Link Budget Analysis for the LGCell System (DCS 1800)

Application Note AN0091Link Budget for WCDMA for Unison

Dr. Adam Schwartz

Chief Scientist

INTRODUCTIONThis note is intended to highlight some issues in dimensioning the coverage of an in-building wireless networking system that is providing WCDMA coverage. There are several factors complicating the WCDMA link budget beyond the issues associated with other standards such as GSM. Conversely, there are similarities to cdmaOne as well as differences. The most important difference concerns the issue of the open-loop power control on the uplink and multiple services and data rates provided by WCDMA.

The following description provides details for the downlink and uplink link budgets provided on page 5 of this document. In this note, we use the word carrier to refer to the RF WCDMA signal and the word channel to refer to an individual traffic or control path within that WCDMA signal. The different channels are specified by their OVSF (orthogonal variable spreading factor) codes.

ACRONYMS USED IN THIS NOTECPICH

Common Pilot Channel

DPCH

Dedicated Physical Channel

DPCCH

Dedicated Physical Control Channel

DPDCH

Dedicated Physical Data Channel

Eb/N0

Energy per bit above noise (and interference)

Ec/Io

Energy per chip above interference

OVSF

Orthogonal Variable Spreading Factor

PCCPCHPrimary Common Control Physical Channel

PICH

Paging Indicator Channel

SCH

Synchronization Channel

SIR

Signal-to-Interference Ratio

RAU

Remote Antenna Unit

UE

User Equipment

GENERAL ISSUESCoverage LimitationIn UMTS networks, the coverage is typically limited by the uplink. This is also generally true for Unison unless several carriers are being transmitted on the downlink. In that case, the downlink power per carrier is reduced and the coverage will be downlink limited.On the other hand, capacity for 3G systems is limited by the downlink. The reason for this is that data-driven applications usually require much higher data rates for downloads. Because the spreading factor is lower for higher data rates, the required signal-to-interference ratio (SIR) is larger. Increasing power would allow for increased coverage but would result in more downlink interference, hence limiting capacity.

Difference with Regard to cdmaOneWhile many of the link budget issues are common between 2G CDMA and WCDMA technologies, there are some notable differences. The differences relevant to our link budget include:1. For WCDMA open-loop power control is not operated in tandem with the close-loop power control law. This is due to many factors including the higher close-loop power control rate of 1.5 Khz for WCDMA (versus 800 Hz for cdmaOne).

2. WCDMA offers different services and data rates over the same air link. Even an individual user can receive or transmit multiple channels at different rates, they can be packet-switched or circuit-switched, and in real-time or non-real-time. This means that (a) power is divided in a more complex manner between different channels, (b) the required Eb/N0 is different for different services, and (c) process gain varies with data rate.

3. WCDMA employs fast power control on the downlink.

FACTORS ON THE DOWNLINKDownlink Power per CarrierThe most important factor on the downlink is the maximum transmit power. The maximum transmitted power per WCDMA carrier is given in the Unison datasheet and the I&R manual. Table 1 is a preliminary list of the power per carrier vs. number of carriers. These values are approximately correct and have not yet been verified.

Number of Carriers

Power per Carrier

1

15 dBm

2

11 dBm

3

8 dBm

4

6 dBm

Table 1: Maximum power per WCDMA carrier vs. number of carriers.

Because there are many channels within the WCDMA carrier, each channel has a different spreading factor, and there are common as well as dedicated channels, it is not obvious how much power per channel should be used to determine the downlink link budget. Typically, common channels consume more than 20% of the downlink power. This includes 10% for the PCCPCH and SCH, 10% for the CPICH and then a bit more for the PICH. Dedicated physical channels (DPCH) use the rest. The amount of power allocated to any DPCH is determined by the number of other DPCHs, the path loss between the mobile and the RAU, interference to that user from other WCDMA signals, and the channels spreading factor. According to Test Model 1 (section 6.1.1 in [1]), which gives a representative spread of different channel uses, the individual channel power level varies relative to the total carrier power. The results are summarized in Table 2.

Number of Channels

Maximum Power Level

Minimum Power Level

16

9 dB

19 dB

32

12 dB

24 dB

64

16 dB

24 dB

Table 2: Representative spread of power levels relative to total carrier power for different user channels.

