M-HCD

Mobile Header Compressor/Decompressor

Protocol
Introduced in Rel-5
A combined entity in the mobile terminal (UE) that performs both header compression for uplink and header decompression for downlink. It is the integrated functional block within the UE's PDCP layer handling bidirectional header optimization.

Description

The Mobile Header Compressor/Decompressor (M-HCD) represents the unified functional entity within the User Equipment's Packet Data Convergence Protocol (PDCP) layer responsible for all header compression and decompression operations. In practice, it encompasses the separate M-HC (compressor) and M-HD (decompressor) functions into a single logical block for implementation efficiency. For uplink traffic, the M-HCD acts as the compressor (M-HC), applying algorithms like Robust Header Compression (ROHC) to IP, UDP, and RTP headers before packets are sent over the radio interface to the network. It maintains compression contexts, tracks packet flows, and uses state machines to ensure robust operation even in lossy radio conditions. For downlink traffic, it acts as the decompressor (M-HD), receiving compressed packets from the network-side compressor (U-HC), using the shared context to reconstruct the full original headers before passing the packets up the protocol stack to the IP layer. The M-HCD manages synchronization between its compressor and decompressor sides, handles feedback messages (if supported by the profile), and executes error recovery procedures when context damage or desynchronization is detected. Its integration into PDCP allows it to leverage security functions (like ciphering and integrity protection) and radio bearer management, ensuring compressed packets are correctly associated with their respective QoS flows and security contexts.

Purpose & Motivation

The M-HCD concept was standardized to provide a complete, bidirectional header processing solution within the UE, addressing the need for efficient IP packet handling in both transmission and reception directions over the radio link. Prior to header compression, symmetric real-time services like VoIP required full headers in both uplink and downlink, doubling the bandwidth waste. By integrating compression and decompression into a single entity, 3GPP ensured a coherent implementation that could manage the stateful contexts required for ROHC in both directions simultaneously. This solved the problem of inconsistent implementation and potential interoperability issues between separate compressor and decompressor modules. It also simplified the UE design and testing, as the M-HCD could be treated as a unified protocol feature within PDCP. The creation of the M-HCD was motivated by the all-IP vision of 3GPP networks, where maximizing spectral efficiency for packet-switched voice, video, and data was paramount for commercial viability and user experience.

Key Features

  • Integrated entity combining M-HC and M-HD functions in the UE PDCP
  • Performs uplink header compression using ROHC and other algorithms
  • Performs downlink header decompression to reconstruct original packets
  • Manages bidirectional compression contexts and state synchronization
  • Handles ROHC feedback channels for robustness and error recovery
  • Tightly integrated with PDCP security (ciphering/integrity) and bearer mapping

Evolution Across Releases

Rel-5 Initial

Introduced alongside HSDPA and IMS as the logical combination of the M-HC and M-HD entities within the UE's PDCP specification. Initially supported RFC 2507 and early ROHC profiles for bidirectional VoIP and multimedia traffic over UMTS.

TS 25.323
Rel-6

Enhanced with additional ROHC profiles and improved feedback mechanisms for both compression and decompression directions. Better support for interactive services.

TS 25.323
Rel-7

Optimizations for continuous packet connectivity and improved battery life. Enhanced error recovery and context management for both uplink and downlink.

TS 25.323
Rel-8

Adapted for the LTE PDCP layer, with optimizations for the new E-UTRAN architecture and lower latency requirements in both directions.

TS 25.323
Rel-9

Support for carrier aggregation, requiring context management across multiple component carriers for both compression and decompression.

TS 25.323
Rel-10

Enhanced for LTE-Advanced, with support for more complex ROHC profiles and integration with relaying and CoMP scenarios affecting both uplink and downlink paths.

TS 25.323
Rel-11

Improvements for HetNet mobility, ensuring seamless header compression/decompression during handovers and cell changes.

TS 25.323
Rel-12

Optimizations for small cells and dual connectivity, managing contexts across multiple transmission points for bidirectional traffic.

TS 25.323
Rel-13

Enhanced for operation in unlicensed spectrum (LAA) and D2D, requiring robust bidirectional compression in shared and direct communication scenarios.

TS 25.323
Rel-14

Further enhancements for eMBB, preparing for 5G NR's bidirectional low-latency requirements.

TS 25.323
Rel-15

Fully adapted for 5G NR PDCP, supporting new ROHC profiles and the requirements of network slicing for both uplink and downlink traffic.

TS 25.323
Rel-16

Enhanced for URLLC and Industrial IoT, with ultra-reliable and low-latency compression/decompression procedures for critical bidirectional communications.

TS 25.323
Rel-17

Extended to support sidelink communication for V2X and public safety, enabling direct device-to-device header compression and decompression.

TS 25.323
Rel-18

AI/ML enhancements for adaptive bidirectional compression, predicting traffic patterns and optimizing contexts for both transmission and reception.

TS 25.323
Rel-19

Optimizations for 5G-Advanced and non-terrestrial networks, handling the unique delay and intermittent connectivity challenges for bidirectional flows in satellite links.

TS 25.323

Defining Specifications

SpecificationTitle
TS 25.323 3GPP TS 25.323