1- For a data (basically compressed voice plus a little overhead plus an error c

Question

1- For a data (basically compressed voice plus a little overhead plus an error correction code) signal at 19.2 Kbps spread to 1.2288 Million chips per second, what is the processing or spreading gain, as an absolute number and in dB?

Note: This is what happens in IS-95 CDMA (this was CDMA for 2G, which was enhanced to a higher rate for 3G where they used close to 5 Million chips/sec) for one of the channel types (the forward traffic channel, not all channel types are the same). The absolute number is the number of chips per bit. In IS-95 out of the 19.2 Kbps FOR ONE CHANNEL half is due to an FEC code at a rate of

Explanation / Answer

In a spread spectrum cellular telephone system, such as illustrated in FIG. 1, the preferred waveform design implemented involves a direct sequence pseudonoise spread spectrum carrier. The chip rate of the PN sequence is chosen to be 1.2288 MHz in the preferred embodiment. This particular chip rate is chosen so that the resulting bandwidth, about 1.25 MHz after filtering, is approximately one-tenth of the total bandwidth allocated to one cellular service carrier.

Another consideration in the choice of the exact chip rate is that it is desirable that the chip rate be exactly divisible by the baseband data rates to be used in the system. It is also desirable for the divisor to be a power of two. In the preferred embodiment, the baseband data rate is 9600 bits per second, leading to a choice of 1.2288 MHz, 128 times 9600 for the PN chip rate.

In the cell-to-mobile link, the binary sequences used for spreading the spectrum are constructed from two different types of sequences, each with different properties to provide different functions. There is an outer code that is shared by all signals in a cell or sector that is used to discriminate between multipath signals. The outer code is also used to discriminate between signals transmitted by different cells or sectors to the mobile units. There is also an inner code that is used to discriminate between user signals transmitted by single sector or cell.

The carrier waveform design in the preferred embodiment for the cell-site transmitted signals utilizes a sinusoidal carrier that is quadraphase (four phase) modulated by a pair of binary PN sequences that provide the outer code transmitted by a single sector or cell. The sequences are generated by two different PN generators of the same sequence length. One sequence bi-phase modulates the in-phase channel (I Channel) of the carrier and the other sequence bi-phase modulates the quadrature phase (Q Channel) of the carrier. The resulting signals are summed to form a composite four-phase carrier.

Although the values of a logical "zero" and a logical "one" are conventionally used to represent the binary sequences, the signal voltages used in the modulation process are +V volts for a logical "one" and -V volts for a logical "zero". To bi-phase modulate a sinusoidal signal, a zero volt average value sinusoid is multiplied by the +V or -V voltage level as controlled by the binary sequences using a multiplier circuit. The resulting signal may then be band limited by passing through a bandpass filter. It is also known in the art to lowpass filter the binary sequence stream prior to multiplying by the sinusoidal signal, thereby interchanging the order of the operations. A quadraphase modulator consists of two bi-phase modulators each driven by a different sequence and with the sinusoidal signals used in the bi-phase modulators having a 90

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