1. INTRODUCTION

CLOVER is a digital communications mode that conveys 8-bit digital data over narrow-band high-frequency radio. It can also transfer ASCII text and executable computer files without using the additional control characters required in other digital modes, which decrease throughput. It measures signal conditions, and automatically changes modulation format and data throughput to match current link quality. Reed-Solomon data encoding provides forward error correction (FEC) within each data block to repair many errors without the need for retransmission.

2. WAVEFORM

The CLOVER waveform consists of four tone pulses, each of which is 125 Hz wide, spaced at 125 Hz centers. The four tone pulses are sequential, with only one tone being present at any instant and each tone lasting 8 ms. Each frame consists of four tone pulses lasting a total of 32 ms, so the base modulation rate of a CLOVER signal is always 31.25 symbols per second. Data is conveyed by changing the phase and/or amplitude of successive pulses at the same frequency. These changes are made only at the instants midway between the peaks of two successive pulses when their amplitudes are zero. The measured CLOVER modulation spectrum is tightly confined within a 500 Hz bandwidth, with outside edges suppressed 50 dB to prevent interference to adjacent frequencies. Unlike other modulation schemes, the CLOVER modulation spectrum is the same for all modulation formats. Additional key parameters of CLOVER modulation include a symbol rate of 31.25 symbol/s (regardless of the type of modulation being used), 2:1 voltage (6 dB power) crest factor, and a ITU emission designator of 500HJ2DEN or 500HJ2BEN.

3. TRANSMISSION PROTOCOLS

CLOVER normally operates over half-duplex links and uses a Reed-Solornon algorithm to provide FEC. This FEC may be used alone in FEC mode or combined with an Automatic Repeat reQuest (ARQ) protocol in ARQ mode to acknowledge each individual data block. The ARQ mode provides an effective adaptive control system which constantly measures the signal-to-noise ratio, frequency offset, phase dispersion, and errors on each block of received data. CLOVER evaluates these measured parameters and selects the best modulation format for the respective propagation conditions. The receive modem sends commands to the transmitting modem indicating which modulation format should be used for the next transmission. This process ensures selection of an optimum modulation format, allowing CLOVER to operate even with multipath and other HF propagation impairments.

4. REED-SOLOMON ERROR DETECTION AND CORRECTION ALGORITHM

Reed-Solomon FEC is used in all CLOVER modes. This is a powerful byte and block oriented error-correction technique, not available in other common HF data modes, and it can allow the receiving station to correct errors without requiring a repeat transmission. Errors are detected on octets of data rather than on the individual bits themselves. This error correction technique is ideally suited for HF use in which errors due to fades or interferences are often bursty (short-lived) but cause total destruction of a number of sequential data bits. Error correction at the receiver is determined by check bytes which are inserted in each block by the transmitter. The receiver uses these check bytes to reconstruct data which has been damaged during transmission. The capacity of the error corrector to fix errors is limited and set by how many check bytes are sent per block.
Check bytes are also overhead on the signal and their addition effectively reduces the efficiency and therefore the throughput rate at which user data is passed between transmitter and receiver. Efficiencies of 60%, 75%, or 90% can be invoked by using successively lower levels of Reed-Solomon encoding for error correction, or 100% efficiency by bypassing this algorithm. Better propagation conditions do not require as much error correction, which means the amount of overhead decreases and the efficiency increases.

5. SIGNAL CHARACTERISTICS

CLOVER normally uses six different modulation formats which are automatically selected to best compensate for the propagation conditions. These formats are described in the following chart:

CLOVER MODULATION FORMATS

Modulation	Description	Data Rate (bit/s)
2DPSM	Dual-diversity Binary Phase Shift Modulation (PSM)	62.5
BPSM	Binary PSM	125
QPSM	Quadrinary PSM	250
8PSM	8-level PSM	375
SPSM/2ASM	8-level PSM, 2-level Amplitude Shift Modulation	500
16PSM/4ASM	16-level PSM, 4-level Amplitude Shift Modulation	750

6. IDENTIFICATION

Call signs are exchanged during linking, and each station identifies every few minutes, all automatically.

7. CLOVER CONTROL BLOCK (CCB)

Each transmission uses a CLOVER Control Block (CCB) to provide synchronization and mode information. Only one CCB is sent per transmitter-ON cycle. The CCB is always sent using a 17-octet block size and a 60% Reed-Solomon encoder efficiency. The CCB uses 2DPSM modulation in the FEC mode and BPSM modulation in the ARQ mode. The CCB is followed by one or more Error Corrector Blocks (ECBs) of data. In FEC mode, the CCB serves as a preamble to the data block which announces the modulation format and the sending station call sign. In ARQ mode, the CCB follows transmission of one or more data blocks and is used to announce the modulation format to be used by the other station during its next transmission.

