ADTRAN 2 x E1 Tracer Description, Channel Selection, Forward Error Correction, Direct Sequence

61280004L2-1C

Section 1 TRACER Description

Direct Sequence

A direct sequence transmitter spreads the signal by mixing the data with the output of a pseudorandom number generator which changes state at a rate higher than the data rate. This rate is called the “chipping” rate. The TRACER chipping rate is twelve times the data rate.

Coding

Many different pseudorandom sequences exist. The sequences are called pseudorandom because, although they appear noise-like, they are determinant and repeat after a specific number of chips. The longer a code is, the better correlation characteristics it possesses. These traits allow multiple spread spectrum systems to operate in the presence of one another with minimal interference if they are operating with different sequences. The TRACER allows the selection of one of ten different 120-bit long sequences.

Channel Selection

The FCC has allocated 83.5 MHz of spectrum in the 2.4 GHz band and 125 MHz in the 5.8 GHz band in which TRACER operates. A TRACER system fully uses the available bandwidth – transmitting in one half and receiving in the other. Figure 1-2illustrates the bandwidth division.

2400 MHz	2441 MHz	2483.5 MHz
	or
5725 MHz	5787 MHz	5850 MHz

Figure 1-2. Bandwidth Division

The transmitter at one end of a link (TxA) will transmit in the lower half of the spectrum. Consequently, the receiver at the other end will receive in the lower half of the band and transmit in the upper half. Thus, a system will operate in one of two frequency plans -- transmit in the upper and receive in the lower or vice versa. These two plans are called Plan A and Plan B. One end of a path will be on Plan A and the other will be on Plan B. Shipment of a link will consist of an A and a B unless specified otherwise.

Forward Error Correction

With the addition of overhead data, error detection and correction capability can be added to a data stream. Error correction can be accomplished by allowing the receiver to request to retransmit of the erred block once detected. The TRACER, on the other hand, implements forward error correction (FEC) which adds enough overhead data for the receiver to detect and correct errors in the data stream. This capability comes at the cost of bandwidth. The addition of FEC decreases the required signal-to-noise (S/N) ratio by approximately 5.5 dB to achieve a given bit error rate (BER).

2 TRACER 2 x E1 User’s Manual