Channels

Physical channels, logical channel mappings, and what 1xBTS currently supports.

Channel Architecture

Forward Link Channels (BTS to Mobile)

The forward link carries signals from the base station to mobiles. All forward channels are code-multiplexed using Walsh codes on the same RF carrier.

F-PICH — Pilot Channel

The pilot is a constant, unmodulated signal on Walsh code 0. It carries no data — its purpose is to provide a timing and phase reference that mobiles use for:

• Acquisition — Finding and synchronizing to the base station
• Coherent demodulation — Phase reference for decoding other channels
• Signal strength measurement — Pilot Ec/Io drives handoff and power control decisions

The pilot is always transmitted at the highest power. Every other channel is measured relative to it.

1xBTS status: Implemented. Gain configurable.

F-SYNC — Sync Channel

The sync channel on Walsh code 32 broadcasts system timing at 1200 bps. After acquiring the pilot, the mobile reads the sync channel to learn:

• System time (synchronized to GPS)
• The base station’s PN offset
• Protocol revision
• Paging channel configuration

The sync channel uses a 26.67 ms superframe (3 frames per 80 ms) with R=1/2 K=9 convolutional coding and 2x symbol repetition.

1xBTS status: Implemented. R=1/2 convolutional coding, block interleaving.

F-PCH — Paging Channel

The paging channel on Walsh code 1 is the primary forward common signaling channel. It carries:

• Overhead messages — System Parameters, Access Parameters, Neighbor List, Extended System Parameters
• Page messages — General Page Message to alert specific mobiles
• Channel assignments — Extended Channel Assignment Message (ECAM) directing mobiles to traffic channels
• Directed orders — Authentication challenges, service redirection

Data rate is configurable at 9600 bps or 4800 bps. At 9600 bps, each 20 ms frame carries 172 information bits after R=1/2 encoding. Long code scrambling provides privacy.

Mobiles use slotted paging — each mobile only monitors its assigned slot (determined by IMSI hash), waking up periodically rather than listening continuously.

1xBTS status: Implemented. 9600/4800 bps, full overhead message broadcast, General Page, ECAM.

F-FCH — Forward Fundamental Channel

The fundamental channel is the primary traffic channel, carrying voice frames and signaling during active calls and data sessions. Walsh code is dynamically assigned from the available pool (typically W8-W63, avoiding reserved codes).

RC1 (IS-95A Compatible)

All rates produce 384 symbols after encoding and repetition. Forward modulation is BPSK.

RC3 (CDMA2000 1x)

Uses R=1/4 K=9 convolutional coding and QPSK modulation. Supports the same 9600 bps base rate with variable rates, plus per-PCG power control. 768 modulation symbols per frame map to 384 QPSK symbol pairs.

1xBTS status: Implemented for both RC1 and RC3. Variable rate, MuxPDU Type 1 framing, dim-and-burst and blank-and-burst signaling.

F-SCH — Forward Supplemental Channel

The supplemental channel provides additional data bandwidth alongside the fundamental channel. It uses a separate Walsh code with a shorter spreading factor for higher rates. F-SCH allocation is not yet active in 1xBTS — SO33 data sessions currently use the fundamental channel only.

Higher rates use shorter Walsh codes, consuming more of the code space but providing proportionally more bandwidth. The supplemental channel has its own long code mask and power control gain offset.

1xBTS status: RC3 at 19.2 kbps implemented. Higher rates require additional interleaver parameters and turbo encoder (planned).

F-DCCH — Forward Dedicated Control Channel

A planned dedicated signaling channel that runs alongside or instead of the fundamental channel. Unlike the FCH which multiplexes voice/data with signaling, the DCCH carries only signaling at a fixed rate (9600 bps for RC3).

Benefits:

• No voice interruption for signaling (no dim-and-burst needed)
• DTX capability — can stop transmitting when idle
• Simpler framing (fixed rate, no blind rate detection)

1xBTS status: Planned. Design document covers 6 implementation phases.