Since the base station will have to allocate more power to a given user the further away it is from an RAU (in terms of path loss), we will use the maximum power level to determine the coverage radius. Thus, if 32 simultaneous users of various data rates are provisioned for, then the maximum power per channel in the link budget will be 12dB less than the total carrier power.

FACTORS ON THE UPLINKAllocation of power amongst uplink channels

Unlike cdmaOne, WCDMA mobiles always transmit at least a control channel (DPCCH) and one or more traffic channels (DPDCH). Thus, not all of the transmit power is allocated to the traffic channel. The relative amount of power given to a traffic channel depends on the number of channels being transmitted and the channels spreading factor. Assuming that the typical usage will be one traffic channel, the following table shows the amount of power allocated to the DPCCH relative to the DPDCH (see Table 11.2, p. 240 in [2]):

Bit Rate

Power of DPDCH minus Power of DPCCH

DPCCH Power Overhead

12.2 kbps speech

3.0 dB

1.76 dB

144 kbps data

6.0 dB

0.97 dB

384 kbps data

9.0 dB

0.52 dB

1024 kbps data

12.0 dB

0.27 dB

Table 3: Power allocated to uplink control channel

Loading

As more users access the system, the noise floor on the uplink rises. This is because each user, while received at a level beneath thermal noise (or, in the case of an active system, beneath the noise level of the active system) is seen as noise to other users. Thus, in order to maintain a sufficient Eb/N0 level, the mobiles must transmit at higher power when the number of users increases. The noise rises geometrically with the number of users. If is the load factor as a percentage of the pole capacity (the theoretical maximum number of users before the noise rise goes to infinity), then the noise rise (also referred to as loading, or interference margin) is given by

(1)noise rise (dB) = 10 log10 ( 1 / (1- ) ) .

Normally, a CDMA network will not be run above 75% loading (6 dB noise rise). Because coverage is uplink-limited while capacity is downlink limited, and because 75% loading would result in capacity limitation on the uplink, it is reasonable to assume that this level will not be reached. Hence, in the link budget we assume = 0.5 (3 dB noise rise).

Eb/N0 RequirementAnother important factor for the downlink and uplink is the Eb/N0. The Eb/N0 is the amount of energy per bit above the noise and interference after de-spreading that is required for satisfactory performance. This number is affected by the availability of diversity and the characteristics of the multipath propagation environment. Diversity can be provided by multipath propagation if the delay spread is greater than one chip duration (about 260nS). However, this is unlikely to be the case inside of buildings (except very large venues) unless delay between different RAUs is intentionally designed in. Therefore, the main effects of multipath are additional fading to the link budget and reduced orthogonality of the channelization codes. Because of this last fact, we will assume that the required Eb/N0 is 8.0 dB (see table Table 11.12, p. 253 in [2]) for speech. This tables assume no receive diversity. However, the ITU Pedestrian A model is a two-tap model with a small weight on the second tap. The in-building environment contains a lot more multipath, and the multipath delays are shorter than a chip duration. Therefore, a larger number for Eb/N0 should probably be used.Note that for higher data rates the required Eb/N0 decreases. There are more bits per second so the required transmit power goes up (power is energy integrated over time). For instance, the difference in spreading factor for 12.2 kbps data versus 1024 kbps results in a process gain difference of

10log10(12.2kbps/1024kbps) = -19.24 dB

So if the Eb/N0 requirement is 2 dB lower than the received power at 1024 kbps, then the received power for 1024 kbps would still have to be about 17 dB higher than the received power for 12.2 kbps.

Link BudgetBelow is a sample link budget for WCDMA in-building applications. The actual Excel file is available from LGC Wireless. Many of the terms in this link budget are defined in [3], section 7.6 as well as [2], chapter 11.

Link Budget for WCDMA MicroCell

PRELIMINARY

chip rate (MCPS)3.84Spreading Factor for given downlink traffic channel

256

For LGC Internal use ONLYUser should fill in BOLD Green cellsSpreading Factor for given uplink traffic channel

