Physical Channels & Signals in LTE 4G

Introduction

The LTE physical layer (TS 36.211) is where bits finally become radio. Above it, the MAC hands down transport channels — abstract pipes like "downlink shared data" or "the broadcast channel" — and the physical layer's job is to code, modulate, and map those pipes onto real OFDM resource elements in a 1 ms subframe. It does that through a small, fixed set of physical channels. Alongside them it transmits physical signals, which carry no higher-layer payload and exist only so the receiver can find the cell, estimate the channel, and measure quality.

This roster is used constantly and everywhere in the UE lifecycle. From the first moment a modem powers on it is reading physical signals (PSS/SSS/CRS) and the broadcast channel (PBCH); once connected, every scheduled millisecond involves PCFICH → PDCCH → PDSCH on the downlink and PUCCH/PUSCH on the uplink. Knowing which channel does what — and which ones carry a transport channel versus pure L1 control — is the mental map underneath almost every other LTE topic.

It matters because nearly every "why won't this decode?" question resolves to one channel. A UE that cannot read PCFICH cannot find the control region; one that cannot read PDCCH never learns where its PDSCH is; one whose CRS estimate is poor mis-equalises everything. Get the roster straight and the rest of the air interface becomes legible.

Why the channel roster is needed

Concretely, a shared radio channel has to solve several problems at once. It must let one base station serve many UEs from the same spectrum, so it needs a scheduler and a way to signal its decisions (PDCCH carrying DCI). It must let a receiver that has never seen the cell find it and lock timing (PSS/SSS) and then estimate the channel before decoding anything (CRS/DMRS). It must carry bulk data efficiently (PDSCH/PUSCH) and small control messages cheaply without stealing a whole data grant (PCFICH, PHICH, PUCCH). And it must give a brand-new UE a way to get onto the network before it has any dedicated resources (PRACH). The roster is exactly the minimal set of specialised channels that covers those jobs.

Channels vs Signals — the two kinds of thing on the grid

LTE maps everything onto a time/frequency grid of resource elements: 12 subcarriers by one slot form a resource block, and each subframe is 1 ms of 14 OFDM symbols (normal cyclic prefix, 7 symbols per 0.5 ms slot; extended CP gives 12 symbols per subframe / 6 per slot). Two categories of thing live on that grid, and getting the split straight is the first thing an interviewer checks.

A physical channel carries information that came down from higher layers — user data, control commands, system information. Physical channels have a payload that originated above the physical layer (or, for a few pure-L1 control channels, was generated inside the physical layer to serve higher-layer scheduling). A physical signal carries no higher-layer payload at all; it is a known pattern that exists purely so the physical layer can function — synchronisation, channel estimation, measurement, and positioning.

Downlink Physical Channels

The downlink carries six physical channels. The trick to remembering them is that most of them exist just to make the one that matters — PDSCH — decodable, plus one specialist channel (PMCH) for broadcast/multicast services.

PDSCH (Physical Downlink Shared Channel). The workhorse. It carries actual user data and most higher-layer signalling, plus paging and the system information blocks (SIBs). "Shared" means it is dynamically split among UEs subframe by subframe by the scheduler. Everything else on this list is essentially plumbing that tells you where your PDSCH is, or acknowledges what you sent. PDSCH lives in the data region of the subframe, after the control region, and is demodulated using either CRS or (in the newer transmission modes) UE-specific DMRS. It supports QPSK, 16-QAM and 64-QAM from Rel-8 (256-QAM added in Rel-12), and turbo coding for the transport block.

