1. OFDM Parameters And Preambles
September 2006
doc.: IEEE yy/xxxxr0
December 2006
OFDM Parameters And Preambles
IEEE P Wireless RANs Date:
Authors:
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Monisha Ghosh, Philips
Carlos Cordeiro, Philips

2. OFDM Parameters Concerns about latest proposal:
December 2006
OFDM Parameters
Concerns about latest proposal:
Too few preamble symbols will lead to performance degradation.
Allows acquisition and synchronization ONLY on the Superframe short preamble, i.e once every 160ms
Robust downlink acquisition, synchronization (frequency offset and timing), and channel estimation are crucial for robust performance.
Relying solely on distributed pilots for channel estimation is problematic for the following reasons:
In a packet data system, OFDM symbols will need to be buffered while channel estimation is performed. Note that in DVB-T one does not need to do this since it is streaming video.
Granularity of packets can be too large (e.g 7 OFDM symbols) for short data bursts, leading to excessive overhead.
Monisha Ghosh, Philips

3. Proposal For Downlink Parameters
December 2006
Proposal For Downlink Parameters
SUPERFRAME PREAMBLE: One OFDM symbol of 4 repetitions of short training, followed by TWO OFDM symbols of long training.
This will allow robust acquisition, frequency offset estimation and one-shot channel estimation because the long preamble will have a training symbol on every frequency.
Negligible additional overhead: one OFDM symbol (about 0.3 ms) every 160 ms.
Will allow robust decoding of the SCH.
FRAME PREAMBLE: Mandatory: One OFDM symbol for channel estimation, with training symbol on every 2nd frequency. Optional: additional OFDM symbol for acquisition, similar to short training in SF.
The channel estimation OFDM symbol could be omitted for the first frame only.
Frames 5 and 10 should have a frame preamble of 2 OFDM symbols.
PILOT DENSITY: Can be lower than 240/OFDM symbol as in the current proposal, since the combination of superframe and frame preambles will be adequate for channel tracking.
Monisha Ghosh, Philips

4. Preamble Sequences Need 3 kinds of preamble sequences:
December 2006
Preamble Sequences
Need 3 kinds of preamble sequences:
Superframe/frame short: Training symbol on every 4th frequency.
Superframe long: Training symbol on every frequency.
Frame long: Training symbol on every 2nd frequency.
802.16e does not provide these different kinds of sequences. Moreover e parameters (no of used tones, frequency of symbols etc.) are different. Cannot use e preamble sequences directly.
We have a simple method (not the one in the current draft) of generating low PAPR (less than 5 dB) preamble sequences for 4-times oversampled sequences for each of the above.
Monisha Ghosh, Philips

5. PAPR Of Random QPSK Data
December 2006
PAPR Of Random QPSK Data
PAPR < 5 dB for the preamble sequences will be adequate.
Monisha Ghosh, Philips

6. Recommendations Preambles: Pilot density:
December 2006
Recommendations
Preambles:
Increase the Superframe preamble from 2 to 3 OFDM symbols.
Frame preamble is 1 OFDM symbol, except on Frame #1 which will have no frame preamble and Frames # 5 and 10, where 2 OFDM symbols will be used.
The above changes will have negligible impact on overhead, but improve robustness and ease of implementation considerably.
Pilot density:
Reduced from 240 per symbol to 192 per symbol.
Monisha Ghosh, Philips