128

DownlinkUplinkBasestationBasestationa1.W-CDMA carrier power (dBm)40.0a.BTS Noise figure (dB)4.0a2.Max power per OVSF code channel (dBm)24.0b.Loss between BTS and Unison (dB)-15.0b.Loss between BTS and Unison (dB)-25.0Unisonc1.Power per channel into Main Hub (dBm) = a2+b0.0c.Unison gain (dB)0.0c2.Power per W-CDMA carrier into Main Hub (dBm)=a1+b15.0d.Unison noise figure (dB)22.0Unisone.System Noise Figure (dB)23.7d.Unison gain (dB)0.0f.Thermal Noise (dBm/Hz)-174.0e.RAU antenna gain (dBi)3.0g.Noise Rise 50% loading (dB)3.0f1.Radiated power per channel (dBm) =c1+d+e2.0h.Receiver interference density (dBm/Hz) = e+f+g-147.2f2.Radiated power per carrier (dBm) = c2+d+e18.0i.Information rate (dB/Hz)44.8Mobile Phonej.Required Eb/(No+Io)8.0e.Noise Figure (dB)7.0k.Multi-RAU shadowing Gain (dB)0.0f.Thermal Noise (dBm/Hz)-174.0l.RAU antenna gain (dBi)3.0g.Receiver interference density (dBm/Hz) = e+f-167.0m.Minimum Received Signal (dBm) = h+i+j-k-l-97.5h.Information rate (dB/Hz)41.8Mobile Phonei.Required Eb/(No+Io)8.0n.Mobile Transmit Power (dBm)24.0j.Receive Sensitivity (dBm) = g+h+i-117.2o.relative power of DPCCH (dB)-3.0k.Multi-RAU shadowing Gain (dB)0.0p.DPCCH overhead1.8l.Multipath Fade Margin (dB)6.0q.Multipath Fade Margin (dB)6.0m.Log-Normal Shadow Margin (dB)10.0r.Log-Normal Shadow Margin (dB)10.0n.Body loss (dB)3.0s.Body loss (dB)3.0o.Minimum Received signal (dBm) = j-k+l+m+n-98.2t.Effective transmitted power (dBm) = n-p-q-r-s3.2Path LossPath LossMax. path loss (dB) = f1-o100.2Max. path loss (dB) = t-m100.7

Spreading Factor

At the top of the link budget is the spreading factor for uplink and downlink. The spreading factor, SF, can be different for any data channel (including multiple channels for a single user). For the downlink the spreading factor can be any multiple of 2 between 4 and 512 while on the uplink the spreading factor can be any multiple of 2 between 4 and 256. A spreading factor of 256 corresponds to a channel symbol rate of 15 ksps = 3.84mcps/256. This corresponds to 15 kbps for the uplink and 30 kbps for the downlink (see tables 4 and 5 below).

Attenuator Selection

The attenuator on the downlink should be chosen so that the Unison system will not be over-driven at the maximum number of WCDMA carriers that will be supported at that site in the future. On the uplink, the attenuator should be chosen so that the uplink coverage area is similar to the downlink coverage area for similar services. By this we mean, for instance, that voice service on the downlink (SF=256) has the same coverage as voice service on the uplink (SF=128). The Unison downlink output power and IP3 and uplink NF typically are well balanced when the uplink and downlink gains are the same. This is not a hard rule, and there is quite a bit of flexibility in the choice of attenuators and Unison gain settings.

Other Items

1. On the downlink, the power per WCDMA carrier (a1) is determined by the maximum power per carrier tables for Unison (see Table 1). The power allocated to any user is a fraction of the total power in the carrier. This is determined from table 2 assuming 64 channels and reflected in the second line (a2). It is this power that determines the maximum path loss available to the user. Please note that (a1) represents the fully loaded power for the WCDMA carrier. The power will be lower if the carrier is not carrying a full traffic load (as much as 10 dB lower than if there is no traffic).

2. Theres a line for multi-RAU shadowing gain (k). This reflects the fact that if there is more than one RAU in a coverage region then the shadow margin can be effectively reduced. This is because the area that is shadowed from one RAU may not be shadowed from the other RAU. A gain of 3 dB is a reasonable expectation in such cases. Note that this gain is also achieved if the output of a single RAU is split and use to drive coax connecting to two separate antennas.

3. On the uplink, the maximum mobile transmit power is assumed to be 24 dBm (21 dBm for some phones) and the Unison noise figure of 22dB is for a full 1-4-32 system.

USER DATA RATESThe actual user data rate achieved on a given channel depends on the spreading factor, the type of convolutional coding used, the number of CRC bits added to each data block, the amount of puncturing or padding bits and, on the downlink, the number of pilot and control bits added in each block.

