Mid-Level Co ntroller
2.4 Experimental Results
2.4.4 Experimental Results - Flights in IMAV2013 Replica
DPCCH
DPCH DPDCH
DCH
Figure 6
Uplink Physical Channels
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1.8 Channel Mapping Options
1.8.1 Logical to Transport Channel Mapping
Before its transmission across the air interface, information presented in logical channels must be mapped into transport channels. This mapping process is very flexible, and for some logical channels there are several options, depending on the function and the type of information being transferred.
1.8.2 Transport to Physical Channel Mapping
The physical layer applies error protection and maps and multiplexes transport channels into physical channels.
It should be noted that some unidirectional channels, i.e. PICH, CPICH and AICH, at the physical layer. These are referred to as physical signals.
1.8.3 Mapping for the Uu Interface
The directions of arrows shown in Figure 7 reflect the mapping process as seen from the UTRAN side. For the channels carrying broadcast information, mapping is direct from BCCH to BCH and from PCCH to PCH.
For the other control- and traffic-carrying channels, mapping is more flexible. For example, downlink DCCH can be mapped either to FACH or to DSCH, depending on information requirements. In the uplink direction DCCH may take information from CPCH, RACH, USCH or DCH. The logical channel DTCH is similar, in that it has access to a range of transport channels. However, the CCCH is simple: it uses only RACH and FACH for bidirectional communication.
BCCH PCCH DCCH CCCH CTCH DTCH
BCH PCH CPCH RACH FACH DSCH DCH
PCCPCH SCCPCH PCPCH PRACH PDSCH DPCH
Air Interface Physical
Layer Transport Channels Logical Channels
MAC
Figure 7
Channel Mapping Options
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Figure 8 shows how a typical UMTS cell may be configured in the uplink and downlink directions.
2.1 Example Downlink Channels
The cell will contain a single PCCPCH. This channel carries the BCH transport channel, which in turn carries system information messages. Phase synchronization for this physical channel is provided by the CPICH. These channels will always be scrambled by the cell-specific primary scrambling code. The two channels will be time-aligned in terms of scrambling code and frame structure, this timing being indicated by the Primary and Secondary SCH.
This example cell contains one SCCPCH. This channel is being used to carry the FACH and PCH. These are variable-rate channels that, in the case of FACH, may contain a mixture of signalling and traffic.
There are several types of physical channels with which a cell may be provisioned that carry only physical layer signalling. Two of these are shown in Figure 8: the AICH, which is used to acknowledge random access probes, and the PICH, which is used to support a discontinuous reception function for the PCH.
There are likely to be multiple DPCHs and PDSCHs in operation on the cell. These are variable-rate channels that may carry signalling or traffic. In general, bursty packet-switched traffic is likely to be carried in the DSCH, while circuit-switched traffic must be carried in a DCH.
2.2 Example Uplink Channels
In the UL direction there are three physical channel types with slightly different code requirements. The PRACH and the PCPCH are always directed at a single cell; soft handover is not a feature of these channels. As a result, the codes used can be cell-specific. Up to 16 PRACH and up to 64 CPCH channels could be provisioned on a cell but the example cell has two PRACHs and four PCPCHs.
The cell also has provision for uplink DPCHs to match those operating in the downlink direction. These channels can use soft handover and therefore the codes are not cell-specific.
PRACH
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2.3 Configuration Options
There are many cell configuration options available to a UMTS operator. The most suitable configuration will depend on location and the likely traffic profile of users in the cell area. At rollout it is likely that all cells will be configured in one way only or perhaps in a limited set of default ways.
Optimization can include consideration of channel provisioning on a cell. Given that different channels are suited to different traffic characteristics it is likely that the channel types available on a cell could be optimally matched to the local traffic requirements.
For example, a cell being used for an indoor coverage solution is more likely to carry high-bit-rate packet-switched data. This means that more CPCHs and DSCHs may need to be provisioned. In addition to the changes to the site database this will also impact the Node B’s physical requirements for channel elements and terrestrial transmission bandwidth.
Another possibility is to build a new cell to provide a specific function. For example, at a sports stadium or in a large public arena a cell could be used to stream audio and visual content, perhaps as a commentary of an event. This would require the CTCH, which is mapped into the FACH. This could be operating at a very high bit rate requiring the construction of a cell with its capacity predominantly dedicated to a SCCPCH carrying the FACH.
Standard configuration In-building – more
provision for packet data with CPCH and
DSCH
Sports stadium – more provision for streamed audio/video
with CTCH in FACH
Figure 9
Configuration Options
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2.4 Using More than One Frequency
Most UMTS licence holders have enough radio spectrum for more than one FDD radio carrier pair. It is possible to add second or even third frequencies to a Node B.
However, this concept is slightly different in UMTS than in GSM.
A Node B can contain one or more cells. A typical arrangement would be to have three cells using appropriate directional antenna on a Node B site. All three cells would be using the same frequency. It would be possible to add more capacity to the Node B by adding a second frequency for each set of antennas. However, each new frequency added carries its own full set of control and traffic channels. This means that the second frequency must be considered as a new cell. Thus a three-cell Node B becomes a six-cell Node B even though only three sets of directional antennas are used.
