System Model

4.2.1 Network Architecture

As mentioned earlier in Chapter 3, the access network of BuNGee, which has been modified in this work to include high power control BSs in each zone (i.e. Zone Base Stations (ZBSs)) is considered. The BuNGee network topology consisting of the ABSs and ZBSs for access, and the BHSSs and HBSs for backhauling is shown in Figure 4.1. Each ABS is co-located and interfaced to a BHSS as shown in Figure 4.2.

Figure 4.1 BuNGee Topology

As mentioned in Chapter 3, each ZBS co-ordinates the ABSs in its zone to provide broadband access to MSs. MS data is backhauled to HBSs through the BHSSs. Ultra high capacity and reliable backhaul is provided between the HBSs and BHSSs through a combination of in-band and millimetre wave (mmWave) backhauling [141], which will enable low latency communication in the network. It is assumed

that the backhaul frequency band is different from the access frequency band, hence there is no interference between the access and backhaul links.

BHSS ABS MS HBS

ZBS Co-located Entities

Access link

Figure 4.2 BuNGee Access and Backhaul Tier

The fixed frequency plan [125] specified in the BuNGee project is used here. According to this plan, the two antennas of an ABS operate in different frequency bands. Four directions are considered - north, south, east and west - and an ABS can have north and south pointing antenna beams or east and west pointing antenna beams. In Figure 4.1, the four frequency bands in the different directions are shown. ABSs located along the east-west streets have antenna beams in the north and south directions like the ABS shaded in black in the Figure 4.1. ABSs located along the north-south streets have antenna beams in the east and west directions like the ABS shaded in grey. As shown in Figure 4.1, in order to mitigate interference, the antennas of adjacent ABSs facing the same direction operate in different frequency bands and two antennas belonging to different ABSs but pointing along the same street also operate in different frequency bands. Four unique frequency bands are assigned to the small cell layer and each frequency band has a bandwidth of 10 MHz. In this work, each frequency band is further divided into 10 unique subchannels and each subchannel on a particular ABS can be assigned to only one MS. Each MS is assigned only one subchannel at a time for uplink transmission.

The MSs are distributed uniformly outdoors in the service area and each MS is equipped with an omnidirectional antenna with a gain of 0 dBi [80]. The service area is divided into nine square zones as shown in Figure 4.1 and also mentioned earlier in Chapter 3. ABSs can be associated with up to a maximum of four zones. ABSs can only communicate directly with their adjacent neighbours through the co-located BHSSs while they can communicate with the ZBSs through the HBSs. The ZBSs share a 10 MHz frequency band that is out of band to the ABS bands.

In line with the separation architecture paradigm, the ZBSs deployed in the zones are always on to provide universal coverage for the MSs while the ABSs, which can be switched on and off, provide data services. Hence, an MS is always connected to the ZBS in its current zone while it can utilise resources on any ABS in the zone depending on the channel quality and RRM scheme adopted. Although, the ZBS can be configured to serve low data services, the case where only the ABSs provide data services is considered in this chapter and subsequent chapters. Also, the control and data plane separation that makes it possible for separation of universal coverage from data services is explained in detail in the next chapter.

As shown in Figure 4.2, each ZBS is connected to an HBS through optical fibre and information exchange is possible with low delay between a ZBS and an ABS through the backhaul links between a HBS and a BHSS. Whenever a MS has to be served in the DL or UL, the serving ZBS requests the ABSs in its zone to send channel quality measurement in respect of the concerned MS. The channel quality measurement used here is the signal to noise plus interference ratio (SINR). The ZBS will then determine the ABS to serve the MS depending on the objective of the RRM scheme adopted. This co-ordination procedure between the ZBSs, ABSs and MSs before data transmission is similar to the one in [19] and it is illustrated in Figure 4.3 for the UL.

The power consumption at small cell ABSs when they are serving MS uplink traffic is considered in this chapter and subsequent chapters. This is important because the full load downlink power consumption of small cell BSs is of similar order of magnitude as the uplink and no load conditions in existing systems. This is due to the lower share of the power amplifier consumption in small cell BSs [40].

The WINNER II B1 propagation model [127] for Urban micro-cell described in Chapter 3 is used in the system evaluation to determine the path loss and shadowing between an ABS and a MS. This is because ABSs and MSs are deployed outdoors. The uplink data transmission rate, , is determined from the Truncated Shannon Bound (TSB) [132] as follows:

(

where α is the attenuation factor, SINRmin is the minimum SINR required for

reception, SINRmax is the SINR at which the maximum throughput, Rmax can be

achieved. The parameters of the BuNGee-specific TSB [80] are α = 0.65, SINRmin =

1.8dB, SINRmax= 21dB and Rmax=4.5bps/Hz.

Request UL resource

Request channel quality measurements Send channel quality

measurements

Inform MS about selected ABS and resource

assignment decision MS synchronizes with selected ABS Uplink Transmission to selected ABS MS ZBS ABSs in ZBS zone

Alert selected ABS and inform about resource assignment decision

Figure 4.3 Co-ordination Procedure for UL Data Transmission

4.2.2 Power Model

The power model proposed by Han et. al. [31], described earlier in Chapter 2, is used to evaluate the overall energy consumption of the ABSs in the network. The ABS energy consumption, EABS is given by the following equation:

ೃೣ,ೕ ೃಷ ೅ೣ,ೕ ೃಷ ೌ್ೞ ೎ (4.2)

where nabs is the number of ABS, Psleep is the power consumed by an ABS when in

sleep state. Pidle is the power consumed when an ABS is on but not receiving or

transmitting, instead it is waiting to serve users. This power is due to the non-radio-frequency components. PRx and PTx are the power consumed in the

receiving and transmitting states respectively. However, PTx is assumed to be zero

here, because the focus is on the uplink traffic and power consumption due to signalling is not considered. tsleep, tidle, tRx, and tTx are the total time the ABS spends

in sleep, idle, receiving and transmitting states respectively. However, the time taken to switch from one state to another is assumed to be negligible. µRF is the efficiency

of the power amplifier while µcrepresents the losses in the power supply and battery. nwakeup is the number of times the ABS switches from the sleep state to the idle state.

Finally, Ewakeup is the energy consumed in the process of waking up the ABS. The

values of the different parameters of the equation are given in Table 4.1.

Table 4.1 ABS Energy Consumption Parameters

Parameter Value

Power in receiving state 5W

Power in sleep state 250mW (assumed 5% of receiving state) Efficiency of RF 20%

Efficiency of supply loss 10% ABS max transmit power 5W Wakeup Energy 50J

The backhaul network tier comprising of HBSs, BHSSs and backhaul links are always active and provide several alternate routes for transferring MS data from/to the access network to/from the core network. Backhaul energy saving is beyond the scope of this work and it is not consider in this chapter and subsequent chapters. The ZBSs are typical macro BSs with maximum transmission power of 40W [131] that operate at 2.6 GHz, while the ABSs operate at a higher frequency of 3.5 GHz. However, the energy consumption of the always on ZBSs is assumed constant because the case of data service support by ABSs alone is considered. The focus is on the energy saving possible with dynamic control of ABS status and the

interference mitigation among the small cell ABS tier of the architecture. Specifically, the goal is to determine the energy saving that can be achieved by switching as many ABSs as possible into sleep state (or off) under a given QoS constraint. It is important to note that switching ABS off is used interchangeably with switching ABSs to sleep state. This should not be confused with turning off the ABS completely such that it consumes no power at all, rather the ABS operates in a sleep state and consumes non-negligible power.