Tables 4 and 5 show the data rates for the downlink and uplink. The user data rate is the DPDCH data rate after decoding and removing CRC and tail bits. On the downlink, the data is transmitted using QPSK (quadrature phase shift keying) modulation. Thus, every symbol contains two bits. Also, on the downlink the DPCCH and DPDCH are multiplexed together. So the actual bit rate of the DPDCH is less than the channel bit rate. On the uplink, the data is transmitted using HPSK (hybrid phase shift keying) modulation with the DPCCH transmitted on the Q-channel and the DPDCH on the I-channel (assuming that there is only one data channel). Thus, there is one DPDCH bit transmitted per symbol. Note that the overhead of the CRC and tail bits becomes insignificant at higher data rates. .

DPDCH SpreadingFactor

Channel Symbol Rate(kbps)

Channel Bit Rate(kbps)

DPDCH Channel Bit Rate Range (kbps)

Maximum User Data Rate with-rate Coding (approx.)

512

7.5

15

36

13 kbps

256

15

30

1224

612 kbps

128

30

60

4251

2024 kbps

64

60

120

90

45 kbps

32

120

240

210

105 kbps

16

240

480

432

215 kbps

8

480

960

912

456 kbps

4

960

1920

1872

936 kbps

4, with 3 parallel codes

2880

5760

5616

2.3 Mbps

Table 4: Approximate downlink data transmission rates for different spreading factors. This does not include CRC overhead, which slightly reduces the user data rates. Note, 1/3-rate turbo codes are also specified for high data rates.

DPDCH SpreadingFactor

DPDCH Channel Bit Rate (kbps)

Maximum Data Rate with -rate Coding (approx.)

User Data Rate (approx.)

256

15

7.5 kbps

128

30

15 kbps

12.2 kbps

64

60

30 kbps

32

120

60 kbps

16

240

120 kbps

8

480

240 kbps

144 kbps

4

960

480 kbps

384 kbps

4, with 6 parallel codes

5740

2.3 Mbps

Table 5: Approximate uplink data transmission rates for different spreading factors. This does not include CRC overhead, which slightly reduces the user data rates. Note, 1/3-rate turbo codes are also specified for high data rates.

PILOT POLLUTION/ESTABLISHING PILOT DOMINANCEThe first step required for the UE to demodulate the WCDMA signal transmitted from the node-B equipment is to acquire and dde-spread the CPICH. Because WCDMA operates with frequency re-use of one it is common for the CPICH from more than one base station to be received at fairly high levels. This situation is the trigger mechanism for soft-handoff. However, in some cases, too many pilot channels are received at relatively high levels and the mobile phone cannot find a dominant CPICH to lock onto. This situation is referred to as pilot pollution

Pilot pollution can occur in high-rise buildings that have line-of-sight to several macro base stations. Pilot pollution may prevent WCDMA phones from accessing the network. To combat this, a dominant pilot channel must be established inside the building using an in-building coverage solution such as Unison. Establishing pilot dominance with the in-building system is important, in general, to limit soft-handoff to the outdoor network.

When designing an in-building coverage solution that combats pilot pollution, the relevant figure of merit is no longer the Eb/N0 level. Rather, the design must provide a sufficient received power level on the downlink so that the received CPICH from the in-building system dominates the other pilots received from the macro network. The relevant figure of merit for measuring the CPICH level is Ec/Io or energy per chip above interference. This is because the pilot signal is measured by the UE before the signal is de-spread. Thus, only the chip energy is available, not the bit energy. Also, interference is used explicitly here because in the case of pilot pollution the interference level will be substantially higher than thermal noise. So thermal noise is not a consideration.

To establish pilot dominance, the Ec/Io for the in-building CPICH should be around 16 dB below the measured interference level inside the building. Since the CPICH occupies about 10% of the power of a fully loaded WCDMA the CPICH level will be about 10 dB lower than the total received power.

Mathematically, the Ec/Io is given by

(2)Ec/Io = power per pilot chip total interference level

where Pcarrier is the power per WCDMA carrier from the in-building system at the point where Ec/Io is being measured (as given by f2 k l m in the link budget) and I0 is the interference level from the outdoor network (i.e., the power level measured with the in-building system turned off). In the extreme cases, equation (2) reduces down to what one would expect. If Pcarrier >> I0 then (2) reduces to Ec/Io -10dB. If, on the other hand, Pcarrier