# Presentation: 276918

## Stretchable Architectures for Next Generation Cellular Networks

- Presented By
- Shashidhar Lakkavalli, Ansuya Negi and Dr. Suresh Singh
- Portland State University

## Relaying

**Notes:**
Relaying is a technique where instead of connecting to the destination in one hop, relays are used to forward the data to the destination. So, here, if the source-destination connection is not possible due to excessive transmission power caused by high pathloss, obstructions, fading, etc., which occurs in wireless connections, we can still establish the connection using relays.

## Path loss reduction with Relaying

- L(in dB) = 128.1 + 37.6log(d)
- ‘d’ = single hop distance in km
- L(Cumulative in Watts) = hops * L(in Watts)

**Notes:**
The advantage of relaying can be best explained from the relationship between the cumulative pathloss and the number of hops.

This figure shows the cumulative path loss incurred, when we vary the number of hops.
Pathloss is shown in dB on y-axis, with the number of hops in the x-axis.

We see here that if we break the single hop connection between transmitter and receiver, the cumulative pathloss decreases exponentially. 

The pathloss formula specified here was taken from UMTS specifications for an urban area.

## Relaying

- Multi-hop connection between source and destination
-  Advantages
- Longer battery life for the source mobile
- Lower Node and System power
- Increased capacity of cell
- Higher system throughput (Energy per bit reduces)
- Improves coverage in dead spots
- path diversity
- Propagation conditions can vary hop by hop.

**Notes:**
So, what are the effects of reduced pathloss for the connection. 
If the path loss reduces, then the required transmit power for source mobile decreases, which means that the battery life of the mobile can be increased. 
Now, when the mobile lowers its transmit power at the node, which means that interference it causes at BS receivers is reduced.
With reduction in inter-cell interference, the capacity is increased in a cell
As distances become large, maintaining achieving the required throughput means higher transmit powers. But higher transmit powers mean more interference. One way to reduce this is to use multi-hop relaying to reduce transmit powers and interference.
Provides coverage to MTs, which are in shadow regions
Can adjust to the different propagation conditions in each hop by using different coding, spreading and modulation techniques
Suitable in urban settings where shadowing is peculiar. So, relays can be used to relay around the buildings.

## 3G Issues

- Battery power utilized for benefit of others
- Mobile-to-Mobile communication
- Interference at relay’s receiver
- Absence of diversity gain at the relay
- Security
- Handoffs between mobiles in multi-hop
- Power Control between relay and MT
- QOS & Overhead
- 3G standards shelves multi-hop protocol ODMA due to excessive overhead from signaling.

**Notes:**

Battery power is utilized for the benefit of others is a problem, unless the relay owner gets some credit either financially or in terms of extra free minutes.
With relaying, the mobile should be able to communicate with other mobiles, which is not currently supported
Now, not only have we got to be concerned with interference at BS receiver in the uplink, we should also consider interference at the receiver of the relay because of mobiles at its vicinity.
Then, there is absence of diversity gain at the relay. BS employs Multi-user detection, Joint detection, smart antennas, which improves the detection of the signals at the receiver. But, these techniques are expensive and cannot be installed in the relays. 
The relay is buffering and forwarding data other user’s data – security hazard
3G supports soft handoff for real time traffic, where the MTs are receiving signals from both source and target BSs during handoff, to ensure continuity of data transfer. So, need to support soft handoffs between MT and relays. 
Power Control between relays and MTs required to ensure that the MT is transmitting just enough power – as power savings at the MT is crucial.
Overhead: Additional resources – code assignment, signaling, very complex in maintaining billing information. It is good if BS is having the control of assigning the relays.
With multihop, the delay becomes variable, and therefore for real time applications QOS cannot be guaranteed

## Our Proposal – Stretched Connection

- Stretched Connection
- AIM: To increase battery life of a Mobile Terminal using relaying in 3G systems.
- Scenario: 2 hop relaying
- Suitable for real time applications with less jitter.
- Requires a “simple handoff/routing algorithm”
- Appreciable “energy savings”.
- Relay: A device with abundant power reserve – cars, full charged MTs
- Relay is called “intermediary”
- Relayed connection is a called “stretched connection”

## Stretched Connection

**Notes:**
A simple scenario of a stretched connection is given in this slide. The user does not have a LOS connection, which has a better performance than a NLOS connection. But, a car ‘a’ has LOS connection to BS, and he has an LOS connection to ‘a’. The combined power consumption of this stretched connection is much less than the direct connection, and so he starts off with a stretched connection.

Then after some time, it is ‘d’, who has a better connection because connection through ‘a’ is no longer efficient. So, a handoff has occurred from a to d.