It is possible to use wideband power amplifiers so that a single power amplifier can amplify two frequencies. This would save cost because although the site has six cells, only three power amplifiers would be needed. However, this means that the power available to each cell is halved. If the cells are downlink limited then this will halve the capacity of the cells.
Cell 4F2 F2 Cell 5
F2 Cell 6
Three sets of antennas Three cells One frequency
Three sets of antennas Six cells
Two frequencies Cell 1F1 F1
Cell 2
Cell 3F1 F1
Cell 1 F1
Cell 2
Cell 3F1
Figure 10
Using More than One Frequency
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3.1 Downlink Power
In the downlink direction the maximum transmit power available from the highly linear power amplifier can be considered constant. The total power available will depend on the vendor and on the type of Node B. For a macro cell product it could be expected to be in the range 20 to 45 W, for a micro cell or pico cell product it would be proportionally lower. The specifications1 require that the power amplifier has a total power dynamic range of at least 18 dB. Maximum transmit power is limited to 50 dBm (100 W).
This power is shared between all downlink channels. Downlink power control is implemented through the adjustment of the weighted sum of the downlink channels.
Broadcast and common control channels are likely to be allocated a fixed proportion of the power available. The remainder of power is then shared between users. The weighting may be used to vary proportions to each user dependent on path loss, interference and required quality of service. For closed loop power control the UE indicates the requested power step changes to the Node B. However, a limit will be set for the power proportion available to each channel type, so the Node B may not obey all power control commands. If the cell is operating at less than full load then the total power transmitted is less than the total power available.
More power is required if:
• there are more channels required
• if users are distant from the Node B
• if users request higher data rates
• if users request a higher quality of service
Thus the total power available in a cell ultimately limits downlink capacity and quality of service.
1 3GPP TS 25.104 BS Radio Transmission and Reception (FDD).
G1
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3.2 Downlink Power Weightings
The power weighting allocated to downlink broadcast and common channels, along with the range for power control available to other channels, is an important part of a cell’s configuration.
All downlink channels will be allocated a fraction of the maximum transmit power available on the cell. This fraction is referred to as the channel code power and can be in the range –3 dB to –28 dB relative to the maximum transmit power available.
Thus the maximum proportion that can be allocated to any individual channel is 50%
of the maximum power.
The starting point and reference for all other channels in the CPICH. The standards allow for the code power in this channel to be set from –10 dBm to 50 dBm.
However, the important consideration for this channel is the percentage allocated to it from the total power available for the cell. A typical value for this percentage is 10%
of total transmit power. Nevertheless, the optimal value may depend on local conditions, so it should be an optimization task to refine this setting. All the other channels are then set as a power relative to the CPICH power.
Figure 12 shows an example of power settings in a macro cell with a maximum transmit power capability of 46 dBm (40 W). The CPICH has been set at 10% (4 W) of the total power. The primary and secondary SCHs have been set at 6% of total power, but they are subject to a 10% duty cycle so they average a combined power of only 0.48 W. The primary and secondary CCPCHs have each been set at 5% (2 W), but it is worth noting that there may be multiple SCCPCHs and that the SCCPCH is potentially a variable rate channel. Higher data rates in the SCCPCH would require a higher power weighting. The PICH and AICH have been set at 1.5% (0.6 W) each, but again it should be noted that this example only shows a single AICH. There is a one-to-one mapping of AICHs to the number of RACH channels configured so there could be up to eight AICHs on a cell.
The total power allocated to control channels on this example cell is 9.68 W, almost 25% of the total power available on the cell.
Maximum transmit power for the cell is 46 dBm (40 W)
Channel Percentage of
Total Power Power
(dBm) Power
(W)
CPICH 10% 36 dBm 4 W
P&S SCH
(inc. 10% duty cycle) 6% (x 2 x 0.1) 33.8 dBm 2.4 W (x 2 x 0.1)
PCCPCH 5% 33 dBm 2 W
SCCPCH 5% 33 dBm 2 W
PICH 1.5% 27.8 dBm 0.6 W
AICH 1.5% 27.8 dBm 0.6 W
Total for control channels 24.2% 39.86 dBm 9.68 W
Figure 12
Downlink Power Weightings
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3.3 Varying the CPICH Weighting
The nature of a CDMA channel is that a receiver must have accurate code phase alignment in order to successfully receive a channel. The required code phase alignment in the downlink direction in UMTS comes from the CPICH. This explains the need for a large power weighting on the CPICH channel.
Although a typical value for this weighting might be 10% there is considerable scope for variation. Variation in this weighting can be used by the optimizer as a tool to influence the balance between coverage and capacity in a cell.
A higher weighting extends cell range because it enables UEs that are further away to decode downlink channels. However, the larger power weighting uses more of the cell’s capacity. Additionally, a larger cell will tend to have a higher proportion of more distant UEs that also require larger power weightings, thus further reducing cell capacity. This setting may be more appropriate in rural areas.
A lower power weighting reduces cell range because UEs need to be closer before they can decode downlink channels. The lower power weighting coupled with less distant UEs allows more capacity in the cell.
