Deploying HSUPA? For Receive Performance, Choose an RRH with Low Rx Noise Figure.

Tuomas Pöyry, TelASIC Communications, Inc.

Introduction

High Speed Uplink Packet Access (HSUPA) enables peak data rates of 5.76 Mbps from the User Equipment (UE) to the UMTS base station (Node-B). Delivering these high uplink speeds to meet user expectations means that performance of the Nobe-B receiver becomes very important.

The purpose of this paper is to describe technical parameters in the Remote Radio Head/Node-B combination that can boost the cell receive performance. This paper also seeks to establish the criteria for making performance comparisons between different RRH receivers for HSUPA deployment.

HSUPA Link Budget parameters

Table 1 presents the key parameters with typical values needed for 990 kbps HSUPA link budget computation. For more information about the parameters, please see Notes at the end of this paper.

Parameter	Value	Notes
Mobile EIRP	21 dBm	0 UE antenna gain, no cable losses or PA backoff assumed
Rx Antenna Gain	18 dBi	Typical UMTS antenna gain[2]
Propagation factors	26.2 dB	Log normal fading margin 10.3dB, Handover gain 4.1 dB, Building Penetration losses 20 dB (urban area)
Cable losses	3 dB
TND	-174 dBm/Hz	Thermal Noise Density
Receiver NF	5 dB	Typical Receiver noise figure [1,3]
TROT	7 dB	Target loading of 80% used.
Data rate	60 dB	10*log(990 kbps)
Eb/No	4.8 dB	Simulation results of HSUPA over 3GPP PA3 channel with target BLER of 1% and 2-way receive diversity [1]

Table 1. Link Budget Parameters with Traditional Node-B

There are three parameters in Table 1 that depend on Node-B deployment and the receiver chain performance. These parameters are

A. Cable losses

B. Receiver Noise Figure

C. Available diversity (contributes to reducing the required Eb/No)

A) Cable losses

Figures 1 and 2 illustrate the cable losses in the link budget parameter list. In Figure 1, the signal captured by the Node-B antenna is attenuated by 3dB due to RF cabling from the antenna to the Node-B. The same attenuation naturally occurs in downlink direction as well.

Figure 1: Traditional Node-B Deployment

Figure 2 Node-B  Deployment with the RRH

Figure 2 shows Node-B + RRH deployment, where the RRH is typically mounted to the mast directly adjacent to the antenna and the received signal data is transferred to the Node B baseband unit through optical fiber without any loss in power in uplink or downlink direction. The direct impact of this is the elimination of the 3 dB term from the link budget equation.

While a Tower Mounted Amplifier (TMA) can compensate for the 3 dB losses in uplink direction, it fails to do so for the downlink. Therefore an RRH is a clearly superior technology for enhancing the cell performance and achieving balanced uplink and downlink performance. 

B) Receiver Noise Figure

Receiver Noise Figure arises from the receiver design and the components of the Rx chain which perform receiver functions such as amplification, frequency translation, analog-to-digital conversion and digital down conversion to baseband. Using an RRH with an advanced receiver design it is possible to achieve a noise figure below 2 dB. This will slice more than 3 dB off the 5 dB standard equipment noise figure. If the effect of deploying such a low noise figure RRH is evaluated against standard Node-B deployment, using values in Table 1 and link budget equations described in Notes, then traditional Node-B deployment allows a maximum path loss of 107 dB whereas the RRH approach permits a path loss of over 113 dB. In a metropolitan area (using the Walfisch-Ikegami propagation model), the 107 dB path loss of the traditional Node-B allows a maximum distance of 300m between UE and Node-B. With the RRH, a 130m increase is achieved enabling a maximum distance of 430m from the UE to the Node-B antenna.

Table 2[3] shows the relation between link budget improvements and the number of cell sites needed. A 6 dB improvement in the link budget achieved from using advanced RRH technology translates to a staggering savings of 54% in site density. For deploying an HSUPA network economically while guaranteeing high uplink speed for as many users as possible, a very low noise figure RRH is clearly needed.  

Improvement in the link budget	Relative number of sites
0.0 dB = reference case	100%
1.0 dB	88%
2.0 dB	77%
3.0 dB	68%
4.0 dB	59%
5.0 dB	52%
6.0 dB	46%
10.0 dB	27%

Table 2: Reduction in the Node-B Site Density with Link Budget Improvements [3]

As UE manufacturers are striving to improve the battery life of mobile handsets, there is constant pressure to lower the transmit power of mobile handsets even more. This further underlines the importance of high performance RRH receive technology where every reduced noise decibel counts. In addition to increased coverage, users also experience higher data throughput and increased battery life. The benefits of advanced RRH receive technology therefore also improve the overall quality of service.