We call the link between the mobile and intermediary as Lower Arm & the link between intermediary and BS as Upper Arm

## Soft Handoff

- BS1

- BS2

- M - Mobile
- A - Active Set
- N - Neighbor Set

**Notes:**
This slide briefly describes how soft handoff occurs in cdma cellular systems.
MTs support soft handoff mechanism when it moves from one cell to another. For doing that, they maintain active, neighbor and candidate list of BS. Based on the strength of the pilots from the BS, it puts them in one of these lists. In active list it contains the identity of the BS whose pilot is the strongest and therefore the one with which it is currently communicating with. The neighbor list contains identities of the neighboring BSs. 
So, here we have a Mobile M, which is in BS 1 currently, and the pilot channel from BS 1 is the strongest, and therefore  has it in its active list. BS 2 and 3 are neighboring BSs and has it in the neighboring list. As it moves towards the handoff region, it moves BS 2 into its active list because the pilot of BS2 is above the threshold required to be in the active list. After the handoff, it puts BS 1 in the neighbor list and has only BS 2 in the active list.
So here the handoff is initiated by the MT and assisted by the BS.
For a stretched connection, we have handoffs between two intermediaries. 
Implementing the same mechanism in the MT for handoff between 2 intermediaries incurs extra overhead. 
Also, the intermediaries can be mobile, so the number of signaling information that has to take place between MT and the intermediaries will become higher.
Also, the intermediaries will now have to transmit unique pilot channels similar to what the BS does, so that the MTs can keep track of them, which means that the relays will have to be actively transmitting.
These are some of the issues that prop up when we try to implement intermediary to intermediary handoff in the MT.

## Intermediary Initiated Soft Handoff (1)

- Handoff is initiated by intermediaries and assisted by BS.
- BS assist candidate intermediaries by passing location of MT, system parameters of the mobile
- Relays listen promiscuously to MT’s transmissions, assuming some changes to uplink ( ODMA specifications)
- Relays maintain 3 sets - active, neighbor and candidate sets of MTs in its vicinity: similar to Soft Handoff mechanism.
- BS selects the best intermediary.
- Note: For multihop scenario, BS cannot choose all the intermediaries – not scalable.
- Use of adhoc networking protocol, with metric being SIR at the intermediary.

**Notes:**
We instead propose a intermediary initiated soft handoff, which is assisted by the BS.

The BS will assist the candidate intermediaries by passing them locations of MT in their vicinity, spreading codes and other information required for detecting signals from the MTs.

3G standards say that there is a pilot-subchannel in the reverse link traffic channel. So, the intermediaries will now have to listen to these pilot channels to determine the strength of the MT signals.

The relays maintain active list, neighbor list and candidate list similar to what the MTs maintain for BS to BS handoff. But here, these lists contain identities of MTs rather than BSs. Using the similar mechanism adopted by the MTs during soft handoff which was just explained to choose the BS, the intermediaries can choose MTs. 

But, there can be many intermediaries in the vicinity of the MT. So, the BS would send the identity of MT to more than one intermediary in the MTs vicinity. The intermediaries repeatedly send the active lists to the BS, which takes into consideration the relative gains of using one intermediary over the other in terms of longevity of the stretched connection, battery power gain, QOS issues etc.,

The BS will then select one of the candidate intermediary to relay the MT’s signal.

This is feasible because it is just 2 hop. We do not require to implement complex routing algorithms to achieve this.

So, here we have both the BS and the intermediaries which are assumed to be abundant in battery power to do the handoff. The MTs, need to know the identity of the intermediaries. The modification in the MT to incorporate intermediary-intermediary handoff would be minimal, because both intermediaries will be using the same parameters like spreading codes, BS code etc., unlike in soft handoff, and so it can assume that signals from multiple intermediaries is actually multipath of the same signal.

## Intermediary Initiated Soft Handoff (2)

- M

- M

- M

- M

- M

- BS

- M

- M

- M

- In1, In2 – Intermediaries
- M - Mobile

- M

**Notes:**
This slide shows this could work

## Advantages of Intermediary Initiated Soft Handoff

- Receiver centric
- The intermediary knows about its “Interference temperature” better than the sender.
- Neighbor discovery and maintenance is done at intermediary
- Reduced overhead for the MT.
- (MT need not know the identity of the intermediary)
- Signals from 2 intermediaries is considered as multipath
- During intermediary soft handoff, MT’s parameters for the connection remain unchanged – unlike soft handoff

**Notes:**
The receiver knows more about this surrounding environment than the sender – can take care of interference temperature to determine if the sender can be part of the active list

## FSM of Node in Simulation

- REQUESTED

- DIRECT

- CARRYING

- IDLE

- STRETCHED

**Notes:**
So far, we have seen how by having just 2 hops, we get a simple way of establishing a stretched connection, with minimal overhead. In this slide we give the FSM of a node.

