph curves
have a cross-point. This also occurs in Figure 5. Is there any theory that explains
this condition?
Answer: Still under investigation.
8. Why in Figure 7 is there NO crossing of the 16 QAM circular and rectangular
graph curves?
Answer: Still under investigation.
9. Filters:
What is the baseband filter that LinkAir uses?
Answer: The baseband filters in LAS-CDMA are raise cosine or square root raise
cosine FIR filters.
Also, from the following it looks that baseband filter design in LAS-2000 can
re-use that of IS-2000 without modification:
o The Baseband filter is used for shaping the digital pulses to fit into the limited
bandwidth for transmission.
o According to Nyquist theorem, given a bandwidth W Hz, the maximum rate at
which symbols can be transmitted without ISI is 2W symbols/s. Thus, if the chip rate
is 1228.8 kcps, the baseband bandwidth should be at least 1228.8/2 = 614.4 kHz.
o The baseband filter in IS-2000 seems to be designed based on the above principle,
with passband frequency fp = 590 kHz and stopband fs = 740 kHz. So the center
frequency is a little greater than 614.4 kHz.
Since LAS-2000 uses the same chip rate of 1.2288Mcps, the design in IS-2000 can be
re-used in our case.
10. Compare BER and FER – there seem to be some inconsistent results between the
indoor and outdoor values. Some companies said that there seems that BER is
much than FER. This may depend on the frame size used … if this is true, then
verify the frame size.
Answer: Still under investigation.
11. (Figures 2 vs Figure 7): How is FER frame size defined? Is it equal to the sum of
all the bits from the BER calculations?
Answer: Still under investigation.
12. Explain the differences in link level simulations and system level simulations.
Companies want to know how LinkAir defines a system level simulation.
Answer: Still under investigation.
13. For system level simulations, does LinkAir use retransmissions with ARQ?
Answer: Still under investigation.
14. Power Control:
Explain why LinkAir uses Forward Power Control (FPC)?
How does LinkAir control the power to one mobile without reducing power to
the others?
Answer: In order to reduce ACI (Adjacent Cell Interference), the best way is to
employ FPC. However, FPC reproduces the near-far effect, which means that
currently CDMA cannot fully use it.
However, LAS-CDMA is able to fully use FPC without producing this effect. LASCDMA performs FPC in the same way as the reverse channel, i.e., with respect to the
step and the total range.
15. MCS: What is the rate for the MCS and what is the delay when MSC is used?
Answer: MCS can be changed on a per data frame (DF) basis. The current design
has 3 sub-frames per DF. So a DF is actually 6.24 ms, and MCS change occurs every
DF or every 2 DFs (6.24 ms to 12.48 ms intervals).
Physical Layer Strawman Presentation Questions
6/6/00
1. What is the “GAP”, outlined with the time-slot, for a subframe? Answer given:
No transmissions (i.e. power transmitted) within this timeframe. Follow-up
question: Can LinkAir transmit “On-Off” within a 4 chip gap?
Answer: The transmission of the GAPs is completely different from powering off,
it similar to that the point where the signal jumps over from one point to another in
the signal constellation, e.g. 16-QAM , QPSK ,even BPSK signals. Just as it is not
necessary, in the case of BPSK or QPSK, to power OFF first then power ON in the
opposite direction when the signal transition is 180 degrees, similarly, gaps do not
influence the spectrum of the modulated waveform, the modulation waveform's
spectrum is only determined by the filtered chip waveform.
2. There are many gaps within a 20 ms frame, this may cause a power gating
problem like GSM. What is our response?
Answer: Still under investigation.
3. What is the number of the code channels for the Forward and Reverse Packet
Channels? What is the data rate?
Answer: Still under investigation.
4. Are the power control and adaptive link control (MCS) used is parallel or just
simultaneous? If yes, then what is the complexity for the mobile? If no, then
under what conditions does LAS-CDMA use power control and adaptive link
control? How is power control handled?
Answer: Still under investigation.
5. Define the peak to average power ratio and the power gain. (Companies really
would like to see these results determined within the simulations.)
Answer: The peak power to average power ratio is just the duty ratio of the LASCDMA code, i.e. 2048/2559 = 0.800.