refactor(802.11): 24-bit OFDM SIGNAL layout helpers

Added [Ieee80211OfdmSignalField.h](src/inet/physicallayer/wireless/ieee80211/packetlevel/Ieee80211OfdmSignalField.h) with shared `packIeee80211OfdmSignalField()` / `unpackIeee80211OfdmSignalField()` helpers for the 24-bit OFDM SIGNAL layout.

Then updated:

- [Ieee80211PhyHeaderSerializer.cc](src/inet/physicallayer/wireless/ieee80211/packetlevel/Ieee80211PhyHeaderSerializer.cc) to use the shared helper instead of its private `packOfdmSignal()` logic.
- [Ieee80211OfdmRadio.cc](src/inet/physicallayer/wireless/ieee80211/bitlevel/Ieee80211OfdmRadio.cc) to use the same unpack helper in `decapsulate()` instead of duplicating the bit masks inline.

Author: mgonzalezlopezudc

src/inet/physicallayer/wireless/ieee80211/bitlevel/Ieee80211OfdmRadio.cc

@@ -12,6 +12,7 @@
 #include "inet/common/packet/chunk/BytesChunk.h"
 #include "inet/physicallayer/wireless/ieee80211/bitlevel/Ieee80211LayeredOfdmReceiver.h"
 #include "inet/physicallayer/wireless/ieee80211/bitlevel/Ieee80211LayeredOfdmTransmitter.h"
+#include "inet/physicallayer/wireless/ieee80211/packetlevel/Ieee80211OfdmSignalField.h"
 #include "inet/physicallayer/wireless/ieee80211/packetlevel/Ieee80211PhyHeader_m.h"
 #include "inet/physicallayer/wireless/ieee80211/packetlevel/Ieee80211Tag_m.h"
 
@@ -45,13 +46,13 @@ void Ieee80211OfdmRadio::decapsulate(Packet *packet) const
     if (!packet->hasBitError()) {
         auto ofdmReceiver = check_and_cast<const Ieee80211LayeredOfdmReceiver *>(receiver);
         const auto& phyHeaderBytes = packet->peekDataAt<BytesChunk>(b(0), B(5), Chunk::PF_ALLOW_IMPROPERLY_REPRESENTED);
-        auto signal = phyHeaderBytes->getByte(0) | (phyHeaderBytes->getByte(1) << 8) | (phyHeaderBytes->getByte(2) << 16);
+        auto signalField = unpackIeee80211OfdmSignalField(phyHeaderBytes->getByte(0), phyHeaderBytes->getByte(1), phyHeaderBytes->getByte(2));
         const auto& phyHeader = makeShared<Ieee80211OfdmPhyHeader>();
-        phyHeader->setRate(signal & 0xF);
-        phyHeader->setReserved((signal & 0x10) != 0);
-        phyHeader->setLengthField(B((signal >> 5) & 0xFFF));
-        phyHeader->setParity((signal & 0x20000) != 0);
-        phyHeader->setTail((signal >> 18) & 0x3F);
+        phyHeader->setRate(signalField.rate);
+        phyHeader->setReserved(signalField.reserved);
+        phyHeader->setLengthField(B(signalField.length));
+        phyHeader->setParity(signalField.parity);
+        phyHeader->setTail(signalField.tail);
         phyHeader->setService(phyHeaderBytes->getByte(3) | (phyHeaderBytes->getByte(4) << 8));
         packet->eraseAtFront(B(5));
         auto mode = ofdmReceiver->getMode(packet);