8. ERROR CORRECTION BLOCK (ECB)

The data field contains one or more ECBs of error-correction encoded data. One of the variable parameters in CLOVER modulation is the length of the ECB which can be 17, 51, 85, or 255 octets long, always sent at a fixed channel rate of 31.25 bit/s. This is analogous to packet length. However, in this case, block length and the number of Reed-Solomon correctable errors are proportional, with longer blocks being able to correct more errors without requiring repeat transmissions. The effective data rate varies with the type of modulation and the encoder efficiency, going up to 750 bit/s under optimum conditions.

9. FEC MODE

This mode allows a sending station to transmit data to one or more receiving stations. FEC mode is a one-way transmission that cannot repeat transmissions for error correction or use adaptive waveform selection. Therefore, the sending station must choose a transmitting modulation format in advance and assume that conditions between the sending station and all other stations are adequate for the chosen mode. The Reed-Solomon algorithm is used to provide receive error correction in FEC mode with a 60% code rate. Both BPSM and QPSM modulations use 85-character blocks, and the default modulation format is 2DPSM in FEC mode.

The first CCB frame begins with 2.048 seconds of carrier. A 32-ms carrier-off gap immediately follows this frame and each of the following frames. This first CCB frame becomes identical to all of the following (recurrent) CCB frames from this point on. The next frame sent is a 64-bit synchronization sequence which also lasts 2.048 seconds. All subsequent blocks are immediately preceded by a 32-ms reference tone pulse sequence. The CCB is next, and it is followed by from 3 to 9 ECBs.

10. ARQ MODE

ARQ is a two-way point-to-point mode which provides fully adaptive and error-corrected communications between two stations that are linked together. As in the case of FEC, a varying number of ECBs are sent in each ARQ time frame. The number of ECBs and other timing parameters are adjusted so that the total time for each ARQ frame is exactly 19.488 seconds, regardless of modulation waveform combination used. The full advantages of adaptive waveform control and error correction via repeat transmission are provided to these two stations. Data is communicated between both ARQ stations by adding a series of ECBs of data following the CCB. Although the CCBs waveform parameters remain fixed, the waveform of the ECBs is adaptively adjusted to match current propagation conditions. The throughput rate during ECB transinissions is generally much higher than that used for the CCB, because the ECB uses longer blocks and high-rate modulation waveforms to expedite data transfer. All ARQ link maintenance operations are performed at the CCB level.

ARQ ECBs are always 255 bytes long. The Reed-Solomon code rate is set for 150, 188, or 226 8-bit data bytes per ECB depending upon the ARQ bias selected (Robust, Normal, or Fast, respectively). The default bias setting is Robust. Both stations send CCB frames, which last 2.72 seconds. A limited amount of data may be exchanged within the CCBs (called Chat Mode), although large quantities of data are transferred through the ECBs. Unlike packet radio, CLOVER selectively repeats only those blocks which fail Reed-Solomon correction, not all blocks following a failed block. ARQ is an adaptive mode that does not use 2DPSM modulation.

11. MONITORING CAPABILITY

LISTEN mode permits additional stations to monitor all traffic between linked ARQ mode stations. Call signs of all CLOVER stations being monitored are also displayed, since CLOVER stations automatically identify every few minutes.

CLOVER REFERENCES

Ford, Steve: HAL Communications PCI-4000 CLOVER-II Data Controller (Product Review), QST, American Radio Relay League, Newington, CT, May 1993, pp. 71-73.

HAL Communications: CLOVER Glossary, Engineering Document E2000, Rev. D, HAL Communications Corp., Urbana, IL, November 1992.

HAL Communications: PCI-4000/CLOVER-II Interface Specifications, Engineering Document E2001, Rev. G, HAL Communications Corp., Urbana, IL, March 1993.

HAL Communications: PCI-4000 CLOVER-II Data Modem Reference Manual and PC-CLOVER Operators Manual, HAL Communications Corp., Urbana, IL, November 1992.

Henry, George W. and Ray C. Petit: CLOVER - Fast Data on HF Radio, CQ, CQ Communications, Hicksville, NY, May 1992, pp. 40-44.

Henry, George W. and Ray C. Petit: HF Radio Data Communication: CW to CLOVER, Communications Quarterly, CQ Communications, Hicksville, NY, Spring 1992, pp. 11 -24.

Horzepa, Stan (ed); George W. Henry and Ray C. Petit: CLOVER Development Continues, Gateway, QEX, American Radio Relay League, Newington, CT, March 1992, pp. 12-14.

Petit, Ray C.: CLOVER is Here, RTTY Journal, Fountain Valley, CA; January 1991, pp. 16-18; February 1991, pp. 12-13; March 1991, pp. 16-17; April 1991, p.10.

Petit, Ray C.: CLOVER Status Report, RTTY Journal, Fountain Valley, CA, January 1992, pp. 8-9.

Petit, Ray C.: The CLOVERLEAF Performance-Oriented HF Data Communication System, QEX, American Radio Relay League, Newington, CT, July 1990, pp. 9-12.

Townsend, Jay: CLOVER - PCI-4000, RTTY Journal, Fallbrook, CA, April 1993, pp. 3-4; May/June 1993, p. 20.

See also: http://www.halcomm.com.

ACKNOWLEDGEMENT

This technical description was prepared by Steven L Karty, N5SK.