PDSCH (Physical Downlink Shared Channel) in LTE

In LTE, the Physical Downlink Shared Channel (PDSCH) is the workhorse of the downlink. While the PDCCH acts like the “control tower,” it is the PDSCH that carries the actual payload, your voice packets, video streams, app data, and even certain signaling messages. Without the PDSCH, LTE would just be a signaling skeleton with no real user experience.

The PDSCH transmits data mapped from the DL-SCH (Downlink Shared Channel), which includes user-plane traffic, paging messages, system information blocks (SIBs), and RRC signaling. Its design ensures high efficiency, flexibility, and adaptability to varying radio conditions.

• LTE downlink communication flows from logical channels → transport channels → physical channels.
• Logical channels define the type of information.
• Transport channels decide how the information is carried.
• Physical channels are the actual radio signals over the air.
• PDSCH is the main downlink workhorse that delivers user data, paging, and system information.

If the PDCCH is the traffic controller, then the PDSCH is the highway itself. Once the controller (PDCCH) tells each car (UE) which lane to use, the cars actually travel down the highway (PDSCH) carrying goods (user data).

For example:

• A user streaming a movie gets video packets over the PDSCH.
• Another user receiving paging messages also gets them via PDSCH.
• Even retry transmissions (HARQ re-transmissions) flow over this channel.

It is the main pipe through which LTE delivers services.

Position and Structure of PDSCH in the LTE Frame

• Time Domain: PDSCH occupies the portion of OFDM symbols in a subframe not reserved for control channels (PDCCH, PCFICH, PHICH).
• Frequency Domain: It spans the allocated resource blocks (RBs) scheduled by the eNodeB. Unlike PDCCH, which uses the full bandwidth, PDSCH can be localized to only part of the spectrum.

Resource Mapping

• Each PDSCH transmission is linked to Demodulation Reference Signals (DMRS) inserted in specific OFDM symbols to allow channel estimation.
• Resource elements (REs) not reserved for DMRS, CRS, or control are filled with modulated PDSCH symbols.

From Scheduling to Transmission

The journey of PDSCH data follows a detailed processing chain:

Transport Block Creation

• User/system data is segmented into one or two transport blocks per subframe (depending on the number of MIMO layers).

Channel Coding

• Turbo coding at rate 1/3 provides forward error correction.
• Rate matching adapts coded bits to fit allocated RBs.

Scrambling

• Bits are scrambled with a sequence tied to the UE’s RNTI, preventing interference between users.

Modulation

• QPSK, 16QAM, or 64QAM (based on MCS from PDCCH).
• Higher-order modulation enables higher throughput under good conditions.

Layer Mapping & Precoding

• Data streams mapped onto MIMO layers.
• Precoding provides spatial diversity or multiplexing.

Resource Grid Mapping

1. Symbols placed in OFDM subcarriers and time slots, with reference signals embedded.

OFDM Transmission

• Final resource grid is transformed into time-domain signals and transmitted.

At the receiver, the UE performs channel estimation, descrambling, demodulation, decoding, and reassembly to recover the transport block.

Example: Streaming in a Cell

Three LTE users in the same subframe:

• User A gets 20 RBs with 64-QAM modulation for a video stream.
• User B receives 10 RBs with QPSK for paging.
• User C is allocated 15 RBs for HARQ retransmission.

All of these allocations were instructed via PDCCH, but the actual data flows over the PDSCH, making it the payload bearer of LTE.

Limitations and Challenges

• Interference Sensitivity-Since PDSCH uses variable bandwidth portions, it can suffer from inter-cell interference at cell edges.
• Control Dependence-Without PDCCH instructions, the PDSCH cannot be decoded.
• Power Allocation-Trade-off between control (PDCCH) and data (PDSCH) power impacts coverage.
• HARQ Retransmissions-While HARQ improves reliability, repeated use of PDSCH for failed packets reduces available capacity.

PDSCH in LTE vs. 5G NR

• In LTE, the PDSCH is closely tied to fixed control region definitions.
• In 5G NR, PDSCH mapping is more flexible: it can occupy variable slots, symbols, and bandwidth parts, adapting better to dynamic numerology.
• NR also supports higher modulation (256-QAM), increasing throughput efficiency.

The PDSCH is the data backbone of LTE, delivering everything from YouTube videos to paging messages. By combining turbo coding, adaptive modulation, MIMO, and OFDM, LTE ensures that PDSCH maintains high throughput and robustness even in challenging radio conditions.

If the PDCCH is the “control tower,” then the PDSCH is the fleet of airplanes carrying the actual passengers and cargo, the part of LTE users experience most directly.

References

• 3GPP TS 36.211 – Defines PDSCH structure and mapping.
• 3GPP TS 36.212 – Covers channel coding and rate matching.
• 3GPP TS 36.213 – Explains modulation, MIMO, and scheduling procedures.
• Sesia, Toufik & Baker – LTE: The UMTS Long Term Evolution.
• Dahlman, Parkvall & Skold – 4G LTE-Advanced Pro and the Road to 5G.
• Dangle