PDCCH (Physical Downlink Control Channel). Carries Downlink Control Information (DCI) — the scheduling messages. A DCI is either a DL assignment ("your PDSCH is at these resources, this MCS, this HARQ process") or an UL grant ("you may transmit PUSCH here"). The main LTE DCI formats are format 0 (UL grant), 1/1A/1B/1C/1D (various DL assignments; 1A is the compact "fallback" format), 2/2A/2B/2C/2D (spatial-multiplexing DL), and 3/3A (TPC power-control commands). Each DCI's CRC (16 bits) is masked with a UE's RNTI — e.g. C-RNTI for dynamic user data, SI-RNTI for system information, P-RNTI for paging, RA-RNTI/Temporary C-RNTI for random access, SPS-C-RNTI for semi-persistent scheduling — so a UE only acts on the DCIs addressed to it. A PDCCH is built from aggregations of Control Channel Elements (CCEs), each 9 Resource Element Groups (36 REs); the aggregation level is 1, 2, 4 or 8 CCEs, with more CCEs giving more robust coding for a UE in poor coverage. It carries no transport channel — the DCI is generated inside the physical layer. (Rel-11 added the EPDCCH, which lives in the PDSCH region rather than the front control region.)

PCFICH (Physical Control Format Indicator Channel). Carries a single value, the CFI (Control Format Indicator), of 1, 2, or 3. That number tells the UE how many OFDM symbols at the start of the subframe form the control region — i.e. how many symbols hold PDCCH/PHICH/PCFICH before PDSCH begins. The two-bit CFI is coded into a 32-bit block and carried in exactly four Resource Element Groups (16 REs) in the first OFDM symbol, quadrature-spread across the whole band so the UE can always find it. The UE must read PCFICH first, because it defines the boundary for everything else in the subframe. (In the narrowest bandwidth, ≤10 RBs, the actual control span is CFI+1 symbols, but the principle of "how big is the control region" is unchanged.)

PHICH (Physical HARQ Indicator Channel). Carries the 1-bit HARQ ACK/NACK for uplink transmissions. When the UE sends data on PUSCH, the eNB replies on PHICH: ACK means "received, done," NACK means "resend." It is the downlink acknowledgement channel for uplink HARQ, it lives inside the control region, and it too carries no transport channel — just the single HARQ indicator bit. Many PHICHs are code-multiplexed into one PHICH group (a set of REGs) using orthogonal length-4 (normal CP) or length-2 (extended CP) sequences; the number of groups is scaled by the Ng parameter (one of {1/6, 1/2, 1, 2}) broadcast in the MIB.

PBCH (Physical Broadcast Channel). Carries the MIB (Master Information Block) — the small, most-essential system parameters: downlink system bandwidth, the PHICH configuration, and the System Frame Number (SFN). The MIB is only 24 bits, protected by a 16-bit CRC, tail-biting convolutionally coded and QPSK-modulated. PBCH sits in the centre 72 subcarriers around the sync signals, in the first four OFDM symbols of slot 1, so a UE that has just found the cell can read it without yet knowing the full bandwidth. Its 40 ms transmission is split into four self-decodable copies, one per radio frame, so a UE can often decode it from a single copy.

PMCH (Physical Multicast Channel). The specialist. It carries MBMS (Multimedia Broadcast Multicast Service) content — the same stream sent to many UEs at once, such as broadcast TV or emergency messaging. PMCH is transmitted in MBSFN (Multicast/Broadcast Single Frequency Network) subframes, where multiple synchronised cells send identical waveforms so the UE sees them as one strong multipath signal. It uses an extended cyclic prefix and its own MBSFN reference signal for demodulation rather than CRS, and it never mixes with unicast PDSCH in the same subframe region.

The Downlink Control Region — an LTE signature concept

Here is the idea that defines LTE's downlink structure. In every 1 ms subframe (14 OFDM symbols in normal CP), the first 1 to 3 OFDM symbols form the control region. That region is spread across the whole system bandwidth and holds PCFICH, PHICH, and PDCCH. Whatever symbols remain form the data region, which is PDSCH. The exact split — 1, 2, or 3 symbols — is signalled per subframe by the CFI on PCFICH. A lightly loaded cell can use one symbol of control and give the rest to data; a heavily loaded cell scheduling many UEs uses three.

