A number of different service options have been defined for use on the CDMA physical channel. A description of each of the service options follows: Service Option 001 – This is the normal, duplex voice conversation mode that uses the original 8 kbps vocoder defined in the TIA IS-96-A specification. Service Option 002 – This … Read more
Forward – Reverse Link Advantages – Disadvantages CDMA The forward link advantages include high transmission power, a pilot channel to provide a timing reference as well as a coherent reference to aid in demodulation, the added time diversity of data bit repetition when transmitting at lower voice data rates, and orthogonal code channel. On the … Read more
The CDMA reverse link uses a different coding scheme to transmit data. Unlike the forward link, the reverse link cannot support a pilot channel for synchronous demodulation (since each mobile station would need its own pilot channel). The lack of a pilot channel is partially responsible for the reverse link’s lower capacity than the forward … Read more
If the modulation data changes, QPSK modulation makes one symbol change each period. If the data is the same, then QPSK and OQPSK do not change state. However, if the data changes, then OQPSK will make at least one symbol change and possibly two changes in a single period. QPSK Makes one Symbol Change Every … Read more
Reverse Channel Long Code Spreading The channelization in the reverse link must provide for unique code assignments for every operational phone. Long Code Spreading Provides Unique Mobile Channelization Mobiles are Uncorrelated but not Orthogonal with Each Other Since the long code is 42 bits in length, this allows 2^42 (4.3 billion) unique channel assignments. Thus … Read more
There are two key reasons that Walsh codes are not used for the channelization in the reverse link: Mobile transmission are not time aligned and therefore cannot be orthogonal, and Walsh codes do not provide enough unique channels (causing more network overhead to manage Walsh code handoffs). All Walsh Codes Arrive Together in Time to … Read more
Pilot Channel Physical Layer The Pilot channel is essentially the short codes operating on their assigned PN offset. This is accomplished by selecting Walsh code 0 which is 64 zeroes (remember, the first code in any Walsh code set is always composed of all zeroes) as the Walsh code modulation data. In addition, the channel … Read more
The Base Station transmitter signal is, however, the composite of many channels (with a minimum of four). The Pilot channel is unmodulated (Walsh code 0); it consists of only the final spreading sequence (short sequences). The Pilot channel is used by all mobiles linked to a cell as a coherent phase reference and also provides … Read more
Auto-correlation is simply a comparison of a signal against itself. For a digital sequence, such as the short codes used in EIA/TIA-95-B, this comparison is a measure of the number of bits that match relative to the number of bits that do not match. Good pseudo-random patterns (such as the short codes) are designed to … Read more
After all of the coding used so far in the forward channel, you’re probably wondering why the data needs to be scrambled again at the same rate. If all cells used the same 64 Walsh codes without another layer of scrambling, the resulting interference would severely limit system capacity. Since all cells can use the … Read more