Repeaters are basically Radio Frequency (RF) heads which receive one or more carriers from the donor BS, and re-transmits them to a remote location. Considering how this process is done, two types of repeaters can be established.
1. On-frequency repeaters or Iso-frequency repeaters. This is the simplest case, they basically are bidirectional amplifiers connected to two antennas. The donor antenna is installed under the coverage of the BS and subsequently the signal is amplified and retransmitted (Figure5.2). One of their main advan- tages is that they can be virtually installed anywhere as long as it provides a suitable mounting area with a power supply, without the need of an equipment room.
Antenna isolation is an essential issue for the performance of this type of repeaters since they can act as oscillators under certain circumstances. The feedback path is formed between its two antennas and, to guarantee an ad- equate protection against self-oscillation, the coupling loss between the two repeater ports should be at least 15 dB higher than the repeater gain [3GPd]. Appropriate alignment of null radiation points in the diagram pattern, verti- cal separation between antennas and modifications in the environment around antennas (e.g. use of shielding grids) are mechanisms that can help to increase isolation. Nevertheless, repeaters will use an Automatic Gain Control (AGC) system as a fall back to prevent this self-oscillation. AGC would function if, for some reason, the isolation between ports is reduced during the network oper- ation. Further details about repeaters gain adjustment and AGC functioning are given in Section5.4.2.
2. Remote sectors consist of antennas and respective amplifiers. As indicated by their name, they are located remote from the BS shelter to which are linked via a link that can be of different types of conduit, e.g. microwaves, fiber optics,
Donor BTS Donor Antenna Coverage Antenna Repeater
Figure 5.2: On-frequency repeaters.
coaxial cable, copper twisted pairs or optical wireless links. In this second type of repeaters, a new equipment denoted as donor unit is needed. It is directly connected to the BS and it is responsible of adapting the RF interface to the link. In the other end, the remote unit converts back the signal to mobile wireless frequencies and transmit them to UEs through the coverage antenna (Figure5.3).
There can be more than one remote sector (this also applies to on-frequency repeater) connected to the BS. On the other hand, there can also be more than one BS, which may be of different operators, in one specific location or BS hotel.
Regarding the different types of conduits that can be used in the link between donor and remote units, repeaters can be classified in three big groups [WC01]. (a) RF repeaters. When the link between the donor and the remote unit is radio based, the frequency band to chose for transmission is one of the most important issues. Desirably, It must be one of those that the operator already exploits. If the radiolink does not over-interfere with the rest of the mobile network, it is possible to interchange the UL and DL WCDMA bands and use them as DL and UL respectively. Thanks to this transposition users will not connect to the network through the radiolink. Another option is using 5 MHz of the GSM band if it is available to the operator. The advantage of this second approach is that the interference would be around 13 dB lower than in WCDMA, since the channels are 22 times smaller in the first system. In any case, frequency planning must always be done carefully and taking into account this new source of interference.
(b) Fiber Optic Repeaters also denoted by the generic term Radio over Fiber (RoF). This case is formed by a combination of a RF repeater and optical
Donor BTS Coverage antenna Donor unit Remote unit Link (unspecified conduit) Figure 5.3: Deployment of remote sectors.
transceivers (laser diodes and photodiodes) that enable signal transmis- sion through optical fibers. This type of repeaters is very appropriate for areas where the geography does not allow having Line of Sight (LOS) between the donor and remote units, they are also suitable if the in- tended coverage area is far from the donor BS and for RF distribution in buildings. Indeed they make possible covering distances several km away from the donor BS (Figure5.4). The main RoF architectures are:
i. Direct modulation of the laser diode with the RF signal.
ii. Idem, but with previous downconversion of the radio signal to Inter- mediate Frequency (IF).
iii. Downconversion plus digitalization for digital transmission through the fiber.
The first approach is the simplest one but requires monomode fiber ca- bles. On the other hand, in many indoor environments it exists mul- timode fiber for the connection of floor distributors with campus (or building) distributors and so the second approach would be more suit- able, at the expend of an extra complexity. Finally, the third architecture can work well over both types of fibers and it has all the advantages of digital transmission, nevertheless it is the most complex and expensive [WWW+04].
The same authors in [WWW+04] propose an optical switching architec- ture to dynamically activate and deactivate remote sectors, thus achiev- ing a dynamic cell shape. B. Lanoo et al. go one step further and develop this idea in several papers (being perhaps [LCPD07] and [LCPD06] the most representative) to design a novel RoF based architecture in which moving cells supply high transmission rates to users inside trains. Instead of the train moving along a fixed repeater cell pattern, the authors con- sider a cell pattern that moves together with the train, so that handovers
Laser Diode Photo- detector IF IF RF
BT
S
RF Fiber Optic(s) Diode detector Donor UnitRemote Unit RF Interface
RF Interface
Figure 5.4: Basic structure of fiber optic repeaters.
are minimized. Preliminary simulations demonstrate the feasibility of this concept. The idea of (de)activating remote sectors was also proposed in [CCB01] but in a RF repeaters environment. However, in this case the reason was the reduction of noise at the BS receiver.
In case it is desired to transmit more than one subcarrier, a multiplex- ing mechanism is mandatory. Subcarrier Multiplexing (SCM) is a viable method in the DL. However, since several frequency conversions must be done, phase noise on oscillators must be carefully considered so that the final phase error in the UMTS signal is not excessive. Although more expensive than SCM, Wavelength Division Multiplexing is also a feasible option, particularly for the UL, since SCM shows several problems in that direction [DU92].
(c) Other alternatives as for example repeaters over coaxial cable or copper twisted pairs are also viable but allow shorter link distances, up to 300 m in the first case and 100 m in the second one. That is why repeaters with this type of backhauls are mainly used indoors. The process of transmis- sion is similar in both cases: firstly the WCDMA signal is shifted to IF, next, it is digitalized and finally it is transmitted through the cable. Echo cancelers are used in twister pairs to differentiate transmission paths. In the coaxial case, UL and DL are separated through the use of different IFs. Phase equalizers are not rare because of the phase distortion caused by coaxial cable.
Finally, optical wireless based repeaters are also gaining focus lately. This technique has matured in the last years and permits establishing links through laser transmission in free space between the donor and remote units [WB01]. Since clear LOS is required, its usability is restricted to limited distances and acceptable weather conditions (particularly non- foggy regions). Its main niche market is dense urban environments. Since a conduit is not needed and the transmission band does not require a license. Thus, fast deployment and cost are its advantages along with wide bandwidth, interference immunity and security.
From the previous paragraphs it can be seen that repeaters can be categorized as well in devices for indoor or outdoor purposes. The main difference between them
is the transmission power, until 20 dBm approximately in the first case, and up to 43 dBm in the second one. The cabinet and complexity of installation is also an obvious difference between them.