Smart Automatic Level Control For improved repeater integration in CDMA and WCDMA networks

Transcription

1 Smart Automatic Level Control For improved repeater integration in CDMA and WCDMA networks The most important thing will build is trust

2 Smart Automatic Level Control (SALC) Abstract The incorporation of RF repeaters as coverage solutions within a CDMA and WCDMA network can unwillingly increase undesired network behaviour phenomena. Such undesired incidents are a result of the fact that the surrounding base stations cannot directly control the repeater gain and power. This paper describes a useful proprietary method, developed by Cobham Wireless Ltd., for automatic control of the CDMA and WCDMA repeater output RF levels invariable network load environments. This innovative solution relieves CDMA and WCDMA network operators from most of the problems encountered when incorporating high-gain repeaters into their systems, including gain adjustment, power-control, isolation and oscillation fine-tuning. Introduction As part of any normal operation of a CDMA and WCDMA system, networks carry out load sharing by managing the load at the base stations (BTS s). This is a function of the active nature of the number of communicating mobile devices and their transmitted power. These systems suffer from "Adjacent Code Interference, a phenomenon handled by a power control scheme that is used for maintaining the received power from all mobiles within a cell at a constant level. This observable fact along with power control and adjacent cell soft-handoff, cause what is known as Cell Breathing (the periodic expansion and reduction of cell coverage) and is sensitive to accurate detection of signals from each of the connected mobile devices in a given cell. What happens when an RF repeater is incorporated into a cellular CDMA and WCDMA system? When incorporating an RF repeater within a CDMA and WCDMA network, a standard procedure is to set the repeater s desired uplink and downlink levels. However, if an inappropriate gain and/or level is chosen, this may result in a nonlinear signal compression, which in-turn may not only disturb the normal performance of the mobile devices but also might partially disable the ability of the base station to control the output power of the mobile devices connected to it. To keep this from happening, the normal Automatic Level Control (ALC) mechanism, which is implemented in most RF repeaters, is used. The standard ALC circuit sees that the repeaters output power is kept at a predefined level (a level in which all components function linearly). The standard ALC, however, does not provide a suitable answer to the Cell Breathing phenomena. The Cell Breathing phenomena Due to the nature of the spread spectrum communication technology, the range of the geographical area covered by a certain CDMA and WCDMA base station changes continuously with relation to the amount of traffic currently using that cell. When a CDMA and WCDMA cell becomes heavily loaded, it shrinks, causing some of its subscriber traffic to be then redirected to a neighbouring cell that is more lightly loaded, (load balancing). The cell shrinkage is also known as Cell Breathing. Cell breathing is commonly encountered in 2G (IS-95) and 2.5G (1xRTT), and is likely to become more critical for the advanced 3G CDMA2000 and WCDMA cellular systems (UMTS), where a very high Quality of Service (QoS) to the end-user will be required to cope with the data services introduced. Multi-Cell Environment In a multi-cell environment, the uplink capacity of a CDMA and WCDMA system is determined by the bit energy-to-noise density ratio, Eb/No (where Eb is the energy of one bit of information and No is the total spectral noise power density, which includes both the background thermal noise and the co-channel interference caused by mobiles in the same cell and adjacent cells). The E b /N o system parameter determines the quality of the signal, i.e. a minimum E b /N o is required for adequate system performance. It can be shown, that as the number of mobiles (n) increases, E b /N o of the system decreases. So there is a maximum number of mobiles, n=n max, for which E b /N o reaches its minimum value, beyond which satisfactory performance of the receiver and its decoding process will not be possible. When the number of users in the cell approaches n max, the cell will reach its physical capacity limit. If this heavily loaded cell can share its load with neighbouring cells by offloading some of the users to some less heavily loaded neighbouring cells, then more users could be active within by the whole system. Overlapping areas are important for mobiles near the

