1. Wireless VoIP System Design Considerations
July 2005
doc.: IEEE /0557r0
Sept 2005
Wireless VoIP System Design Considerations
Date:
Authors:
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

2. July 2005
doc.: IEEE /0557r0
Sept 2005
Introduction
Wireless VoIP systems based on dot11 have been designed and deployed in the enterprise for the past few years
In this presentation we will share the design criteria used in these deployments
Techniques are described which enhance the user experience
We discuss requirements for next generation wireless handheld devices
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

3. July 2005
doc.: IEEE /0557r0
Sept 2005
Design Criteria
From a customer satisfaction perspective the key metrics and criteria in order of importance are:
Talk time, standby time, battery life
Call quality
Seamless handovers
Coverage, range
Capacity
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

4. Talk Time, Standby Time, Battery Life
July 2005
doc.: IEEE /0557r0
Sept 2005
Talk Time, Standby Time, Battery Life
Like other wireless telephony systems, the amount of talk time and standby time between a battery charges is critical for positive user experience
Many hours of talk time and many days of standby time allow the user to be un-tethered from the charger
U-APSD in 11e allows for substantial power saving in the handset in a distributed fashion making for easy deployment
At a high level, the handset initiates the VoIP packet exchange with the AP.
During a voice call, the handset comes out of sleep mode every speech frame interval and sends the speech frame to the AP
This action indicates to the AP that the handset is awake and responds by sending any buffered packets down to the handset
The handset waits to receive the packets and then goes back to sleep for the duration of the speech frame interval
Back of the envelope calculation:
Packet exchange takes 1-2 msec, so for a 20msec speech frame interval the wireless part of the handset is powered down over 90% of the time
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

5. July 2005
doc.: IEEE /0557r0
Sept 2005
Call quality
Minimum Received
Signal Level
Dropped calls or poor call quality will also frustrate the end user
To improve quality, design links with PER better than 10%
Rate shifting is also important for maintaining good call quality
Within the coverage region, higher data rates are used
The data rate starts shifting down to lower data rates in the transition region.
Data rate shifts down to the lowest rate and the call is maintained down to the minimum received signal level
Size the cells such that boundaries have margin in receive sensitivity so calls are not dropped at the edge
Diversity transmitters and/or receivers significantly increase robustness of the link AP AP AP AP AP AP AP
Transition Region
Minimum Coverage
Level
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

6. July 2005
doc.: IEEE /0557r0
Sept 2005
Seamless Handovers
Users will be frustrated by wireless VoIP service if calls are dropped when transitioning from one cell to another
A simple solution is to design adjacent cells with overlap in coverage
Calls must be maintained with good quality by current AP with plenty of time for transition to neighboring APs AP AP AP AP AP AP AP
Transition Region
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

7. July 2005
doc.: IEEE /0557r0
Sept 2005
Coverage / Range
Coverage is similar to call quality in that it is improved by similar techniques, such as diversity
The trend in carpeted enterprise is towards dense deployments for high network capacity, so range is not a primary concern
However, VoIP deployments in other environments with lesser network capacity requirements, such as in small office or home office, will be more focused on enhanced range and coverage with few AP’s
In these environments absolute range may be improved by diversity or coding techniques and longer preambles for improved acquisition and synchronization at lower SNR
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

8. July 2005
doc.: IEEE /0557r0
Sept 2005
Capacity
The end user only notices capacity limitations when not able to place or receive a call
Current 11ag based deployments can reasonably support 30 calls per channel
IP and dot11 MAC overhead play a large role in the achievable capacity
11n preambles will be necessarily longer than 11a preambles for training and signaling
However, the impact on call capacity will be small as seen on chart in next slide
Modest downstream aggregation in the MAC will more than make up for impact of longer preambles with short VoIP speech frames
The chart on the following slide shows the substantial improvement in capacity even by only aggregating two speech frames
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

