Radio Frequency

*******************************************************************

FREQUENCY HOPPING

Frequency Hopping: is a method to increase frequency reuse and improve the system's ability to cope with adjacent channel interference. The frequency hopping algorithm can be either random or cyclic. Associated uplink and downlink frequency are always +/- 45MHz. there are two type of frequency hopping

1. Baseband frequency hopping
2. Synthesized frequency hopping.

Frequency hopping improves BSS MS performance by providing two type of diversity:

1. Frequency Diversity: Frequency diversity average the effects of signal fading by using several frequency to improve transmission performance. Obstacles such as building produce fading by reflecting the signal out of phase with the main signal. Each frequency is affected differently by fading.After error correction information is added to the data, it is encoded so that the data is split into packets and the information is repeated. This creates redundant information which is transmitted in bursts on the Um. With frequency hopping, each redundant information burst is transmitted on a different frequency. This enable the original data to be reconstructed from the received flow, even if error occur due to fading. In this way frequency hopping improves transmission performance.
2. Interference diversity: Interference diversity spreads the Co-CH interference between several MS. In high traffic areas, the capacity of a network system is limited by its own interference, that is, the interference caused by frequency reuse. Interference diversity min the time during which a given user on a given MS will experience the effect of such interference.

BASEBAND FREQUENCY HOPPING

A MS allocation is a set of all the frequency available for frequency hopping. When the frequency hopping procedure is implemented, a group of MS is assigned to a MS allocation. When a TCH is setup in a cell where frequency hopping is active, the TCH is assigned by four part such as:

1. A particular time slot
2. An FHS. An FHS is defined as the subset of frequency within the MA to be used by a given cell for frequency hopping.
3. A MAIO. The MAIO indicates the initial hopping frequency of the TCH within the FHS. Use of the maio ensure that each TCH is assigned a different frequency during hopping.
4. An HSN. The HSN supply the identifying number of an algorithm which is used to calculate the next frequency in the FHS on which the TCH transmits. There can be up to 63 different HSN algorithms, all of which are pseudo random. Within a given FHS, only one algorithm is used to avoid collisions. An HSN of 0 mean a cyclic use of the frequency.

Ex of frequency hopping is shown in the figure below. Because HSN=0, hopping occurs in a sequential manner. With a non 0 HSN, each of the three TCH would hop in a random fashion determined by the algorithm corresponding to the HSN.

***********************************************************************