3G’s standby mode when handoff from stretched-direct... 

## Simulation Setup

- Discrete Event simulation
- Recursive path loss formula for Pedestrian and vehicular radio channel in a Manhattan type terrain (UMTS 30.03 Selection Procedures)
- Block size = 20 meters
- Frequency = 1.9Ghz
- Poisson call arrival ( Call rate = 1,2 per hour)
- All nodes moving with velocity 1.5m/s
- Stretched connection pathloss is always less than direct connection.

**Notes:**
We developed a discrete event simulator in java, and used the path loss model specified for pedestrian and vehicular radio channel for a Manhattan type terrain. We used frequency of 1.9 Ghz, a poisson call arrival model and a node mobility model, where all the nodes are moving constantly at 1.5m/s

## Terrain Map

**Notes:**
This slide shows an urban terrain map used to run simulations. The block grids denote obstructions like buildings and the white grids between them are roads. The yellow grids denote intersections. The nodes are initialized at the intersections uniformly. The intersections assign direction and velocities to each of the node. Here you can see an example of the values assigned 
Intersections assign direction probability

## Experimental Setup

- Metrics chosen: Energy, # of handoffs, Time spent in relaying
- Factors chosen: Number of nodes and location of BS
- 1000 sec per run, with 10 repetitions
- Mean and 90% confidence interval plotted
- Diversity gain only between Intermediary and BS
- Pair-wise comparison between stretched and direct connections
- Selection of intermediary based on greedy method

**Notes:**
The metrics we chose to measure are energy comparison between direct and stretched, number of handoffs and time spent in relaying, and the factors were number of nodes, the location of BS and velocity.

We ran 1500 sec simulation, removing the first 500 second simulation to get measurements, when the simulation is in a stable state. We did 10 repetitions for every set and found the mean and calculated for 90% confidence interval.

We did not consider diversity for the intermediary, by removing the cell gain factor.

The selection of intermediary is greedy, where an intermediary is chosen which provides the least cumulative path loss for both upper arm and lower arm. 

## Energy for BS at center

**Notes:**
Scenario: Smaller cell with Omni-directional BS.
Energy savings 3-7 times for call rate 2

## Energy for BS at Corner

**Notes:**
Scenario: Case simulates for a larger cell, with sectoring.
Units: System energy in joules = power * time in 1000 second( duration of a run)
More energy shown here because we used the same grid as in BS at center, and just moved the BS to the corner.
Energy savings  of 4 times – higher savings when the MT is farther from the BS. – call rate 2
Upto 8 times for call rate 1

## Handoff

**Notes:**
Units: Number of handoffs per node during 1000 seconds.
The number of handoffs increase linearly, which means that without the intermediary initiated soft handoff, the overhead can be very high for the power starved MT.

## Time spent in stretched connection by MT

**Notes:**
Units: Comparison of amount of time spent by nodes in direct, stretched and carrying.
The carrying time is percentage of their idle time

With more number of nodes, more stretched connections are possible and more energy savings.

## Related Work

- ODMA & Intelligent Relaying: Multihop relaying
- Integrated Cellular & Ad hoc Relaying Systems (iCAR)
- Relay stations at strategic locations
- To enable rerouting of congested traffic
- Increase capacity
- Uses 802.11 between MTs and relays, and cellular between relays and BS
- Mobility Increases the Capacity of Ad hoc Wireless Networks.
- Multi-user diversity:  Relays “carry” traffic from source to destination.
- Non-real time applications
- 2 hop
- Advantage: O(1) throughput, independent of number of users.

**Notes:**
iCAR: Uses 2 different technologies, requires infrastructure to be installed and also excessive overhead.

MUD: This is for non-real time applications, where delay is unbounded.
IF there are plenty of candidate relays which can be other MTs in the vicinity, data can be transmitted to them with a high throughput.

## Conclusions

- A 2-hop stretched connection yield significant power savings between 3X – 7X!!!!
- The amount of time spent in relaying is 10-15% of idle time – not a significant overhead!!!!
- Handoffs increase linearly with number of nodes
- Intermediary initiated handoff reduces overhead for MT.
- No overhead of ad-hoc networking protocol for 2-hop stretched connection.
- 2-hop stretched connection suitable for real time applications

## Future Work

- Implementing intermediary based soft handoff.
- Capacity and throughput analysis.
- Developing optimizations to choose the best intermediaries