CDMA

Uses Half-Rate Convolutional Encoder Outputs Two Bits of Encoded Data for Every Input Bit Unlike many digital cellular systems, CDMA provides powerful error correction to all voice data bits. This is desirable in CDMA since the idea is to increase the occupied bandwidth (spread the data). The forward link uses a half-rate convolutional encoder to … Read more

How Frame Formats in CDMA Once the analog voice is compressed by one of the vocoder processes, some additional data is added to produce a frame. Each frame in CDMA is 20 milliseconds regardless of the data rate used. This figure shows all of the possible frame configurations for both the 8 kbps and 13 … Read more

Vocoders Convert Voice to/from Analog Using Data Compression There are Three CDMA Vocoders: IS-96A  Variable Rate (8 kbps maximum) CDG  Variable Rate (13 kbps maximum) EVRC  Variable Rate (improved 8 kbps) Each has Different Voice Quality: IS-96A  – moderate quality EVCR  – near toll quality CDG  – toll quality All digital communication systems use various … Read more

Voice data at 9600 bps or 14400 bps (full rate) is first passed through a convolutional encoder, which doubles the data rate for the 9600 bps case or increases it by 1.33 times for the 14400 bps case. It is then interleaved, a process that has no effect on the rate, but does introduce time … Read more

The base station uses a slightly different scheme when the vocoder moves to lower rates. First, EIA/TIA-95-B CDMA base stations do not pulse their transmissions. Rather, base stations repeat the same bit patterns as many times as needed to get back to the full rate of 9,600 bps. So, if the vocoder selects a frame … Read more

Each 20 millisecond frame in EIA/TIA-95-B CDMA is divided into sixteen “power control groups”. When the mobile transmits, each power control group contains 1536 data symbols (chips) at a rate of 1.2288 Mbps. When the voice coder moves to a lower date rate, the CDMA mobile bursts its output by only sending the appropriate number … Read more

CDMA takes advantage of quiet times during speech to raise capacity.  A variable rate vocoder is used; for the original vocoder the channel is a 9,600 bps when the user is talking.  When the user pauses, or is listening, the data rate drops to only 1,200 bps (CDMA Variable Rate Speech Coder). Data rates of … Read more

Closed loop power control is used to allow the power from the mobile unit to deviate from the nominal as set by open loop control. This is done with a form of delta modulator.  The base station monitors the power received from each mobile station in reverse link and commands the mobile to either raise … Read more

The Open Loop Power Control require in cdma due to following reason. Assumes Loss is Similar on Forward paths and Reverse Paths Receive Power + Transmit Power = -73 All Powers in dBm Example: For a Received Power of -85 dBm Transmit Power = (-73) – (- 85) Transmit Power = +12 dBm Provides an … Read more

Power Control Required in Reverse link cdma due to following reason. Maximum System Capacity is Achieved if: All Mobiles are Power Controlled to the Minimum Power for Acceptable Signal Quality in Reverse Link. As a Result, all Mobiles are Received at About Equal Power at the Base Station Independent of Their Location in reverse Link. … Read more