6
Table II
AP PLICABLE MULTI-RU COMBINATIONS F OR DIFFERENT BANDWIDTH MODES IN EHT.
Type Definition Allowed Combinations
Small-size RU 26-tone, 52-tone, 106-tone
- 26-tone RU + 106-tone RU for 20/40 MHz
- 26-tone RU + 52-tone RU for 20/40/80 MHz
Large-size RU
242-tone, 484-tone, 996-tone,
2 × 996-tone, 3 × 996-tone (new
additional)
- 242-tone RU + 484-tone RU for 80 MHz
- 484-tone RU + 996-tone RU for 160 MHz,
242-tone RU + 484-tone RU + 996-tone RU for 160 MHz
- 484-tone RU + 2 × 996-tone RU for 240 MHz, 2 × 996-tone RU for 240 MHz
- 484-tone RU + 3 × 996-tone RU for 320 MHz, 3 × 996-tone RU for 320 MHz
multi-RU signaling designs. To achieve the trade-o ff between
combination complexity and spectral efficiency, it is allowed
to use some limited multi-RU combination s f or the case with
the bandwidth less or e qual to 160 MHz, that is, small-size
RUs (less than 242 tones) can only be combined with small-
size RUs an d la rge-size RUs (more tha n or equal to 242 tones)
can only be combined with large-size RUs, but the mixture of
small-size RUs and large-size RUs is unallowed [4]. Table
II lists the applicable multi-RU combinations for different
bandwidth modes in EHT, where the combination of small-
size RUs shall not c ross 20 MHz channel boundary, and the
combination of 26-tone RU and 52-tone RU for 20/40/80 MHz
PPDU form at an d the combination of 26-tone RU and 106-
tone RU for 20/40 MHz PPDU for mat are permitted. In terms
of large-size RUs, the allowed la rge-size RU combina tions
are 242-tone RU + 484-tone RU for 80 MHz OFDMA/non-
OFDMA PPDU format and 484-tone RU + 996- tone RU
for 160 MHz OFDMA/non-OFDMA PPDU format. In IEEE
802.11ax, OFDMA only supports 2/4/8/16 users, while in EHT
the multi-RU sup port could allow more flexible support for
other values of the number of users, such as 5 or 6 users,
and allowing up to 3 RUs to be assigned to a sin gle user
was proposed in [14]. However, till now, the EHT task group
has not reac hed a consensus on the maximum number of RUs
assigned to a single user.
In terms of the data transmission in multiple RUs, the same
or different set of transmission parameters, such as modulation
and co ding schemes (MCSs), interleaving schemes and the
number of space-time streams, may be applied to comb ined
RUs assigned to a user. There are four approaches to transmit
the d a ta in combined RUs [15]: (1) all RUs are encoded
and interleaved independently, (2) multiple RUs are encoded
together, but e ach RU is interleaved in dependently, (3) multiple
RUs need interleaver across RUs regardless of encoding, and
(4) multiple RUs act as one logic/continuous RU. However,
these potential approaches need to be analyzed and further
evaluated unde r the con straints of hardware complexity and
signaling overhead.
In addition to RU sizes/multi-RU combinations/multi-RU
transmission, the E H T ta sk group need s to make more efforts
in signaling support for multi-RU PPDU, such a s how to
reuse/optimize the existing signaling methods (e.g., Bandwidth
field, Allo c ation field or User field) in IEEE 802.11ax to in-
dicate size/tone-mapping/combination of multi-RU. Different
from IEEE 802.11ax where a u nique STA-ID is used for one
RU assigned to an STA, a matching STA-ID in EHT needs to
be envisioned for the use of multi-RU.
C. EHT Preamble Design
Observing the WLAN development pr ocess, each WLAN
standard has its specific prea mble, which provides functions
including syn c hronization, channel estimation, auto-detection
and ne c essary sign aling, etc. Similar to IEEE 802.11ax, to
support different te chnolog ie s and scenarios, EHT should
define at least a new preamble format for possible PPDU
formats, such as EHT SU PPDU, EHT Trigger-based PPDU,
EHT ER (extended r a nge) SU PPDU and EHT MU PPDU.
As shown in Fig. 4, an EHT PPDU consists of a legacy
part field (namely non-HT Short Training field (L-STF), legacy
LTF field (L-LTF), legacy SIG field (L-SIG) and repeat legacy
SIGNAL field (RL-SIG)), a un iversal SIG (U-SIG) field,
an EHT-SIG field, an EHT Short Training field (EHT-STF),
an EHT Long Training field (EHT-LTF) and a Data field
[4]. Spec ifically, to keep backward compatibility with legacy
PPDUs operating in 2.4 GHz, 5 GHz, and 6 GHz bands, the
legacy part field should be applied to the beginning of th e EHT
PPDU, which is used f or frame detection, sy nchroniz ation and
carrying the necessary indication information (e.g., MCSs and
frame length). For a PPDU with a bandwidth of 160 MHz or
less, the legacy part is duplicated and can r euse the existing
tone rotation [16]. However, for a PPDU with a bandwidth
wider than 160 MHz, the tone rotation is still not determined.
To spoof IEEE 802.11ax devices and respect the length in
the L-SIG field, the first symbol after L -SIG should be BPSK
modulated in an EHT PPDU [17]. To improve the r obustness
of L-SIG in outd oor scenario s and identify the EHT PPDU
through automatic detec tion, the RL-SIG field is necessary
and should be different from the RL-SIG field in the IEEE
802.11ax PPDU [18].
Following the RL-SIG field, the EHT PPDU includes a
two-OFDM-symbol U-SIG field like the HE SIGNAL A (HE-
SIG-A) field in IEEE 802.11ax, which is used to carry the
necessary informatio n for the interpretation of EHT PPDUs
[4]. The U-SIG field c ontains both version independent fields
and version d e pendent fields. The version independent fields
can be composed including PHY version identifier, UL/DL
flag, BSS color, PPDU type, MCS, bandwidth, transmission
opportunity (TXOP), etc. The version de pendent fields likely
consist of the similar information included in HE-SIG-A
except the information included in the version ind ependent
fields as well as new information fields, such as the guard
interval duration, EHT-STF/LTF size, space-time block coding