Frequency Hopping

GSM and DCS both employ frequency hopping technique for two main advantages:

• To reduce Mulitpath Fading effect which is dependent in carrier frequency
• To randomize Co-Channel Interference (CCI)

The hopping algorithm is continuously broadcast on BCCH and MS/BTS transmits TDMA frames on different hopping carriers during a call on hopping TRX. Hopping is used on TCH, SDCCH and all Packet Data channels while BCCH and common control channels are not allowed to hop. Frequency hopping types are:

• Baseband Hopping (BBH)
• Synthesizer Hopping (SFH)
• Discrete Hopping

Sector Swap Issue in a Cellular Network

Sector Swap issue refers to the situation when cables of connected to wrong antennas, for example, sector 2 cables are connected with the sector 1 antenna and sector 1 cables are connected to sector 2 antenna. This issue can cause degraded quality, call drop, illogical hand over etc.

Power Control RxLev for GSM and DCS

The upper and lower bounds of Power Control RxLev for Downlink should be set such as signal strength is maintained slightly above the threshold of noise floor. This would provide for enough signal strength to maintain a good voice quality and keeping the interference level to the minimum. Thus, these values are kept more aggressive for DCS, being a clearer band than GSM, for instance, the upper and lower bounds for DCS are generally set to -80dBm and -90dBm as compared to GSM values of -75 dBm and -85 dBm.

Antenna Tip of the Day!

Omni-directional antennas have the advantage of more coverage while directional ones provide higher gain. Phased arrays combine the advantages of both of these by providing higher gain in all directions turn by turn. 

Array Signal Processing

Methods used to estimate the Angles of Arrival (AoA) of signals at a base station can be generally classified into two categories.

1. Maximum Likelihood Based Algorithms
2. Subspace Estimation Based Algorithms

MUSIC (Multiple Signal Classification) is one of the methods based on subspace estimation that can be used to estimate the angles of arrival of multiple impinging signals. In this method array of antenna elements is used, all the impinging wave fronts are recorded on all antennas and processed to find out the angles of arrival. Maximum number of impinging wave fronts that can distinguished using this method is given by M-1, where M is the number of elements in the antenna array. The reason is that this method is based on Noise Subspace and we need at least one vector for noise subspace to exist, which is made sure when the number of impinging wave fronts is at least one less than the number of antenna elements in the array.