M-HD

Mobile Header Decompressor

Protocol
Introduced in Rel-5
Entity in the mobile terminal (UE) that performs header decompression on downlink data packets received from the network. It reconstructs full IP/UDP/RTP headers from compressed formats, enabling efficient use of radio bandwidth for downlink services.

Description

The Mobile Header Decompressor (M-HD) is a functional entity within the Packet Data Convergence Protocol (PDCP) layer of the User Equipment (UE) in 3GPP systems. Its specific role is to process downlink packets received over the radio interface from the network-side header compressor (U-HC). These packets arrive with compressed headers, often reduced to a few bytes containing only differential changes or context identifiers. The M-HD uses a locally maintained decompression context, which must be synchronized with the compressor's context, to reconstruct the original full IP, UDP, RTP, and potentially other protocol headers. The decompression context contains the static fields and the last known dynamic field values for a given packet flow. Upon receiving a compressed packet, the M-HD interprets the header format, applies the delta updates or references the context, and rebuilds the complete header as it existed before compression. This process is governed by the same ROHC state machines (e.g., Initialization and Refresh, First Order, Second Order) as the compressor, ensuring robustness. The M-HD may also generate feedback packets (acknowledgments, negative acknowledgments, or static context updates) to send back to the U-HC to request retransmissions or signal context damage, depending on the ROHC profile and mode in use. Once decompression is successful, the packet with its full header is passed up to the IP layer in the UE for further processing. The M-HD is essential for maintaining the end-to-end transparency of IP-based services while leveraging the bandwidth savings of header compression on the downlink.

Purpose & Motivation

The M-HD was created to complement the uplink compressor (M-HC), enabling full bidirectional efficiency for header compression over the radio link. In early 3G packet data services, downlink traffic (e.g., web browsing, video streaming) often constituted the majority of bandwidth usage. Transmitting full headers for every downlink packet was a significant waste of scarce radio resources. The M-HD solved this by allowing the network (RNC in UMTS, eNB in LTE) to compress downlink headers, with the UE efficiently decompressing them. This was critical for making high-bandwidth services like video streaming and large file downloads economically feasible over cellular networks. It addressed the asymmetry of early mobile internet use, where downlink capacity was a key bottleneck. By standardizing the decompression function within the UE's PDCP layer, 3GPP ensured interoperability and consistent performance, allowing network operators to deploy header compression transparently, improving user data rates and network capacity without requiring changes to end-user applications.

Key Features

  • Resides in the UE's PDCP layer for downlink processing
  • Reconstructs full IP/UDP/RTP headers from ROHC-compressed formats
  • Maintains decompression contexts synchronized with the U-HC compressor
  • Implements ROHC state machines for robust operation over lossy links
  • Generates and processes feedback packets for context repair and synchronization
  • Ensures packet order and integrity after decompression before IP layer delivery

Evolution Across Releases

Rel-5 Initial

Introduced as part of the HSDPA enhancements in UMTS. The M-HD was defined as the decompressor entity in the UE PDCP, initially supporting decompression of headers compressed using RFC 2507 and early ROHC profiles from the U-HC in the RNC.

TS 25.323
Rel-6

Enhanced ROHC decompression support with additional profiles. Improved feedback mechanisms and error recovery procedures for downlink traffic.

TS 25.323
Rel-7

Optimizations for continuous packet connectivity, improving decompression reliability and context management for always-on services.

TS 25.323
Rel-8

Adapted for LTE, where the M-HD in the UE PDCP decompresses packets from the eNB's U-HC. Optimized for the lower latency and higher data rates of E-UTRAN.

TS 25.323
Rel-9

Support for carrier aggregation, requiring decompression context management across multiple downlink component carriers.

TS 25.323
Rel-10

Enhanced for LTE-Advanced features like relaying, requiring robust decompression from potentially multiple network points.

TS 25.323
Rel-11

Improvements for HetNet mobility, ensuring seamless decompression during handovers between macro and small cells.

TS 25.323
Rel-12

Optimizations for dual connectivity, managing decompression from master and secondary eNBs simultaneously.

TS 25.323
Rel-13

Enhanced for operation in unlicensed spectrum (LAA), requiring robust decompression in environments with higher interference and potential packet loss.

TS 25.323
Rel-14

Further enhancements for high-speed downlink in eMBB scenarios, preparing for 5G NR's downlink requirements.

TS 25.323
Rel-15

Adapted for 5G NR PDCP, decompressing packets from the gNB's U-HC. Support for new ROHC profiles and network slicing on the downlink.

TS 25.323
Rel-16

Enhanced for URLLC downlink, requiring ultra-reliable and low-latency decompression for critical control and data packets.

TS 25.323
Rel-17

Extended to support decompression of sidelink packets in V2X and public safety scenarios, where devices receive directly from other UEs.

TS 25.323
Rel-18

AI/ML enhancements for predictive decompression and adaptive context management, improving efficiency for downlink-intensive applications like XR.

TS 25.323
Rel-19

Optimizations for non-terrestrial networks, handling long propagation delays and intermittent connectivity in downlink satellite communications.

TS 25.323

Defining Specifications

SpecificationTitle
TS 25.323 3GPP TS 25.323