This is a favourite contrast question. In LTE the control region is time-multiplexed at the front of the subframe and spans the full band. In 5G NR that changed: NR replaced the always-full-width front-loaded region with the CORESET (Control Resource Set), configurable in both time and frequency, so control need not occupy the whole bandwidth. LTE's fixed front porch is simpler; NR's CORESET is lower-overhead and more flexible.

Downlink Physical Signals

Signals carry no user data — they are known patterns the UE measures against. The downlink has synchronisation signals, reference signals, and one positioning signal.

PSS / SSS (Primary / Secondary Synchronisation Signals). These are how the UE finds a cell in the first place. PSS gives coarse timing (symbol/slot alignment) and part of the physical-cell-ID — the in-group identity N_ID(2) ∈ {0,1,2}, carried by one of three length-63 Zadoff–Chu sequences (roots 25, 29, 34). SSS gives frame timing and the cell-ID group N_ID(1) ∈ {0…167}, built from two interleaved length-31 m-sequences. Together they yield the Physical Cell ID, PCI = 3×N_ID(1) + N_ID(2), giving 504 values (0–503, i.e. 168 groups × 3 sectors), and let the UE align to the cell in both time and frequency. They sit in the centre 62 subcarriers so any UE can find them regardless of the cell's bandwidth.

CRS (Cell-specific Reference Signal). The defining LTE reference signal. It is a known pilot pattern spread across the whole band in (nearly) every subframe, on antenna ports 0–3 (a cell uses 1, 2, or 4 ports). The UE uses CRS for channel estimation — to equalise PDSCH and PDCCH — and for measurements: RSRP, RSRQ, and the CQI report all derive from it. The CRS pattern is shifted in frequency by v-shift = PCI mod 6, so neighbours with suitably chosen PCIs do not stack their pilots on the same subcarriers. Because CRS is "always on" and cell-wide, it is also LTE's biggest source of constant overhead, a problem NR deliberately avoided by making all its reference signals on-demand.

DMRS (UE-specific Demodulation Reference Signal). A reference signal that travels with a specific UE's PDSCH (in transmission modes 7–10, on antenna port 5 and ports 7–14) and is precoded exactly the same way the data is. Because the pilot is precoded identically to the data, the UE can demodulate a beamformed transmission without ever being told the precoder — it simply estimates the effective channel from the DMRS. This is what enables transparent beamforming and is the demodulation basis for the later transmission modes.

CSI-RS (Channel State Information Reference Signal). A sparser pilot introduced in Rel-10, transmitted periodically (configurable periods of several to tens of milliseconds) rather than every subframe, used purely for channel-state feedback. The UE measures CSI-RS to compute and report CQI/PMI/RI, especially for higher-order and multi-antenna MIMO (up to 8 ports in Rel-10, more in later FD-MIMO releases) where CRS alone is not enough. It separates the "measure the channel for feedback" job from the "demodulate my data" job that DMRS handles.

PRS (Positioning Reference Signal). A special reference signal designed for location, added in Rel-9. It is transmitted in configured positioning subframes with a diagonal pattern and high power/muting coordination across neighbour cells, so the UE can hear and time-of-arrival many cells (not just its serving one). It underpins the OTDOA (Observed Time Difference Of Arrival) positioning method — the UE measures reference-signal time differences (RSTD) between cells and the network trilaterates its position.

Uplink Physical Channels and Signals

The uplink mirror is smaller: three physical channels and two signals. LTE uplink uses SC-FDMA (a single-carrier variant of OFDMA, also called DFT-spread-OFDM) to keep the UE's power amplifier efficient, but the channel roster is what you must know cold.

PUSCH (Physical Uplink Shared Channel). The uplink workhorse — carries user data and higher-layer signalling from the UE, and can piggyback control information too. The UE may only transmit on PUSCH when it holds a valid UL grant (a DCI format 0 on PDCCH, or a semi-persistent/configured grant). PUSCH normally sits in the central portion of the band, leaving the edges for control, and supports QPSK/16-QAM/64-QAM (256-QAM from Rel-14).