Other Forward Channels (Not Implemented)

Reverse Link Channels (Mobile to BTS)

The reverse link carries signals from mobiles to the base station. Unlike the forward link, reverse channels from different mobiles are separated by long code masks rather than Walsh codes.

R-ACH — Reverse Access Channel

The access channel is how mobiles initiate contact with the network. It carries:

• Registration messages
• Origination messages (MO calls, data, SMS)
• Page response messages

The access channel operates at 4800 bps with 20 ms frames. Encoding uses R=1/3 K=9 convolutional code. Modulation is 64-ary orthogonal — each group of 6 coded bits selects one of 64 Walsh functions, providing processing gain.

Access is random with collision avoidance:

• Mobile picks a random access slot
• Transmits preamble + data
• Waits for acknowledgment
• If no ack, increases power and retries (access probe sequence)

1xBTS status: Implemented. Full access probe detection, L3 decode, ARQ acknowledgment.

R-FCH — Reverse Fundamental Channel

The reverse traffic channel, carrying voice frames and signaling from the mobile during active sessions.

RC1

Uses 64-ary orthogonal modulation at variable rates (9600/4800/2400/1200 bps). The base station performs blind rate detection — it Viterbi-decodes at each rate and checks the CRC to determine which rate the mobile transmitted.

Encoding: R=1/3 K=9 convolutional.

RC3

Uses BPSK modulation with a continuous reverse pilot. The pilot provides a coherent reference for the base station’s receiver, enabling better demodulation than RC1’s non-coherent detection.

Per-PCG power control operates at 800 Hz (one power control bit per 1.25 ms power control group).

1xBTS status: RC1 implemented with full pipelined receiver (pilot search, frame alignment, Walsh despreading, Viterbi decode, CRC check). RC3 reverse partially implemented.

R-PICH — Reverse Pilot Channel

Present only for RC3 and higher. The reverse pilot is a continuous (or gated) reference signal that the base station uses for coherent demodulation of the reverse traffic channel. It also carries the power control subchannel.

In gated mode, the pilot is only transmitted during active power control groups, saving mobile battery when the traffic channel is idle.

1xBTS status: Partial support (framework for RC3 gated pilot).

Other Reverse Channels (Not Implemented)

Logical Channels

CDMA2000 defines logical channels that map to physical channels depending on the system configuration:

Forward Logical Channels

In current 1xBTS, f-csch maps exclusively to F-PCH and f-dsch maps to F-FCH (since F-DCCH is not yet implemented).

Reverse Logical Channels

Similarly, r-csch maps to R-ACH and r-dsch maps to R-FCH today.

Composite Channels

A “traffic channel” in CDMA2000 is actually a composite of multiple physical channels:

Forward Traffic = F-FCH + F-DCCH (optional) + F-SCH (optional)

Reverse Traffic = R-FCH + R-DCCH (optional) + R-SCH (optional) + R-PICH (RC3+)

In 1xBTS today, forward traffic consists of F-FCH only. F-SCH allocation is not yet active (pending ESCAM validation). Reverse traffic is R-FCH (plus R-PICH for RC3).

Walsh Code Allocation

On the forward link, all channels share the same RF carrier and are separated by orthogonal Walsh codes. The standard SR1 system has 64 Walsh codes available:

When a traffic channel is established, the BSC allocates a Walsh code from the available pool. For supplemental channels (F-SCH) at higher rates, shorter Walsh codes are used (W32, W16, W8, W4), which consume proportionally more of the code space.

The maximum number of simultaneous traffic channels depends on Walsh code availability and the rates in use. At 9600 bps (W64), up to ~60 traffic channels are theoretically possible. With supplemental channels active, the capacity decreases.

Convolutional Encoders

All channels use constraint-length 9 (K=9) convolutional encoders with different rates:

Higher-rate supplemental channels (153.6 kbps) use turbo coding instead of convolutional coding for better performance at low Eb/No. Turbo encoding is not yet implemented in 1xBTS.

Implementation Status Summary