9. VoIP Call Capacity Sept 2005
July 2005
doc.: IEEE /0557r0
Sept 2005
VoIP Call Capacity
Min data rate, max codec rate (solid red line)
Longest packet time on air
Increasing preamble from 20 to 40usec drops the capacity by 5%
Two destination downstream aggregation recovers capacity loss (red dot)
Max data rate, min codec rate (red line)
shortest packet time on air
Increasing preamble from 20 to 40usec drops the capacity by 17%
Two destination downstream aggregation recovers capacity loss improving capacity by 30% (blue dot)
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

10. Parameters for Capacity Calculation
July 2005
doc.: IEEE /0557r0
Sept 2005
Parameters for Capacity Calculation
Time on air computation based on
DIFS, backoff, packet, SIFS, ACK
Bi-directional packet flow
UDP and dot11 MAC overhead
Codec:
20msec period
64kbps and 4kbps
Aggregation
Two packets aggregated
Downstream aggregation only
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

11. Next Generation Wireless Handheld Devices
July 2005
doc.: IEEE /0557r0
Sept 2005
Next Generation Wireless Handheld Devices
To properly define the 11n standard for effective use with handheld devices, we need to understand the following:
Applications
Environments and usage scenarios
Device Requirements
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

12. July 2005
doc.: IEEE /0557r0
Sept 2005
Applications
It has been presented in the past that applications for handheld devices will include VoIP, internet anywhere, and video streaming.
In order to quantify the benefit of algorithms or compare competing algorithms, we must define the details of these applications specific to handheld devices.
VoIP
What codec (bit rate, period)?
Are there capacity requirements (number of simultaneous calls)?
What target PER?
What target minimum receive sensitivity?
What target data rates?
Internet anywhere
What are the throughput requirements?
Video streaming?
What target application bit rates?
Are there capacity requirements (number of simultaneous streams)?
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

13. Environments and Usage Scenarios
July 2005
doc.: IEEE /0557r0
Sept 2005
Environments and Usage Scenarios
Enterprise
Dual mode handset will allow for a handoff between a cellular system and the in-building WLAN VoIP network, providing coverage for cellular dead spots indoors.
The current push in enterprise is high density deployments with smaller cells.
The important design criteria is aggregation of multiple VoIP packets to multiple receivers for improved capacity.
Home or small office
Achieve full home coverage with one access point (or few access points in small office)
11n handsets provides VoIP telephony bridging on DSL or Cable throughout the home
Will these be dual-mode devices or a specialized version of a WLAN-based “cordless phone”?
Hotspot
Offload capacity from cellular system to WLAN
Fill in coverage holes in cellular system (e.g. train station)
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

14. Device Requirements Minimize power consumption Size limitation
July 2005
doc.: IEEE /0557r0
Sept 2005
Device Requirements
Minimize power consumption
Size limitation
Reduce cost
Multiple air-interfaces
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

15. Techniques to Address Requirements
July 2005
doc.: IEEE /0557r0
Sept 2005
Techniques to Address Requirements
Aggregation
Multi-receiver aggregation improves capacity for VoIP application.
Power save
To maintain long battery life of handheld devices, the power save algorithm must be integrated with multi-receiver aggregation.
Extended range
To extend the range for SOHO applications, we must consider that for indoor applications 10dB of gain is required to double the range.
Investigations should include
STBC, TxBF, advanced coding to extend data detection
Preamble design to extend acquisition and synchronization
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant

16. July 2005
doc.: IEEE /0557r0
Sept 2005
Summary
We described the criteria for designing wireless VoIP systems and equipment in the enterprise
The most important to the user is the talk time and standby time of the handheld device, no dropped calls, and good call quality
With respect to system performance, modest aggregation substantially improves call capacity making the impact of longer 11n preambles negligible
For next generation wireless devices, improvements should include aggregation integrated with power save for enterprise and extended range for SOHO applications
Eldad Perahia (Intel), Brett Douglas (Cisco Sytems)
Bruce Kraemer, Conexant