Chip
Understanding Chip
In a direct-sequence spread-spectrum (DSSS) system, the transmitter multiplies each data bit by a high-rate pseudo-random code, converting each bit into a sequence of chips. If the data rate is Rb and the chip rate is Rc, the spreading factor SF = Rc/Rb determines both the bandwidth expansion and the processing gain. The chip waveform shape (typically NRZ or raised-cosine) determines the power spectral density of the spread signal, which appears noise-like to any receiver that does not know the spreading code.
At the receiver, a correlator or matched filter multiplies the incoming signal by a synchronized replica of the same spreading code. Chips from the desired signal add coherently (SF-fold gain), while interference and noise remain uncorrelated and gain no benefit. This is the fundamental mechanism behind CDMA multiple access: all users share the same frequency band simultaneously, distinguished only by their unique chip codes. For GPS, the chip also serves as a ranging element: the time offset between transmitted and received chip sequences, multiplied by the speed of light, yields the pseudo-range measurement. The C/A code chip duration of 977.5 ns corresponds to a range resolution of 293 meters before carrier-phase refinement.
Processing Gain and Chip Rate
Gp = 10 · log10(Rc / Rb) = 10 · log10(SF) [dB]
Chip Duration:
Tc = 1 / Rc [seconds]
Range Resolution (GPS):
ΔR = c · Tc = c / Rc [meters]
Where Rc = chip rate (chips/s), Rb = data bit rate (bits/s), SF = spreading factor (chips/bit), c = speed of light (3 × 108 m/s). Higher chip rates improve both interference rejection and ranging precision.
Chip Rate Comparison Across Systems
| System | Chip Rate | Spreading Factor | Processing Gain | Chip Duration |
|---|---|---|---|---|
| GPS C/A | 1.023 Mchip/s | 1,023 | 30.1 dB | 977.5 ns |
| GPS P(Y) | 10.23 Mchip/s | ~10,230 | 40.1 dB | 97.75 ns |
| IS-95 CDMA | 1.2288 Mchip/s | 64 to 128 | 18 to 21 dB | 813.8 ns |
| WCDMA (3G) | 3.84 Mchip/s | 4 to 512 | 6 to 27 dB | 260.4 ns |
| Galileo E1 | 1.023 Mchip/s | 4,092 (BOC) | 36.1 dB | 977.5 ns |
Frequently Asked Questions
What is the difference between a chip and a bit?
A bit carries user information at the data rate (e.g., 9.6 kbps), while a chip is one element of the spreading code running at a much higher rate (e.g., 3.84 Mchip/s in WCDMA). Each data bit is multiplied by SF chips, spreading bandwidth by that factor. The receiver correlates with the known sequence to recover the bit, rejecting interference by the processing gain of 10·log10(SF). For SF = 128, the processing gain is 21.1 dB.
How does chip rate affect GPS receiver performance?
GPS C/A code at 1.023 Mchip/s gives a chip duration of 977.5 ns and range resolution of 293 meters. The P(Y) code at 10.23 Mchip/s improves range resolution to 29.3 meters. Higher chip rates provide wider bandwidth for better interference rejection and more precise pseudo-range measurements, but require faster ADCs and wider receiver front-end bandwidth. Military M-code uses 10.23 Mchip/s with BOC(10,5) modulation for enhanced anti-jam performance.
Why does CDMA use different spreading factors?
Variable spreading factors trade data rate for processing gain. Voice at 12.2 kbps uses SF = 128 (21 dB gain), while 2 Mbps data uses SF = 2 (3 dB gain). The chip rate stays fixed at 3.84 Mchip/s. Lower SF means higher throughput but less interference margin. The Orthogonal Variable Spreading Factor (OVSF) code tree maintains orthogonality between channels at different rates, managed dynamically by the base station.