diff --git a/src/target/trx_toolkit/trxd_proto.py b/src/target/trx_toolkit/trxd_proto.py
new file mode 100644
index 000000000..aef3351da
--- /dev/null
+++ b/src/target/trx_toolkit/trxd_proto.py
@@ -0,0 +1,120 @@
+# -*- coding: utf-8 -*-
+
+'''
+TRXD PDU definitions based on declarative codec.
+'''
+
+# TRX Toolkit
+#
+# (C) 2021 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+# Author: Vadim Yanitskiy <vyanitskiy@sysmocom.de>
+#
+# All Rights Reserved
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+import codec
+
+class Header(codec.BitFieldSet):
+	''' Header constructor for TRXD PDUs. '''
+
+	def __init__(self, ver: int):
+		codec.BitFieldSet.__init__(self, len=1, set=(
+			codec.BitField('ver', bl=4, val=ver),
+			codec.BitField.Pad(bl=1), # RFU
+			codec.BitField('tn', bl=3),
+		))
+
+class MTS(codec.BitFieldSet):
+	''' Modulation and Training Sequence. '''
+
+	DEF_LEN = 1
+	STRUCT = (
+		codec.BitField('nope', bl=1),
+		codec.BitField('mod', bl=4),
+		codec.BitField('tsc', bl=3),
+	)
+
+	@staticmethod
+	def get_burst_len(mod: int) -> int:
+		''' Get burst length by modulation type. '''
+
+		GMSK_BURST_LEN = 148
+
+		if (mod >> 2) in (0b00, 0b11): # GMSK or AQPSK
+			return 1 * GMSK_BURST_LEN
+		elif (mod >> 1) == 0b010: # 8-PSK
+			return 3 * GMSK_BURST_LEN
+		elif (mod >> 1) == 0b100: # 16QAM
+			return 4 * GMSK_BURST_LEN
+		elif (mod >> 1) == 0b101: # 32QAM
+			return 5 * GMSK_BURST_LEN
+		elif mod == 0b0110: # GMSK (Access Burst)
+			return 1 * GMSK_BURST_LEN
+		raise ValueError('Unknown modulation type')
+
+class BurstBits(codec.Buf):
+	''' Soft-/hard-bits with variable length. '''
+
+	def __init__(self, name: str, *args, **kw):
+		codec.Buf.__init__(self, name, *args, **kw)
+
+		# This field has a variable length that depends on moduletion type
+		self.get_len = lambda v, _: MTS.get_burst_len(v['mod'])
+		# This field is not present in NOPE / IDLE indications
+		self.get_pres = lambda v: not v['nope']
+
+
+class PDUv0Rx(codec.Envelope):
+	STRUCT = (
+		Header(ver=0),
+		codec.Uint32BE('fn'),
+		codec.Uint('rssi', mult=-1),
+		codec.Int16BE('toa256'),
+		codec.Buf('soft-bits'),
+		codec.Buf('pad'), # Optional
+	)
+
+	def __init__(self, *args, **kw):
+		codec.Envelope.__init__(self, *args, **kw)
+
+		# Field 'soft-bits' is either 148 (GMSK) or 444 (8-PSK) octets long
+		self.STRUCT[-2].get_len = lambda _, data: 444 if len(data) > 148 else 148
+
+class PDUv0Tx(codec.Envelope):
+	STRUCT = (
+		Header(ver=0),
+		codec.Uint32BE('fn'),
+		codec.Uint('pwr'),
+		codec.Buf('hard-bits'),
+	)
+
+
+class PDUv1Rx(codec.Envelope):
+	STRUCT = (
+		Header(ver=1),
+		codec.Uint32BE('fn'),
+		codec.Uint('rssi', mult=-1),
+		codec.Int16BE('toa256'),
+		MTS(),
+		codec.Int16BE('cir'),
+		BurstBits('soft-bits'),
+	)
+
+class PDUv1Tx(PDUv0Tx):
+	# Same structure as PDUv0Tx, only the version is different
+	def __init__(self, *args, **kw):
+		PDUv0Tx.__init__(self, *args, **kw)
+		self.STRUCT[0]._fields[0].val = 1