B. Technical issues

Operating different technologies in the same band – guard bands

• In the renewal of spectrum rights, attention needs to be paid to guard bands and the possibility that a different allocation of the band may result in additional and/or different guard bands.
• There are two kinds of guard bands: inter-band and inter-technology. Inter-band guard bands.
• There is already an inter-band guard band in place between the 850 and 900 MHz bands because the CDMA 850 and GSM 900 bands are adjacent to each other. Specifically, the CDMA downlink is adjacent to the GSM uplink. This means that legitimate out-of-band emissions from GSM mobiles fall into the CDMA mobile secure band.
• The duplex directions of the 850 and 900 MHz bands were never designed for adjacent operation in the same geographical area. In order to mitigate the interfaces, there is a 4 MHz guard band (890-894MHz). Inter-technology guard bands.
• Inter-technology guard bands could arise if technologies belonging to different standards operate in adjacent bands.
• Assume that the 850 MHz band is divided in a manner such that UMTS and CDMA are in adjacent bands as shown below:

Even though in this case, the duplex directions of CDMA and UMTS are harmonised, there is a need for inter-technology guard bands. There are four kinds of interferences arising:

• CDMA mobile transmit falling into UMTS base receive.
• UMTS mobile transmit falling into CDMA base receive.
• CDMA base transmit falling into UMTS mobile receive.
• UMTS base transmit falling into CDMA mobile receive.
• A worst case arises when the offending transmitter is transmitting at high power and the victim receiver is at its cell edge, especially when the different systems are operated by different operators (base stations are not co-located).
• In order to mitigate this interference, an inter-technology guard band of 625 KHz may be needed. Effectively this means that a guard band equivalent to one CDMA carrier bandwidth (1.25MHz) may be needed.

• Therefore it is not advisable to use the 850 MHz spectrum for different technologies especially if different operators are involved.

Inter-working with new operators

• The arrival of a new technology into the same band would require establishment of working arrangements between parties to ensure potential interference issues could be resolved in a timely manner.