src/inet/physicallayer/wireless/ieee80211/packetlevel/Ieee80211OfdmSignalField.h

@@ -0,0 +1,59 @@
+//
+// Copyright (C) 2026
+//
+// SPDX-License-Identifier: LGPL-3.0-or-later
+//
+
+#ifndef __INET_IEEE80211OFDMSIGNALFIELD_H
+#define __INET_IEEE80211OFDMSIGNALFIELD_H
+
+#include <cstdint>
+
+namespace inet {
+
+namespace physicallayer {
+
+struct Ieee80211OfdmSignalField
+{
+    uint8_t rate = 0;
+    bool reserved = false;
+    uint16_t length = 0;
+    bool parity = false;
+    uint8_t tail = 0;
+};
+
+inline uint32_t packIeee80211OfdmSignalField(uint8_t rate, bool reserved, uint16_t length, bool parity, uint8_t tail)
+{
+    // IEEE Std 802.11-2024 Figure 17-5 and 17.3.4: SIGNAL bit 0 is RATE bit
+    // R1 and the LSB is transmitted first.
+    return static_cast<uint32_t>(rate & 0xF) |
+            (reserved ? 0x10 : 0) |
+            (static_cast<uint32_t>(length & 0xFFF) << 5) |
+            (parity ? 0x20000 : 0) |
+            (static_cast<uint32_t>(tail & 0x3F) << 18);
+}
+
+inline Ieee80211OfdmSignalField unpackIeee80211OfdmSignalField(uint32_t signal)
+{
+    Ieee80211OfdmSignalField field;
+    field.rate = static_cast<uint8_t>(signal & 0xF);
+    field.reserved = (signal & 0x10) != 0;
+    field.length = static_cast<uint16_t>((signal >> 5) & 0xFFF);
+    field.parity = (signal & 0x20000) != 0;
+    field.tail = static_cast<uint8_t>((signal >> 18) & 0x3F);
+    return field;
+}
+
+inline Ieee80211OfdmSignalField unpackIeee80211OfdmSignalField(uint8_t byte0, uint8_t byte1, uint8_t byte2)
+{
+    uint32_t signal = static_cast<uint32_t>(byte0) |
+            (static_cast<uint32_t>(byte1) << 8) |
+            (static_cast<uint32_t>(byte2) << 16);
+    return unpackIeee80211OfdmSignalField(signal);
+}
+
+} // namespace physicallayer
+
+} // namespace inet
+
+#endif

src/inet/physicallayer/wireless/ieee80211/packetlevel/Ieee80211PhyHeaderSerializer.cc

@@ -8,6 +8,7 @@
 #include "inet/physicallayer/wireless/ieee80211/packetlevel/Ieee80211PhyHeaderSerializer.h"
 
 #include "inet/common/packet/serializer/ChunkSerializerRegistry.h"
+#include "inet/physicallayer/wireless/ieee80211/packetlevel/Ieee80211OfdmSignalField.h"
 #include "inet/physicallayer/wireless/ieee80211/packetlevel/Ieee80211PhyHeader_m.h"
 
 namespace inet {
@@ -16,18 +17,9 @@ namespace  physicallayer {
 
 namespace {
 
-uint32_t packOfdmSignal(uint8_t rate, bool reserved, uint16_t length, bool parity, uint8_t tail)
-{
-    return (rate & 0xF) |
-            (reserved ? 0x10 : 0) |
-            ((length & 0xFFF) << 5) |
-            (parity ? 0x20000 : 0) |
-            ((tail & 0x3F) << 18);
-}
-
 void writeOfdmSignal(MemoryOutputStream& stream, uint8_t rate, bool reserved, uint16_t length, bool parity, uint8_t tail)
 {
-    auto signal = packOfdmSignal(rate, reserved, length, parity, tail);
+    auto signal = packIeee80211OfdmSignalField(rate, reserved, length, parity, tail);
     stream.writeByte(signal & 0xFF);
     stream.writeByte((signal >> 8) & 0xFF);
     stream.writeByte((signal >> 16) & 0xFF);
@@ -38,11 +30,12 @@ void readOfdmSignal(MemoryInputStream& stream, uint8_t& rate, bool& reserved, ui
     uint32_t signal = stream.readByte();
     signal |= stream.readByte() << 8;
     signal |= stream.readByte() << 16;
-    rate = signal & 0xF;
-    reserved = (signal & 0x10) != 0;
-    length = (signal >> 5) & 0xFFF;
-    parity = (signal & 0x20000) != 0;
-    tail = (signal >> 18) & 0x3F;
+    auto field = unpackIeee80211OfdmSignalField(signal);
+    rate = field.rate;
+    reserved = field.reserved;
+    length = field.length;
+    parity = field.parity;
+    tail = field.tail;
 }
 
 } // namespace