# Presentation: 112222

## INNOVATIONS IN OFDM AND DSSS FOR VERY HIGH-PERFORMANCE, VERY HIGH BIT RATE WLAN/802.11 

* **RAWCom Laboratory*
*Department of ECE*
*  **Colorado State University*
*  **Fort Collins, CO - 80523-1373*

***INNOVATIONS IN OFDM AND DSSS FOR VERY HIGH-PERFORMANCE, VERY HIGH BIT RATE WLAN/802.11***** **

## Introduction

- WLAN’s Motivation
- Part I.
- IEEE 802.11b PHY and DSSS
- Part II.
- IEEE 802.11a PHY and OFDM

## WLAN’s Motivation...

- Cost Effectiveness
- Major portion of cost in LANs is interconnecting
-     end users
- Growth and Reconfiguring Flexibility
- Recabling cost and planning for additional nodes is minimized
- Upgrades to the network become easier
- Portability
- Computer and Printers no longer need designated network connections
- Need
- Popularity of portable notebook computers

*Then:*

*Now:*

- Wireless connectivity to the internet
- Voice over IP
- Wireless home devices

## Part I:  Introduction to IEEE 802.11b

- Quick Hits on DSSS
- IEEE 802.11 and its use of DSSS
- Quick Taste of the CI chip shaping benefits
- What is CI-DSSS with respect to 802.11 WLAN
- What does CI-DSSS mean at the receiver
- Simulation
- Performance Benefits and Analysis
- Conclusions

## Quick Hits on DSSS

*Spreading Sequence*

## What happens after the transmission through a wireless channel?               Multipath Fading

*What happens after the transmission through a wireless channel?               *Multipath Fading

*Path 0:*

*Path 1:*

*Path 2:*

## RAKE Receiver

*How to deal with *Multipath Fading?

## IEEE 802.11b DSSS Wireless LAN

- A system supporting 1, 2, 5.5 and 11 Mbps data rates
*1 Mbps Basic Access Rate*:  11-Chip Barker Sequence
*2 Mbps Enhanced Access Rate*:  11-Chip Barker Sequence
- 5.5 *Mbps*:  8-Chip CCK
- 11 *Mbps*:   8-Chip CCK
- Utilizes 11 Channels within the 2.400-2.4835 GHz ISM band
- i.e., (Channel 1: 2.412 GHz) to (Channel 11: 2.462 GHz)
- Supports 3-4 coexisting channels with little interference at 30 MHz separation
- 22 MHz bandwidth (Corresponding to 2 x Chip Rate)

## What is the CI-DSSS Modification?

**What is the CI-DSSS Modification?**

- (sinc)

- (raised cosine)

***f***

***f***

- (CI)

*Novel chip shaping filter: exploit frequency diversity*

## What does this mean at the receiver?

- CI-DSSS Receiver must
- Receive each chip and remove the phase offsets
- Recombine the signal while effectively dealing with inter-chip interference
- Combine the chips and make final decision

- Generalized CI-DSSS Receiver

- Chip Receiver

## But what about in IEEE 802.11?

- Transmitter:  CI Chip Shaping Filter prior to modulation and 2 sets of orthogonal frequency carriers

- Receiver:  Replace equalizer or sophisticated RAKE with CI-DSSS Receiver

## Pseudo-Orthogonal CI Chips

*solid line: first set of N chips*

*dashed line: second set of N chips*

*Support 2N chips within one symbol duration?*

## Pseudo-Orthogonal Chips

*Orthogonal CI Chip Shape Set 1*

*Orthogonal CI Chip Shape Set 2*

*Pseudo-orthogonal*

*Interference between two sets*

*Orthogonal Carrier Set 1*

*No Interference between two carrier sets*

*Orthogonal Carrier Set 2*

**Bandwidth Efficient Orthogonal Carriers**

## What was the system for simulation?

**Traditional DSSS**
- DBPSK
- Used 6 finger RAKE receiver
**CI-DSSS**
- CI chip shaping filter with DBPSK (Therefore allowing use of orthogonal carriers and giving x4 data rate)
- Frequency Combining scheme:  Mimimum Mean Squared Error Combining (MMSEC)

## What was the channel model employed?

- UMTS Indoor Channel Models A and B (6 Path Models)
- Channel Model A with 35ns delay spread
- Channel Model B with 100ns delay spread
- Demonstrating frequency selectivity over the entire bandwidth (BW)
- Demonstrating flat fading over each of the *N* carriers

## Performance

- x1

- x2

- x4

## Performance

- x1

- x2

- x4

## Analysis

**Analysis**

*4x throughput by*
*Double bandwidth efficiency*
*Double chips/symbol via PO positioning*
*Novel Chip Shaping is introduced to exploit frequency diversity instead of path diversity.*
*Better performance is achieved by exploiting frequency diversity, even with 4x throughput.*
*Complicated RAKE receiver structure is avoided.*
*The novel chip shaping can be implemented via FFT algorithm. *