CPICH Lower weighting for CPICH reduces cell radius but increases capacity
Figure 13
Varying the CPICH Weighting
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3.4 Utilizing Soft Capacity and Dynamic CPICH Power
The maximum capacity available in a cell is governed in part by the amount of interference that can be tolerated. Yet the level of interference is not simply a product of the load on the cell itself. Interference is also contributed from neighbour cells.
Thus a cell’s potential capacity at any moment is partly influenced by the load on its neighbour cells. This means that a cell can carry more traffic if its neighbours are carrying less and vice versa.
In real networks the offered traffic is not evenly spread over the ground, therefore neighbouring cells will tend to carry different loads. A busy cell will be forced to transmit more power in the downlink direction because there are more established channels. This creates more interference to neighbour cells, limiting their potential capacity. It would be desirable to balance the load as far as possible between cells in order to distribute interference more evenly. This should lead to a higher total capacity.
Load balancing can be achieved by varying the CPICH weightings among cells.
Busier cells would be given lower CPICH weightings to reduce coverage area and load. Quieter cells would have higher CPICH weightings to increase coverage and capture more offered traffic.
Varying the CPICH weightings can be performed as an optimization function by setting fixed values based on average conditions. However, traffic characteristics in real networks are variable, making it hard to find a truly optimal setting. Furthermore, without great care it would be easy to create coverage holes by setting values that are too low, or to increase the proportion of soft handovers with settings that are too high. Either way this would reduce rather than increase overall capacity.
Some vendors may have features that enable the dynamic control CPICH power weighting. This uses an algorithm in the RNC to dynamically adjust power weighting on cells to suit current traffic conditions. The optimizer’s input would then relate to setting the triggers and constraints for the dynamic weighting control algorithm. They could, for example, influence whether coverage or capacity is the dominant factor.
Simulations of such systems show useful gains in capacity. They also show considerable variation in optimal CPICH weightings as high as 60%. This again suggests that a optimal static value would be hard to find.
Unbalanced load
CPICH Power weightings can be used to balance load
Figure 14 Soft Capacity
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3.5 Pilot Pollution
Pilot pollution occurs in areas of overlapping coverage between multiple cells.
Specifically, it is an area where signal strength is good but there is also a large number of non-dominant servers. Signal strength in this case can be considered to be the CPICH RSCP. In such an area the receiver is not able to decode the downlink channels because the multiple good servers interfere with each other to the extent that the signal-to-interference ratio for the CPICH, Ec/Io, is not good enough despite the high CPICH RSCP.
In effect, this will result in a coverage hole, where UEs are not able to obtain service from the network. Most 3G planning tools will be able to plot and account for areas of pilot pollution. Nevertheless, planning tools are limited by the accuracy of the propagation model. This means that a key early optimization task may be to identify and rectify significant areas of pilot pollution in the built network.
Adjustment of CPICH weightings is one option for dealing with pilot pollution. It can be used to create a dominant server in affected areas. Other techniques to consider may be antenna adjustments including orientation, downtilt and height.
Pilot Pollution
Multiple non-dominant servers CPICH RSCP is good
CPICH Ec/lo is poor
Adjust CPICH power weightings
Adjust antenna azimuth, downtilt or height
Figure 15 Pilot Pollution
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4.1 Use of Downtilt
In the interests of controlling interference and coverage, it is common to use antennas whose vertical beams are tilted down towards the ground by a few degrees.
Looking at Figure 16a, a directional antenna with 0° downtilt is illustrated. For comparison, Figure 16b shows a sector antenna with x° in-built (electrical) downtilt.
Note that all lobes are downtilted. Figure 16c shows an antenna with 0º electrical downtilt which has been mechanically downtilted by x°. Note that the back lobe is tilted up and side lobes in the horizontal plane will not be fully downtilted.
Accordingly, electrical downtilting is often preferred, although a combination of electrical and mechanical tilting is common, as shown in Figure 16d.
Another technique involves mechanically uptilting an electrically downtilted antenna, as shown in Figure 16e. This can be used to create an antenna with a heavily depressed back lobe, which could be useful for interference rejection in some cases.
A typical configuration for UMTS at rollout is to use minimal downtilt to maximize coverage. As the network matures, downtilt is applied when in-fill cells are built.
Ideally, therefore, variable electrical downtilt antennas should be used to facilitate this. Some estimates are that an antenna’s downtilt could need changing between three and four times in the first five years of operation. This may mean that the most economical solution would be remotely adjustable downtilts. This would greatly reduce the number of site visits required. It would also allow for the potential dynamic adjustment of downtilts based on load conditions.
Normally, omni antennas can only be tilted by electrical means, as shown in Figures 16f and 16g. (Mechanically tilted omni antennas are very rare indeed, but may be seen on steep hillside sites). Typical tilts in use vary from 0 degrees (no tilt) to over 10º. Electrical tilt angles of 0º, 2º, 4º and 8º are common, but others are available.
In general, the majority of tilt should be achieved using electrical tilting, with final fine adjustment being mechanically made.