College of Computer Science

Northeastern University

COM3525: Wireless Networks

Spring 2001

May 1, 2001

 

Homework 1 (Due May 14th, 2001).

 

Exercise 1 (FH-CDMA): 3pts

What is the difference between a Slow Frequency Hopping and a Fast Frequency Spread Spectrum system?

 

How does Frequency Hopping (FS) CDMA allow multiple users to use the same frequency bandwidth simultaneously? Give an example of three hopping sequences that allows FH-CDMA.

 

Exercise 2 (DS-CDMA): 3pts

Consider a Direct Sequence CDMA system where the codes of users are the 8x8 Hadamard matrix rows. Assume that user 1, 3, and 4 are transmitting simultaneously:

user 1 data sequence: 1 0 0 1

user 3 data sequence: 1 1 0 0

user 4 data sequence: 0 1 1 0

 

1. Show the data chip sequence (after spreading) for each user.

2. Show the signal at the receiver.

3. Assume that the receiver wants to extract information from user 3. Show how this is done.

 

Exercise 3 (DSSS): 3pts

Assume that we are using a spread spectrum system in a non CDMA environment (e.g., IEEE802.11 WLAN). What is the advantage of such scheme over narrowband communication? What properties should the spreading sequence have (i.e., PN sequence)?

 

Exercise 3 (IEEE802.11: Chapter 7): 5pts

IEEE802.11 is a WLAN protocol that allows the Internet Protocol stack to run with no changes (i.e., IP, TCP/UDP, etc.).

1. How can real-time services (or isochronous traffic) be provided in IEEE802.11? There is almost no implementation of IEEE802.11 that supports this functionality. Give at least two reasons why it is not supported.

 

2. Wireless devices are generally power constrained. What is the solution of IEEE802.11 to such constraint?

 

3. What basic mechanism is used by IEEE802.11 to prevent a collision between an acknowledgment and a new station attempting to start a new transmission (after finishing its back-off)?

 

Exercise 4 (Wireless MAC): 6pts

Assume that you are requested to design a Medium Access Control protocol for a set of low power wireless (mobile) nodes that have to communicate with a wireless access point. Your constraints are:

1.       You can only use FDMA and TDMA like schemes (CDMA is not considered for cost and complexity reasons).

2.       The nodes clocks can drift in time by some maximum known value.

3.       Nodes can have three modes: Transmit, Listening/Receiving, Sleep. While transmitting nodes consume twice in comparison to when they are receiving. In sleep mode nodes consume 1% power in comparison with transmit mode. Only the mobile nodes are power limited (the access point is not power limited).

4.       You have two types of traffic: emergency (delay constrained and requires high reliability: for example alarms), and best-effort traffic (with a time to live. Which means that if the frames are not transmitted within TTL they can be dropped). The emergency traffic is small in comparison with best-effort traffic. You may have different emergency-type connections with different delay bounds.

5.       You goal is to design a MAC that satisfies both constraints while maximizing battery life.

 

A. There is no single or simple solution. The goal of this exercise is to make you think about the main issues in WMAC design. Propose one or several schemes. Discuss the limitations of your proposed scheme(s). You can also make additional assumption when comparing schemes.

 

B. Assume now that you may have the same system running in multiple neighboring locations. Assume also that these systems implement the same protocols but cannot communicate or explicitly cooperate (e.g., each one running   in a home and belongs to a different person). What techniques can you use to achieve co-existence?

 

Exercise 5: 3pts

Today’s wireless data systems design is mainly focusing on the physical layer and MAC layer. Some research work is being done at higher layers. List three issues that involve higher network layers and that require special attention because the physical layer is wireless.