## PART II

- CI Enhanced IEEE 802.11a

## Part II:  Introduction to IEEE 802.11a

- IEEE 802.11a PHY and its use of OFDM
- Quick Hits on OFDM
- Carrier Interferometry’s (CI) Introduction
- What is CI/OFDM (and therefore CI/WLAN)
- What does CI mean at the receiver
- Simulation
- Performance Benefits and Analysis
- Conclusions

*(Extendible to 802.11g)*

## Quick Hits on IEEE 802.11a?

**Quick Hits on IEEE 802.11a?**

- A high speed system operating in 20 MHz bands in the 5 GHz region
- OFDM system modulation scheme
- 52 Subcarriers (48 information bearing and 4 pilot for coherent detection)
- Subcarrier spacing:  312.5 kHz
- System bandwidth:  16.25 MHz (Occupied BW = 16.6 MHz)
- Subcarrier modulations:  BPSK, QPSK, 16-QAM, and 64-QAM
- Rate 1/2, constraint length 7 convolutional coding (generator polynomials 133, 171)
- Rate 2/3 and 3/4 achieved through puncturing the rate 1/2 mother code
- Supports 6, 9, 12, 18, 24, 36, 48 and 54 Mbps data rates
- i.e.,  BPSK subcarrier modulation with rate 1/2 CC = 6 Mbps data rate
- Channel coded bits are interleaved to benefit from frequency diversity in the channel

- 	         Binary Input                Coder                 Interleaver             Mapper              Modulator            Channel

## Quick hits on OFDM (Transmitter)

- (3) Each data bit gets its own carrier,
-  then all are added together and sent out
- over the carrier frequency

**Quick hits on OFDM (Transmitter)**

## Quick hits on OFDM

- 		         (each with a different data bit)             the bit on each carrier

**Quick hits on OFDM**

## ...

*In CI/OFDM, serial to parallel convert (S/P) your bits then put **all** bits on **all** carriers*
*	**	**	**	**	**	**               **at the same time.*

*puts each bit on **all** the carriers*

**Serial-to-Parallel**

- Input Data

**...**

- Input Data

**What is CI/OFDM?**

## ...

**Serial-to-Parallel**

- Input Data

**...**

**...**

**CI/OFDM Continued**

## Introduction of CI to 802.11a

***f***

*+*
*+*

*=*

***t***

*... *

## 		          Channel	

*	**	**          *_*Channel*_*	*

*1. Frequency selective*
*over the entire bandwidth*
*2. Flat over each of *
*the individual carriers*
*3. Each carrier experiences *
*a flat fade that is different *
*than the flat fade of the other carriers*

**   ****OFDM**

- Bit k

- Bit k

*1. Frequency selective*
*over the entire bandwidth*
*2. Flat over each of *
*the individual carriers*
*3. Each bit experiences freq.*
*DIVERSITY** benefit.*

**  ****CI/OFDM**

_*Send*_

_*Receive*_

## CI/OFDM Receiver

**CI/OFDM Receiver**

*In OFDM receivers,*
*we separate the*
*received signal into*
*carriers and then make a *
*decision on each carrier. *

*In CI/OFDM receivers,*
*we separate the received*
*signal into carriers, then*
*combine across carriers*
*to create a frequency *
*diversity benefit for each bit.*

## But what about in IEEE 802.11a?

**But what about in IEEE 802.11a?**

*puts each bit on *
*all** the carriers*

_*Transmitter*_

## What was the system for simulation?

*N = 48* Carriers
- (Rate 1⁄2, Constraint Length 3 Convolutional Coder)
- Carrier Modulation: Binary Phase Shift Keying (BPSK)
- CI/OFDM utilized a Minimum Mean Squared Error Combining (MMSEC) architecture at the receiver
- Offers good performance as shown in other multi-carrier system literature
- Jointly minimizes the inter-bit interference and noise
- Allows exploitation of frequency diversity in the channel

## What was the channel model employed?

- UMTS Indoor Channel Models A and B
- Demonstrating frequency selectivity over the entire bandwidth (BW)
- Demonstrating flat fading over each of the *N* carriers

## Performance

-    3 dB Gain at BER 10-3 and 4 dB gain at BER 10-4 for Tm=100ns
-    1 dB Gain at BER 10-3 and 2 dB gain at BER 10-4 for Tm=35ns

## Performance

**Performance**

*With NO CODING*

-    3 dB loss at BER 10-3 and even less at lower BERs at Tm=100ns

## CI Enhanced 802.11a Conclusions

- CI/OFDM has been introduced as an alternative for the current method of OFDM in IEEE 802.11a WLAN
-  Offers 3 dB gain over traditional WLAN at BER of 10-3
- Benefits:
- Offers greater range capabilities
- Offers flexibility with the regard to coding
- Minimum architectural complexity gains