3 CDMA and WCDMA cell boundaries where soft-handoff and counteract fluctuations of received signal power are performed. One way to achieve load sharing is for the heavily loaded cells to handoff some of its users in the overlapping region to less heavily loaded neighbouring cells. In terms of energy of the measured Radio-Wave transmission from the base station, the overall temporal sum of energy in a given location within the range of a particular cell goes up with traffic build up. The result is that the cell shrinking is inversely proportional to the measured intensity. The role of a repeater in a CDMA AND W-CDMA network If used for covering RF black spots or as an integral part of the overall network planning, the role of a repeater (also known as cell-extenders or Bi-Directional Amplifiers or BDA s) in a CDMA and WCDMA cellular network is to serve as an uplink/downlink signal enhancer, amplifying the received signals by a predefined gain factor. It is therefore most important for proper cellular network operation that the repeater will not modify the received signals nor compress their dynamic range, thus affecting the cell-breathing phenomenon. A repeater is best functioning when it is as transparent to the network as possible. Having said that, the transparency goal is difficult to achieve. A major barrier is meeting the requirements of setting an upper limit to the downlink transmitted power. The need is clear due to environmental considerations and neighbouring cell interference issues (RF coverage design) that demand optimal power levels. The standard Automatic Level Control (ALC) mechanism As explained above, the intensity of the RF signal at a location (i.e. the location of a given repeater) increases proportionally to the number of actively connected mobiles at a given time. For this reason, a repeater that is set to amplify the uplink/downlink signals by a given gain factor, can reach or even exceed its preset power limit (sometimes causing an undesired non-linear amplifiers operation). The standard procedure for control of the transmitted power known as Automatic Level Control, or ALC, is the process of automatically reducing the repeater gain when the transmit power reaches a predefined level. This ALC process leads to Signal Compression, meaning that while the traffic in a cell drives the repeater to the plateau of the ALC (see fig. 1), its output becomes constant and indifferent to the changes in its input power levels. Although this feature keeps the repeater output from getting into its non-linear saturation area, the transmit fluctuations in the repeater s input port are not reflected, thereby preventing the base station and individual mobiles from conducting their power control 1 mechanisms. Another serious drawback introduced by the use of the standard ALC is the tendency to disturb the uplink/downlink gain balance 2. Since the ALC operates on the downlink only, the downlink/uplink gain balance is disturbed. Disruption of this functionality is reflected in a reduced base-station dynamic range, reduced coverage area and over all improper network operation as cell breathing is interfered with. The Smart Automatic Level Control (SALC) mechanism Confronting the challenge of producing a network friendly, cell transparent, CDMA and WCDMA repeater, Cobham Wireless has developed its proprietary Smart Automatic Level Control (SALC). The idea behind this technology within the development of RF repeaters is to incorporate a digital RF Gain Controller that, when combined with advanced control algorithms can perform gradual learning of traffic load characteristics and adjust the repeater gain accordingly (according to the actual levels as sampled during the network actual activation and following their changing paths). In practice, this automatic operation substitutes the need of special settings during installation and/or initial setup that usually require a knowledgeable technician and some laboratory equipment. It practically removes the need for initial settings at maximal traffic load conditions as well as repeated site visits for the purpose of gain adjustment and allows for a virtual plug and play deployment. 1 Power Control - Depending on attenuation and interference, the base station transmits control messages to the handsets in order to set the smallest possible power that meets the quality target. This reduces interference towards other users and increases the battery lifetime. 2 Gain balance - Spread spectrum, type systems (CDMA and WCDMA being one of them) incorporate the information from the mobiles in a wide band frequency channel that is deciphered per each mobile by an independent code. Keeping the gain and power balanced is crucial for letting the base stations control their connected mobiles and keeping them operating in a way that they are received with an equal power by the base stations, and then preserving a stable E b /N o ratio for all subscribers.

4 Digitally controlled repeater By combining the analogue standard ALC with a built-in micro-controller, the SALC algorithm is implemented and gain is set in such a way that it is optimized for use in the CDMA and WCDMA network environment. The CDMA and WCDMA SALC allows the repeater to sustain an operator s pre-defined, traffic related (subscriber related) maximum output power. In contrast to the standard ALC that keeps the gain at its predefined levels for low signals and decreases its value as the input signal reaches the forbidden region (and restores its previous value when the input goes down again), the SALC actually modifies the pre-set gain level periodically depending on the downlink output power which in line follows the traffic existing in the specific cell. The outcome of this process is that from this point on, the SALC will keep the repeaters gain at a level that matches the optimal downlink input signal, regardless of temporary (traffic driven) transients and cell breathing effects that may appear at the input signal. In practice this means changing the gain from its initial value to a new value that matches the traffic conditions (see Fig. 1). Advantages of the SALC The different behaviours of the repeater s output power can be seen in figure 2. On Cobham Wireless CDMA and WCDMA repeater, equipped with a Smart ALC (SALC) system, the repeater output power is maintained linearly and proportional to the input at all times. The dynamic range of the signal power is preserved regardless of network load, hence the base station can communicate power control signals to the mobiles and measure their response, and gain balance is maintained. The benefits from deploying the SALC digital level control are numerous: - The Smart ALC optimizes gain factors while enabling the repeater to remain transparent to the network (both the base stations and the mobile units). - Uplink and downlink balance is preserved. If uplink and downlink are transmitting at different levels (not balanced), then the base station will see phone signals that are coming through the repeater as transmitting in a higher gain than the rest of the mobile units. This bares the risk of reducing the dynamic range of the base station itself. - Reduction of isolation problems The automatic reduction of gain when sudden gain rise effects are caused. This feature is integral for avoiding oscillation problems. - When installing the repeater, the standard procedure for the technician is to tune the repeater to the desired isolation and gain values. With SALC, the tuning procedure is accomplished automatically, thus saving precious workmanship and equipment usage time for a plug & play, foolproof deployment. - The SALC benefits to a single-channel scenario are magnified with far more importance when considering a multi-channel cell environment. - The SALC periodically learns traffic conditions, avoiding the need to remember and store the last state after power failure.

5 - Standard ALC is the operation of maintaining linear gain until reaching certain output level, but not allowing the output transmit power to exceed the predefined output level. This causes signal compression that in turn conflicts with the CDMA and WCDMA power control scheme. SALC solves this problem. - Repeaters that blindly amplify the pilot code (minimal transmission) of a certain base station might generate network problems (unnecessarily increase the noise in a certain cell). By keeping the same gain for pilot only transmissions as well as for full traffic situations, the Cobham Wireless CDMA and WCDMA SALC feature minimizes this effect. Summary The SALC feature is an innovative solution for automatic repeater gain adjustment. It reduces CDMA and WCDMA cell shrinking due to repeater power emission, and enables the repeater to achieve the best gain value according to the actual network traffic. In addition to solving the level control problem, SALC also emphasizes derivative advantages in the form of reduced isolation problems, reduced sync. amplification, reduction of installation and first time tuning related costs, etc. It also helps in maintaining uplink/downlink power balance. Cobham Wireless advanced Research and Development group (R&D) is in constant search for nonstandard solutions to standard problems. The ability to integrate digital and analogue design in a common dimension previously dominated by analogue engineering, enables us to introduce innovative solutions that have led Cobham Wireless products to be recognized as State of the Art, and the preferred solutions by many cellular operators worldwide.