stds-802-16-tg3: Mode OFDMA2
Dear BeamReach Friends,
After reading your contribution C802.16a-02/40, I thought of four issues
regarding mode OFDMA2. The first three suggest additions to your proposed
text for the standard; the last one questions the expected performance.
Here they are:
1. FRAMING
In IEEE P802.16a/D2-2002, for OFDM mode, we have two figures which show the
framing in detail. Figure 206 on pg 154 of shows the framing when using TDD
and Figure 207 on pg 155 shows the framing when using FDD. SC2 has two
similar figures, which are Figures 177 and 178 in P802.16a/D2-2002.
Presently, neither OFDMA nor OFDMA2 have such figures. Even if just for
editorial consistency, OFDMA and OFDMA2 should probably have such figures.
This should also satisfy the people who requested better explanation of the
framing.
2. EXPLAIN BEAMFORMING IN INITIAL RANGING
Please review the following process for Initial Ranging, and somewhere in
your text, explain (a) exactly when in this process the BS gets the
information it needs to form a receiving beam, and also (b) exactly when the
beam is formed:
Step 1: The SS sends an Initial Ranging CDMA code on the ranging channel
(note: Ranging Channel = two subchannels), for a duration of two OFDM
symbols, at an arbitrary time.
Step 2: The BS sends a ranging response (RNG-RSP) MAC messge, giving the SS
a time advance based on its observation of the Initial Ranging code, and
also an allocation in which to send a MAC header requesting network entry.
Step 3: The SS adjusts its time advance.
Step 4: The SS sends a MAC header requesting network entry.
3. EXPLAIN BEAMFORMING IN PERIODIC RANGING
Ditto for Bandwidth Requesting. Note that, in this case, the BS may already
have the measurements "on file" to form a beam from this SS. But I presume
that, if this SS has become inactive, it will not be "listening" with this
beam but will somehow reactivate this beam sometime. When and how? Here is
the process:
Step 1: The SS sends a Bandwidth Requesting CDMA code on the ranging
channel (note: Ranging Channel = two subchannels), for a duration of one
OFDM symbol, with its proper time advance which it already knows.
Step 2: The BS sends a ranging response (RNG-RSP) MAC messge, giving the SS
an allocation in which to send a normal MAC header with its payload.
Step 3: The SS sends a MAC header with its first payload.
4. NUMBER OF SUBCHANNELS ALLOCATED TO RANGING, AND CDMA RANGING PERFORMANCE
In IEEE P802.16a/D2-2002, pg. 182 line 25, we explain that "The MAC shall
define a single ranging channel. This single ranging channel is composed of
one or more subchannels." Later, we say that the default number of
subchannels allocated to the ranging channel is two. If too many SS try to
send ranging signals, the coding gain available with two subchannels, giving
a code length of 2x53 = 106, may be insufficient, resulting in many missed
detections and false alarms. Under these conditions, BS MAC policies will
presumably increase the ranging channel to include more subcchannels, say 3,
4 or 5, which will give longer code lengths of, say, 159, 212 or 265.
You may recall that, last year, I and my colleagues submitted a contribution
showing that, in OFDMA mode, it will be difficult for the BS to receive more
than one ranging SS at a time under SUI-4 or worse channel conditions,
because the channel will distort the codes.
http://grouper.ieee.org/groups/802/16/tg3_4/contrib/80216abc-01_24.pdf
[You may also recall that Runcom disagrees with me on this. We have their
presentation, but are not satisfied; therefore this disagreement remains.]
Anyhow, in the text you submitted on Thurssday, it says that the ranging
channel allocation is fixed at two subchannels and therefore the code length
is fixed at 106. Thus, the BS is not able reduce congestion on the ranging
channel by allocating more subchannels, as it is able to in OFDMA mode.
However, I expect this allocation of additional channels to be even more
necessary in OFDMA2 than in OFDMA for two reasons.
FIrst, since it has higher capacity due to spatial multiplexing, there will
be more SS per BS, and hence more ranging signals. This looks like a
fundamental problem with OFDMA2. Since beams cannot be formed until
sometime during or after ranging, there is no way to spatially multiplex the
ranging channel like the other subchannels. The ranging channel does not
get the gain in capacity.
The second reason is AWGN. It could be argued that AWGN will not be much of
an issue in OFDMA ranging, because of the robustness of the BPSK signalling
and because of the power concentration available when transmitting on only a
few subchannels. Therefore, to be easy, I did not add AWGN in the
simluations I did last year which were included in our contribution
80216abc-01_24. However, in OFDMA2, AWGN will probably be significant in
ranging signals from outlying SS, further increasing the missed detections.
To summarize, because of (1) the fixed allocation of two subchannels to
ranging, (2) the higher number of SS and (3) the weaker signals due to
extended cell radii, the CDMA ranging performance in OFDMA2 is going to be
even more challenging than in OFDMA.
I think that we need some simulations of the CDMA ranging (initial ranging
and bandwidth requesting) in OFDMA2.
--
Jerry Krinock <jkrinock@radiacommunications.com>
Radia Communications
275 N. Mathilda Suite A
Sunnyvale, CA 94086
phone/voicemail: 408-830-9726 ext 239