PUCCH (Physical Uplink Control Channel). Carries Uplink Control Information (UCI) when the UE has no PUSCH to send. UCI is three things: the HARQ-ACK for downlink PDSCH, the Scheduling Request (SR) asking for an uplink grant, and channel feedback (CQI/PMI/RI). It uses one of several PUCCH formats (1/1a/1b for SR and HARQ-ACK, 2/2a/2b for CSI, and Rel-10's format 3 for larger ACK payloads under carrier aggregation). PUCCH lives at the two band edges, hopping between them across the slot for frequency diversity, so it does not fragment the central PUSCH region. It carries no transport channel — UCI is L1/L2 control. If the UE is already transmitting PUSCH, the UCI is multiplexed onto PUSCH instead of using PUCCH.

PRACH (Physical Random Access Channel). Carries the random access preamble — the very first uplink transmission a UE makes when it has no connection, or has lost uplink timing sync. It is how the UE gets onto the network and obtains a timing advance before any dedicated resources exist. It is preamble-only: it carries a chosen preamble sequence (a Zadoff–Chu root sequence with a cyclic shift), not user data — up to 64 preambles per cell, of which some are reserved for contention-free access. The eNB detects the preamble, replies with a Random Access Response, and only then can the UE send real data on PUSCH.

Uplink signals: DMRS and SRS. The uplink demodulation reference signal (DMRS) travels with PUSCH and PUCCH so the eNB can estimate the channel and demodulate what the UE sent — every uplink data or control transmission carries its own DMRS (for PUSCH, in the middle symbol of each slot, symbol 3 under normal CP). The sounding reference signal (SRS) is a wideband pilot the UE sends, normally in the last symbol of a subframe, so the eNB can measure the uplink channel across frequency, even in bands where the UE is not currently transmitting data. SRS feeds uplink frequency-selective scheduling and, in TDD, downlink channel estimation via reciprocity. Both uplink pilots are Zadoff–Chu-based for low peak-to-average power ratio.

Physical Signals at a Glance

Signals never carry a transport channel and never carry user bits — they are the scaffolding. Here they are side by side across both links, so you can see which job each one does.

Mapping Physical Channels to Transport Channels

Physical channels do not invent their payload — most of them carry a transport channel handed down by the MAC layer (TS 36.212 defines the coding for each). Knowing which physical channel carries which transport channel is a classic exam question, and the mapping is almost one-to-one, with two instructive exceptions.

Notice the two lessons. First, the asymmetry: both DL-SCH and PCH ride on the same physical channel, PDSCH — the DCI on PDCCH (and the RNTI its CRC is masked with) is what tells the UE which one a given PDSCH instance actually is. Second, some physical channels carry no transport channel at all: PDCCH, PCFICH, and PHICH in the downlink, and PUCCH in the uplink. These carry pure physical-layer control — the DCI, the CFI, the HARQ indicator, and the UCI respectively — generated for and by the physical layer rather than passed down as a transport block.

Summary

LTE's air interface is a fixed roster you can hold in your head. On the downlink, six physical channels: PDSCH carries the data (plus paging and SIBs); PDCCH carries the DCI that schedules it; PCFICH tells you how big the control region is; PHICH acknowledges your uplink; PBCH carries the MIB; and PMCH carries MBMS. On the uplink, three: PUSCH for data, PUCCH for UCI, and PRACH for the access preamble. Everything else on the grid is a physical signal — PSS/SSS to find the cell, CRS/DMRS/CSI-RS/PRS downlink and DMRS/SRS uplink — carrying no user bits.

Two facts do most of the heavy lifting in interviews: the read order in a subframe (PCFICH → PDCCH → PDSCH), and the transport-to-physical mapping with its two twists (DL-SCH and PCH both ride PDSCH; the four control channels carry no transport channel). Keep those straight and you can reason about almost any LTE downlink or uplink behaviour from first principles — and you will immediately see what NR kept, moved, or deleted.

Quick Q&A