C) Receive diversity

Receive diversity from the RRH design point of view means more than one Rx chain. This allows the use of multiple antennas at the Node-B site that capture two or more ‘versions’ of the UE signal, one typically stronger than the other due to fading. If the RRH has two low noise Rx chains available, the Node-B baseband is able to choose the stronger signal and achieve a diversity gain. This improvement impacts the link budget in the form of a reduction in the required Eb/No and therefore improves the overall Node-B receive sensitivity. In baseband signal processing, many different methods exist today that can take advantage of two or more Rx chains (and antennas), so it is very important for the RRH to support receive diversity.

Conclusions

We have shown that Remote Radio Head technology improves the link budget and receiver sensitivity through the following three factors:

• Elimination of cable losses compared to traditional Node-B deployment
• Receiver Noise Figure
• Availability of receive diversity in the RRH

Therefore, when comparing the performance of two RRH’s receivers, noise figure and availability of receive diversity are the two parameters that should be considered.

TelASIC’s RRH technology features one of the lowest receiver noise figures in the industry (< 2 dB) and also provides receive diversity to boost sensitivity, allowing potential improvements of more than 6 dB to the link budget compared to traditional Node-B deployment.

Please contact info@telasic.com for further information on low noise figure RRH.

References

1.  White Paper “Aspect of HSUPA network planning”, April 6 2007, Qualcomm, http://www.qualcomm.com/esg/media/pdf/HSUPA_NetworkPlanning_Aspects.pdf

2. http://www.umtsworld.com/industry/antenna.htm

3. Holma H, Toskala A, WCDMA for UMTS: Radio Access for Third Generation Mobile Communication, Wiley, 2002

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Notes:

Receive link budget computation

The goal of the uplink link budget analysis is to answer the following question:

For a given user equipment maximum transmission power and a given minimum radio link quality on the uplink, what is the maximum allowed distance between the user equipment and the Node B receiver antenna?

The receive link budget is calculated as follows:

Maximum Loss = Mobile EIRP – Node-B Rx sensitivity + Rx Antenna Gain – Cable losses - Propagation factors

where:

Maximum Loss = the maximum allowable loss in the signal path between UE and Node-B.

Mobile EIRP = the UE Equivalent Isotropically Radiated Power, or in other words, the UE transmit power after cable losses, maximum power reduction (from PA back-off) and UE antenna gain have been taken into account.

Node-B Rx sensitivity = the minimum required received signal power at the Node-B receiver antenna for the radio signal to be correctly demodulated.

Rx Antenna Gain = the gain achieved in Node-B Rx antenna relative to an isotropic antenna.

Cable losses = the attenuation to the received signal due to RF cables between antenna and Node-B transceiver unit.

Propagation factors = the sum of building penetration loses, log normal fading margin, handover gain and body loses that the signal experiences between UE and Node-B.

Node-B Rx sensitivity

Sensitivity is calculated as follows:

Node-B Rx sensitivity = TND + Receiver NF + TROT + Data rate + Eb/No

where:

TND = Thermal Noise Density, which is a natural constant at a given temperature and is independent of a particular RRH.

Receiver NF = Receiver noise figure. This is the noise that the receiver chain electronics adds to the received signal. This is a key performance metric for the RRH.

TROT = Target Rise Over Thermal, which is the set maximum interference level from all of the UEs in the cell and interference from neighboring cells.

Data rate = Is the data rate of the HSUPA link.

Eb/No = Is the minimum Energy per Bit to Noise Power Density ratio required to achieve a certain BLER.

For HSUPA the required Eb/No   figure depends on the following factors[1]:

• Outer loop power control target BLER.
• Transport Time Interval (TTI)
• Transport Block Size (TBS)
• The number of Hybrid Automatic Re-reQuests (HARQ)
• The mobility channel
• Traffic to pilot power ratio
• Receiver Diversity
• Demodulator Design

These parameters are used in link level simulations to compute the required Eb/No figure.

Target Rise Over Thermal (TROT) depends on the loading of the cell, and can be described as the margin left in the sensitivity equation to account for the interference from other UEs and Cells.